- Email: [email protected]
Superconducting wiggler with semi-cold beam duct at Taiwan light source
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 556 (2006) 607–615 www.elsevier.com/locate/nima
Superconducting wiggler with semi-cold beam duct at Taiwan light source C.-S. Hwang, C.-H. Chang, H.-H. Chen, F.-Y. Lin, T.-C. Fan, M.-H. Huang, J.-C. Jan, K.-T. Hsu, J. Chen, S.-N. Hsu, G.-Y. Hsiung, H.-P. Chang, C.-C. Kuo, Y.-C. Chien, F.-Z. Hsiao, J.-R. Chen, C.-T. Chen National Synchrotron Radiation Research Center (NSRRC), 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu 30076, Taiwan Received 10 August 2005; received in revised form 27 October 2005; accepted 27 October 2005 Available online 6 December 2005
Abstract A 3.2 T superconducting wiggler with a periodic length of 6 cm and 32 poles was designed and fabricated as an X-ray source. The beam duct of this magnet is a semi-cold, ultra-high vacuum chamber that consists of an aluminum and stainless steel taper. The number of poles in this magnet design is even, to minimize the integral strengths of the multipole components. Two measurement systems— involving room-temperature and cryogenic Hall probes—were set up to measure the field of the superconducting wiggler. A cryogenic plant that supplied liquid helium and nitrogen to the superconducting wiggler has already been established. The performance of magnet construction is good and the commissioning of the superconducting wiggler in the storage ring has been successful. No trim coil compensation on the magnet is required to adjust the electron beam orbit. Furthermore, the electron beams exhibit no loss and remain highly stable after the superconducting wiggler has been quenched. r 2005 Elsevier B.V. All rights reserved. PACS: 41.60.Ap; 41.85.Lc; 41.60.Cr Keywords: Superconducting wiggler; Magnet field measurement; Heat load analysis; Magnet cryostat; Cryogenic plant
1. Introduction The protein crystallography and advanced material science is growing up in Taiwan. However, no available straight section can be used to install any insertion device in Taiwan Light Source (TLS). Hence, a short length of 146 cm at the downstream of the superconducting RF cavity (SRF) is selected to accommodate an insertion device. A 3.2 T superconducting wiggler (SW6) [1,2] with a period of 6 cm, has been designed and constructed to fulfill the requirement. Table 1 lists the main specifications of the SW6 and Fig. 1 compares the spectral fluxes of the different insertion devices in the TLS 1.5 GeV storage ring. The X-rays flux, between 2 and 28 keV, from the SW6 exceeded that from other insertion devices. The SW6 was Corresponding author. Tel.: +886 35780281; fax: +886 35783890.
E-mail address: [email protected] (C.-S. Hwang). 0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.10.116
installed in the middle of January 2004 and was tested successfully at the end of April 2004. It was also tested with SRF successfully at the end of December 2004. The SW6 provides three protein crystallography beamlines with energies from 5 to 20 keV. Currently, the SW6 is operated routinely and the beamlines are open to general users. The SW6 was designed with an even number of poles, so the small integral field strength can be easily obtained and the electron trajectory offset adjusted almost to zero, by optimizing the end pole design. This SW6, which has a semi-cold beam duct, has 32 pairs of racetrack NbTi superconducting coils. The semi-cold beam duct is an ultrahigh vacuum (UHV) chamber so a pressure of 109 torr can easily be achieved. The temperature of the beam duct is distributed from 100 to 300 K. Table 1 shows the specifications and parameters of the constructed magnet. Meanwhile, in the magnet, a pair of vapor-cooled copper leads and a gas-electrode insulation array are used to
ARTICLE IN PRESS 608
C.-S. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 607–615
first and second-field integrals are quite close to the designed values. The results of magnet testing show that 13 quenches are required to yield a nominal field of 3.2 T at an excitation current of 285 A. The real boil-off rates of liquid helium and liquid nitrogen are around 2.5 and 1.1 L/ h, respectively. Following the commissioning of the magnet in the storage ring, the electron beams pass through the SW6 and remain in a stable orbit without any trim coil compensation. Hence, this work elucidates the design concept and the performance of the SW6, the semi-cold UHV beam duct, the magnetic field measurement and the results of commissioning the magnet in the TLS storage ring.
Table 1 Specifications of the SW6 magnet in the TLS 1.5 GeV storage ring Number of period Physical length (cm) Magnet period (cm) Magnet gap g (cm) Magnet total dimension L W H (cm) Horizontal (vertical) aperture of beam duct (cm) LHe boiling off (l/h) Beam duct temperature (K) Peak filed (T) Average excitation rate (A/s) Deflection parameter K ¼ 0.934 Bl Radiation angle (mrad.) (68% of flux at 0 mrad & 15 keV) Electron beam size and divergence sx (sy), sx0 (sy0 ) (mm), (mrad) Photon beam size and divergence srx (sry), srx0 (sry0 )(mm), (mrad) at 15 keV Photon flux (brilliance) at 15 keV (0.5 A) Total power (kW) at 500 mA
16 140.6 6.04 1.8 141 120 208 8 (1.1) 2.5 300–100 X3.2 X0.5 X19.1 76 (73) 0.46(0.065), 0.044(0.022) 0.01(0.04), 0.86(0.17) 13
15
6.5 10 (1.2 10 ) 6.4
2. Construction and performance of the magnet The SW6 was fabricated by Wang NMR Inc. (Livermore, USA) and the National Synchrotron Radiation Research Center (NSRRC). The construction of the magnet involves coils, iron poles, return yokes, quench protection components, coil impregnation with an aluminum block, cryogenic components at 4.2, 80 and 300 K, and the semi-cold UHV beam duct. The number of poles in this magnet is even. An aluminum block is impregnated with poles and coils in each up and down magnet array. A magnet gap separator made of aluminum bars, maintains a gap of exactly18 mm between the up and down magnetic array. The construction and performance of the magnet are discussed below. 2.1. Design and construction of magnet
Fig. 1. Intensity spectra of light from the X-ray source at the NSRRC.
recover the warm helium gas and reduce the consumption of liquid helium. Six pairs of back-to-back cold diodes are used for quench protection. Eight suspensions were used to fix the magnet on 300 K vessel. Additionally, a cryoplant [3] was built to provide liquid helium and nitrogen for the superconducting wiggler. Meanwhile, the cryoplant can be switched to provide LHe for the SRF cavity, to support backup in case of emergency or maintenance of the SRF cryoplant. Nowadays, the cryogenic system is used to provide liquid helium and nitrogen to the SW6 and SRF for normal operation. Highly accurate Hall probe [4] and stretch wire systems [5] were employed to measure the roll-off of the central pole, the multipole components and the field distribution along the longitudinal axis. The measurements are compared to the field calculations, and indicate that the
The RADIA code [6] of the 3D model was utilized to design the magnet circuit and optimize the end poles. Table 2 presents the optimal parameters and the dimensions of the flux return yoke, the iron pole and the coil cross-section. Table 3 lists the characteristics of the NbTi superconducting wire. The racetrack coil was constructed by winding a continuous (uncut) superconducting wire over the 32-pole. Accordingly, just a few welded joints are present between the superconducting wires. However, the overlap between the two superconducting wires is 30 cm, guaranteeing that the contact resistance is about 0.01 mO. The 32-pole SW6 has 28 central poles, two-first (1st) end poles (the outside poles) and two-second (2nd) end poles (near the outside poles). Fig. 2 schematically depicts the cross-section of the coils and the poles. Table 2 presents the dimensions. Fig. 3 presents the optimized magnetic field roll-off, DB/B, of the central pole along the x-axis. The DB/Bp0.15% of the central pole is about 720 mm. This range is sufficient to accommodate some misalignments and prevent the accumulation of quadrupole and octupole components. The integral field distribution on the transverse axis always remains at zero, because of the number of poles is even. Therefore, the first integral strength and the integral multipole components are only from errors in the construction of the magnet. Meanwhile, optimizing the end
ARTICLE IN PRESS C.-S. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 607–615
609
Table 2 Principle parameters that govern the design and construction of the magnet
No. of turn/layer Total layers Design current, I0 (A) Coil peak field Bm (T) Pole field strength (T) Current density (A/mm2) Lcx (mm) Rmax (mm) Rmin (mm) Rpole (mm) d (mm) h (mm) t (mm)
Main pole
1st end pole
2nd end pole
26 14 278 4.9 3.2 680 90 14.9 7.62 7.52 0.1 20.323 20.523
26 2 278 2.0 0.8 680 90 8.76 7.72 7.62 0.1 20.323 20.523
26 8 278 4.5 2.3 680 90 14.9 10.74 10.64 0.1 20.323 20.523
Fig. 3. Calculated magnetic field roll-off, DB/B, of the central pole along the x-axis.
Table 3 Specifications of conductor in magnet Cu/SC ratio Twist pitch (mm) Peak field of short sample at 380 A (T) Bare dimensions (mm2) Dimensions including insulation (mm2) Filament size (mm) Number of filaments R.R.R.
1.35 2672 5 0.46 0.72 0.52 0.78 54 54 70
Fig. 4. Calculated and measured (a) magnetic field and (b) second integral field distributions in the longitudinal direction (in the 1.5 GeV storage ring).
Fig. 2. Schematic drawing of design coil, pole and return yoke.
pole structure yields a small second integral strength. Fig. 4 depicts the magnetic field and the second integral field distributions in the longitudinal direction after the end pole has been optimized. It demonstrates that the calculated first and second integral fields are closed to zero. Fig. 5
plots the stored energy and inductance versus the excitation current. The figure shows that the energy and inductance at a nominal field of 3.2 T are 29.2 kJ and 0.75 H, respectively. This information is useful for designing the power supply, the current charging rate, and the quench protection components. Fig. 6 shows in three dimensions the overall structure of the designed magnet array, cryostat, current leads, service tower, and the adjustable support of the SW6. Eight temperature sensors monitor the status of the magnet. The temperature sensors, T1–T4, on the beam duct and T5–T8 on the magnet are PT-100 and Cernox RTDs, respectively.
ARTICLE IN PRESS 610
C.-S. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 607–615
Fig. 7. Cross-section of designed aluminum UHV beam duct and 4.2 K vessel aperture duct (unit: mm).
Fig. 5. Stored energy and inductance versus excitation current.
electric resistance of the 64 coils was 8.370.15 O. These data reveal that the mechanical and winding performance was quite good. Fig. 7 depicts the cross-sections of the designed aperture of the semi-cold beam duct and the liquid helium vessel duct (4.2 K duct). The vertical width and the thickness of the aluminum beam duct in the central region are 11 and 1.2 mm, respectively. A 1.2 mm gap between the Al beam duct and the 4.2 K duct prevents the former from touching the latter. Sixteen G10 rod bumpers with a diameter of 1 mm and a length of 3.5 mm were fixed on the UHV beam duct. The gap between the bumper and the 4.2 K duct was approximately 0.6 mm, to ensure that the only point of contact between the beam duct and the 4.2 K duct was the G10 rods. This design reduces the heat conducted on the coil when the Al beam duct touches the 4.2 K stainless steel duct. The Al beam duct was supported and fixed at both ends of the 4.2 K aperture duct, using a G10 sheet with a thickness of exactly 1.15 mm. The Al beam duct was thermally intersected at 100 K by two pieces of flexible copper plates connected to the liquid nitrogen (LN2) vessel, at both outside of the 4.2 K vessel. 2.2. Design and construction of UHV beam duct
Fig. 6. Overall 3D structure of the SW6, including the magnet array, cryostat, current leads, service tower and adjustable support.
The poles and coils are impregnated with aluminum block [2]. The flatness of the pole and coil in the longitudinal direction, the pole tilt in the transverse direction, the variation of the magnet gap and the periodic length were mechanically measured before the magnetic array was assembled. The measured data indicate that the rms variation of the periodic length was around 20 mm. The maximum variation of the magnet gap was about 40 mm. The maximum pole tilt was 0.1 mrad. The variation in the
Fig. 8 shows that the semi-cold UHV beam duct that the 304 stainless steel taper is connected to the A6061T5 aluminum alloys chamber, by welding the SS/Al bimetal pieces. The location of temperature monitor on beam duct is shown in Fig. 8(b). Temperature sensor of T1 is fixed at center of the Al beam duct and T2 and T3 locate at the two sides of the 80 K thermal intersection. The temperature sensor T4 is fixed at the upstream stainless steel taper. Fig. 8(a) also displays the dimensions of the bimetal and the thickness of the Al chamber. Fig. 8(b) shows a bellow between the flange at 300 K and the beam duct that is used to absorb the thermal contraction of the beam duct. The aperture of the semi-cold bore in vertical and horizontal is 11 and 80 mm, respectively. The aluminum chamber is used to improve the thermal conductivity and the electronic resistance of the beam duct. However, the stainless steel taper is used to reduce the thermal conductivity. Therefore, this beam duct can solve the heat load problem from the image current and the conduction of heat from the room
ARTICLE IN PRESS C.-S. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 607–615
611
operate at temperatures between 300 and 100 K. The length over which the temperature exhibits a gradient from 80 to 300 K on both sides of the beam duct taper is only 100 mm, which is just about to suppress the flow of heat from the duct at room temperature. Finally, the measured temperature of the beam duct is 100 K, which exceeds 80 K. After the SW6 had been installed in the electron storage ring, the UHV beam duct was maintained at room temperature for several weeks to clean the beam duct; cleaning the interior surface of the beam duct reduced the yield of PSD prior to cooling. Accordingly, the number of molecules was reduced and the molecules were absorbed on the cryogenic surface when the magnet was cooled. 2.3. Cryostat design and heat load analysis
Fig. 8. Semi-cold UHV beam duct: (a) dimensions of the Al chamber, Al/ SS bimetal and the stainless steel, (b) overall structure, showing Al chamber, Al/SS bimetal and stainless steel taper with 300 K flange. T1, T2 and T4 locate on beam duct at which temperature are measured.
temperature. The pressure inside the cryogenic beam duct below 20 K [7] is increased by the re-absorption of the accumulated gas molecules that originally resided on the inner surface of the chamber, and their stimulated desorption by scattered synchrotron light. The dominant residual gases in the NSRRC electron storage ring are H2 and CO. Therefore, when the beam duct is kept above 100 K, the number of re-absorbed molecules is small and H2 and CO are not then frozen on the surface but are pumped out. The beam duct is designed to have sufficient space to make an insulating vacuum barrier between the beam duct and the 4.2 K vessel duct. Fig. 7 shows the cross-section of the UHV beam duct and the 4.2 K duct. The Al beam duct is formed by extrusion and the outside surfaces are then machined to yield a distortion of smaller than 0.2 mm. CNC-machining is applied to the external surfaces of the Al beam duct in an ethanol-sprayed environment to prevent contamination of the oil. The beam duct is chemically cleaned thoroughly before TIG welding. The TIG welding of the Al beam duct is performed in the clean room, in which the amount of dust and the humidity are carefully controlled. The beam duct transition tapers of the end section, at between 100 and 300 K, are joined to the Al beam duct by welding the Al/SS bimetal pieces. The temperature of the UHV beam duct was designed to
Fig. 6 schematically depicts the SW6 magnet cryostat. An 80 K aluminum thermal shielding surrounds the 4.4 K cold mass, which consists of the pole pieces, the coils, the aluminum block, the return yoke, the 240 L liquid helium reservoir and a 50 L liquid nitrogen aluminum reservoir. One service tower on the top of the cryostat has a copper current lead feedthrough port, the pumping port, the warm helium gas return port, a vent port with a burst disk, a pressure gauge, a safety valve, and the electrical feedthrough port. A total of 60 (30) super-insulation layers cover the 80 K (4.4 K) thermal radiation shield with a vacuum gap of 25.4 mm (12.7 mm). The 4.4 K vessel, with a cold mass assembly, is suspended by four suspension links near the top of vacuum chamber at 300 K, and by four near the bottom. The suspension links point downward 301 from the vertical direction. The eight suspension links point horizontally away from the center to facilitate the rotational alignment of the 4.4 K cold mass. The links are thermally intercepted at 80 K and can be adjusted from outside the vacuum insulation chamber through eight vacuum ports. All suspension links are made of unidirectional fiberglass and have a racetrack shape. One pair of vapor-cooled leads inside the 4.2 K vessel is used to remove the heat load by counter-flowing cold helium gas. The cryogenic-ceramic breaks with 9 mm diameter hole were welded to the tubular array for passing through cold helium gas to form a gas-electrode insulation array. The gas-electrode insulation array electronically insulates between the power current lead and the whole magnet was connected to a VCR tube that is double-layered suction line. The helium gas was returned to the compressor of the cryogenic system via the suction line. The calculated heat load includes heat from conduction, thermal radiation, synchrotron radiation, heat from the vapor-cooled current lead and the image current. Table 4 presents the estimated heat load, including the extra heat load produced by the image current. The estimated total heat loads at 80 K and 4.2 K are 12.3 W (0.3 L/h) and 1.46 W (2 L/h), respectively. However, the boiling-off rate of the cryostat is 1.1 and 2.5 L/h under normal operating conditions, respectively. The formula for the heat load
ARTICLE IN PRESS C.-S. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 607–615
612
determined from the image current under the narrow gap condition is [8] PðW Þ ¼
128ðrho1=2 ÞT 0 L I 2 . M b Sig z3=2
Under the TLS conditions (E ¼ 1.5 GeV, I ¼ 200 mA, Al resistivity rho ¼ 1.7 108 Om at 100 K, chamber length L ¼ 1 m, period of revolution T0 ¼ 0.4 ms, number of bunches M ¼ 140, beam aperture b ¼ 0.0055 m and bunch length Sig z ¼ 0.006 m), the heat load estimated from the image current is 0.6 W. Furthermore, the heat load from the synchrotron radiation and the scattered light is about 0.17 W. The ANSYS calculation showed that the UHV beam duct temperature is a function of the heat load on the beam duct. The calculations reveal that the beam duct temperature increases by 4 K/1 W on the beam duct [9]. The heat load on beam duct raises the temperature of the beam duct, generating the irradiative heat load on the coil. The ANSYS calculation shows that the coil temperature will increase by 0.05 K/1 W on the beam duct [9]. Therefore, the total heat load on the beam duct is 0.77 W (Table 4), which increases the beam duct temperature by 3.1 K. Fig. 9 plots the measured variation in temperature of the beam duct at different electron beam currents. The temperatures T1 of the beam duct with and without the electron beam are 101.2 and 97.3 K, respectively. The measured results are strongly consistent with the results of ANSYS. 2.4. Quenching protection and power supply Three bipolar power supplies are needed to charge and discharge the magnet. One main power supply with an output 350 A and 15 V is connected in series with all 64 coils to generate the nominal peak field. Hence, one pair of vapor-cooled leads through which is passed 350 A is connected between the superconducting wire at 4.2 K and the normal copper current leads at room temperature. The length and diameter of the vapor-cooled leads are 35 and 0.95 cm, respectively. Additionally, two trim power supplies, each with an output of 20 V and 20 A, are connected in series to the two first end side coils, respectively, to nullify the first and second field integrals. Two pairs of
Table 4 Budgeted cryogenic heat loads of magnet Source of heat
At 4.2 K
At 80 K
Thermal radiation (W) Penetration tubes (W) Suspension support (W) Beam duct conduction (W) Vapor-cooled leads (W) Image current (W) Synchrotron radiation (W) Total heat load (W)
0.01 0.3 0.13 0.01 0.91 0 0.1 1.46
2.1 6.6 1.5 1.3 0 0.6 0.2 12.3
Fig. 9. Variation of beam duct temperature with electron beam current: (a) electron beam current, (b) T4 of stainless steel beam duct taper and (c) T1 of Al beam duct in the central region, and temperatures T2 and T3 at the thermal intercept locations of the Al beam duct.
copper leads with a diameter of 1 mm are used for the trim superconducting wire. If quenching occurs on the superconducting coil, the quenching-protection circuit is designed to reduce the coil voltage and the coil temperature, and to minimize the induced current. The stability of the main power supply should be better than 0.001%/h and that of trim power supply should be better than 0.01%/h. The required resolution of the power supply is about 1 mA, to the first and second integral strengths to be adjusted to zero. Six pairs of back-to-back R620 cold diodes with 5 mO stainless steel resistors are installed. Each pair of back-to-back diode arrays is connected across the 14 coils to form the hardware quench-protection circuit [11]. The diodes do not conduct during normal operation. If a magnet coil is quenched and become resistive, then the voltage across this coil increases, causing the diode to conduct and providing an alternative path through which the current bypasses the resistive coil. This limits the voltage and power dissipated in the coil. The resistors in series with the diodes assist in dissipating the storage energy and limit the build-up of current in the other coils. This quench protection circuit is active when the voltage across any coil exceeds 4 V (forward voltage drop across each cold diode is 0.7 V). Any quenching of the magnet will be detected by comparing the voltage drops. Eight voltage and three temperature signals are also available for use in software quench protection against the coil voltage and temperature over 300 V and 200 K, respectively.
ARTICLE IN PRESS C.-S. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 607–615
2.5. Field measurement Several field measuring methods [4,5], including the Hall probe and the stretch wire method, have been developed to characterize the quality of the field. Before the magnet was assembled in the magnet cryostat, the magnet was put in a test dewar and a cryogenic Hall probe (Lake Shore MCA3160-WN) with an automatic driving and data acquisition measurement system [4] was used to measure the magnetic field along the longitudinal axis at x ¼ 0 mm and that parallel to the axis at x ¼ 20 mm. The measured field in the test dewar is quite consistent with the calculated field. The measured magnetic field roll-off, DB/B, at x ¼ 0 and 20 mm is similar to the designed value. After the magnet was assembled with the magnet cryostat, a room temperature Hall probe [4] was used to measure the field distribution of the magnet at room temperature. A 1 mmthick flat warm duct with an aperture that was 57 mm wide and 5.9 mm high was employed to isolate the vacuum from the atmosphere. The Hall probe was inserted into the flat warm duct to measure the magnetic field at room temperature [4]. Fig. 4 presents the measured field on the on-axis and the distributions of the second field integrals along the longitudinal axis. The trim current of the end pole does not apply to correct the measured field. The measured field strength is almost the same as the designed field strength. However, the second integral field distribution exhibits an orbit offset of 18 mm. This orbit offset can be corrected by the end pole trim current. Accordingly, the construction performance is quite consistent with that of the ideal design. The measured field strength as a function of the current is used to determine the transfer function of the peak field and to establish an integral field look-up table for use during the commissioning of the magnet. 3. Cryogenic system A helium cryogenic system that is based on the modified Claude cycle was built up to supply liquid helium and nitrogen for SRF and SW6. The cryogenic system [3] includes one 315 kW compressor, one 45 kW recovery compressor, one 10 kW refrigerator, one 2000 L dewar and two 100 m3 helium gaseous buffer tanks. Inside the cold box, two expansion turbines connected in series provide most of the cooling power for the helium gas stream. The cold box is equipped with two 80 K absorbers and one 20 K absorber. The two 80 K absorbers are fully automatically switched and regenerated without disturbing the cold box. Two analyzers monitor the impurity of the flowing helium. One analyzer monitors the oxygen and humidity levels on the side of the cold box; the other monitors the oil aerosol and the nitrogen level on the side of the compressor. The designed thermal dynamic state and preliminary test results concerning this helium system can be found in Refs. [3,10]. The system provides a maximum liquefaction rate of 134 L/h when liquid nitrogen is used for pre-cooling and a liquefaction rate of 52 L/h without liquid nitrogen. The
613
cooling power of this cryogenic system has been measured and the data shows that the cooling power in refrigeration mode with and without liquid nitrogen for pre-cooling is 469.5 and 327.2 W, respectively. The stand-alone operation of the cryogenic system has the following features. The interlock logic protects the cryogenic system from utility failure and the recovery compressor returns the helium to the storage tanks, maintaining a suction line pressure of 1.1 bar. The emergency power ensures the continuous operation of the control system and the recovery compressor minimizes helium loss in the event of an electrical failure. As the dewar level approaches the setting value (80%), the turbines slow down and the heater in the dewar is turned on to prevent an over-level event. Liquid helium and liquid nitrogen are supplied to the SW6 through a multi-channel transfer line. The cold gases from the dewar and the multi-channel transfer line are returned to the cold box. However, the cold gas from the SW6 cryostat is warmed up by a passive warmer before returning to the compressor. The operating pressures of the SW6 and the dewar are 1.08 and 1.4 bar, respectively. 4. Magnet testing and commissioning The commissioning of the SW6 with the cryogenic system has been successful. A special LHe transfer line was designed to be operated during both the pre-cooling and the normal operation stages. The SW6 can be filled with liquid helium and nitrogen in the PID control mode (continuous-filling mode) and the on/off-filling mode. However, the control valve box, the multi-channel line, the flexible transfer line and the connectors, are estimated to lose heat at a total rate of 30 W. Filling the SW6 cryostat in the PID control mode consumed liquid helium at a rate of over 60 L/h. Hence, the on/off-filling mode is selected to keep the LHe level at (75710)% to reduce the rate of consumption. Consequently, the mean value of the liquid helium consumption is reduced from 60 to be 4 L/h. For the magnet alone (without liquid filling), the boil-off rates of LHe and LN2 are around 2.5 and 1.1 L/h, respectively, at a magnet excitation current of 285 A. Magnet quenching vaporized a maximum of 10 L of liquid helium within 15 s. The stored energy in the coil is 29 kJ, some of which is dumped to the flywheel diode. During the period of training of SW6, the recovery compressor was kept running to recycle the large amount of the gaseous helium from the SW6 cryostat. The operating temperature (T5) of the magnet is about 4.3 K. Moreover, two sensors, T6 and T7, on the two protection diodes detect the temperature of the diodes when the magnet is quenched. The heat load from the synchrotron light, the scattering light and the image current increases the temperature of the beam duct. The temperatures of the beam duct with (without) a 200 mA electron beam are around T1 ¼ 101 K (97 K), T2 ¼ 102 K (100 K), T3 ¼ 101 K (99 K) and T4 ¼ 233 K (229 K). The temperature of the beam duct is a function of the electron
ARTICLE IN PRESS 614
C.-S. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 607–615
beam current and the magnet will be quenched when the temperature of the beam duct (T1) exceeds 136 K at the LHe vessel pressure of 15 psi. Cold helium gas is used to cool the vapor-cooled current lead. Thus, the measured temperature of the gas-electrode insulation pipe is between 60 and 200 K. The cold helium gas is warmed using a warmer that consists of a multi-channel aluminum pipe, and the warm helium gas is recycled to the recovery compressor. The testing of the magnet in the storage ring was successful. Thirteen training events of SW6 yielded a field of 3.2 T at the nominal current of 285 A. SW6 quenching only weakly affects the cryogenic system; the pressure fluctuates by 72 mbar in the suction line and by 73 mbar in the main dewar. The temperature of the cold gas returned from the valve box is increased by 0.3 K, and the frequency of the cold turbine fluctuates by 710 Hz. The vacuum pressures of the SW6 beam duct change only a little with different temperature on the beam duct (Fig. 10). The pressure of the UHV beam duct is maintained at an almost constant value (0.23–0.19 nTorr), regardless of whether the beam duct is warm or cold, or of whether the beam current (of up to 200 mA) is stored. Fig. 10 plots the pressure of the UHV beam duct in the various electron beam current and temperatures. Trim coils on the SW6 magnet can be used to compensate for the first and second field integrals, but this option is not exploited because the field quality is already quite consistent with the designed
Fig. 10. Vacuum pressure of the UHV beam duct with and without an electron beam current of 200 mA: (a) electron beam current, (b) T1 of the beam duct and (c) vacuum pressure of the SW6 Al UHV beam duct.
value. The commissioning results indicate that the tune shift effect of the SW6 is very consistent with the value predicted by the model, and the magnetic field causes no
Fig. 11. Horizontal and vertical tune shifts versus the magnetic flux density of the SW6.
Fig. 12. Deviation of (a) electron beam current, (b) electron beam size, (c) lifetime and normalized photon flux and (d) electron orbit, due to SW6 quenching.
ARTICLE IN PRESS C.-S. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 607–615
unexpected horizontal tune shift. Fig. 11 presents the measured horizontal and vertical tune shifts. When the magnet is quenched or the power supply is tripped off, the electron beam current remains without beam loss, as displayed in Fig. 12(a) and the orbit of the electron beam does not detectably change (Fig. 12(d)). However, the horizontal and vertical beam sizes changed in steps of 6 mm (Fig. 12(b)) and the lifetime (Fig. (12(c)) changes a little when the magnet is quenched. Although, the photon flux deviation ratio (DI/I0) (where I0 is measured using the beam line photon monitor) changes by around 2.5% when the magnet is quenched, the photon flux deviation ratio recovers to 0.04% after 10 min and the photon beam is kept stable again (Fig. 12(c)). The even-pole design of the SW6 is very effective for preventing the loss of electron beams when the magnet is quenched.
5. Conclusions An aluminum chamber is an effective UHV electron beam duct when the temperature is maintained at 100 K. The beam duct does not need to be coated with Cu to solve the heat load problem. The results of commissioning the magnet in the storage ring confirm that magnet performs well and the real magnetic field is close to the ideal value. Additionally, the even-pole design of a superconducting wiggler and undulator is preferred, especially for use in high-field insertion devices. The authors believe that the even-pole design helps to prevent loss of the electron beam current and to keep the beam when the magnet is quenched. The pumping and purging of the magnet system involves liquid transfer and return gas lines to maintain at a low dew point; purify impure gases and reduce the number of particles, to prevent a fault of the cryogenic system. This process prevents damage to the refrigerator of the cryogenic system.
615
Acknowledgements The authors would like to thank the National Science Council of Taiwan for partially supporting this research under Contract No. NSC 94-2112-M-213-018. Mr. C. J Lin and Dr. D. J. Wang are appreciated for their technical support as well as Ms. W. P. Li, and P. H. Lin for processing the measured data. References [1] C.S. Hwang, B. Wang, et al., Design of a superconducting multipole wiggler for synchrotron radiation, IEEE Trans. Appl. Supercond. 13 (2) (2003) 1209. [2] C. S. Hwang, B. Wang, et al., Construction and performance of a superconducting multipole wiggler, in: Eighth International Conference on Synchrotron Radiation Instrumentation, AIP, CPC05, 2004, p. 199. [3] Hsiao, F. Z., et al., The liquid helium cryogenic system for the superconducting cavity in SRRC, in: PAC2001, Chicago, Illinois, 2001. p. 1604. [4] T. C. Fan, F. Y. Lin, M.H. Huang, C.H. Chang, C. S. Hwang, Magnetic field measurement on superconducting multipole wiggler with narrow duct, in: PAC’2003, Oregon, Portland, 2003, p. 1047. [5] C.S. Hwang, F.Y. Lin, T.C. Fan, Integral magnetic field measurement using an automatic fast long-loop-flip coil system, IEEE Trans. Instrum. Meas. 52 (3) (2003) 865. [6] P. Elleaume, O. Chubar, J. Chavanne, Computing 3D magnetic field from insertion devices, in: PAC1997, Vancouver, Canada, 1997, p. 3509. [7] V. Baglin, I.R. Collins, O. Gro¨bner, C. Gru¨nhagel, B. Jenninger, Molecular desorption by synchrotron radiation and sticking coefficient at cryogenic temperatures for H2, CH4, CO and CO2, Vacuum 67 (2002) 421. [8] Private communication with Dr. Alex Chao. [9] H.H. Chen, C.S. Hwang, et al, Heat load analysis of superconducting wiggler with Semi-cold UHV beam duct, IEEE Trans. Appl. Supercond, in press (2006). [10] F.Z. Hsiao, et al., the pilot-runs of the helium crygoneic system for the TLS superconducting cavity, in: PAC2003, Oregon, Portland, 2003, 2402. [11] C.S. Hwang, B. Wang, R. Wahrer, H.H. Chen, C.H. Chang, F.Y. Lin, T.C. Fan, C.T. Chen, Construction and performance of a compact cryogen-free superconducting wavelength shifter, IEEE Trans. Appl. Supercond. 12 (1) (2002) 686.
Nuclear Instruments and Methods in Physics Research A 556 (2006) 607–615 www.elsevier.com/locate/nima
Superconducting wiggler with semi-cold beam duct at Taiwan light source C.-S. Hwang, C.-H. Chang, H.-H. Chen, F.-Y. Lin, T.-C. Fan, M.-H. Huang, J.-C. Jan, K.-T. Hsu, J. Chen, S.-N. Hsu, G.-Y. Hsiung, H.-P. Chang, C.-C. Kuo, Y.-C. Chien, F.-Z. Hsiao, J.-R. Chen, C.-T. Chen National Synchrotron Radiation Research Center (NSRRC), 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu 30076, Taiwan Received 10 August 2005; received in revised form 27 October 2005; accepted 27 October 2005 Available online 6 December 2005
Abstract A 3.2 T superconducting wiggler with a periodic length of 6 cm and 32 poles was designed and fabricated as an X-ray source. The beam duct of this magnet is a semi-cold, ultra-high vacuum chamber that consists of an aluminum and stainless steel taper. The number of poles in this magnet design is even, to minimize the integral strengths of the multipole components. Two measurement systems— involving room-temperature and cryogenic Hall probes—were set up to measure the field of the superconducting wiggler. A cryogenic plant that supplied liquid helium and nitrogen to the superconducting wiggler has already been established. The performance of magnet construction is good and the commissioning of the superconducting wiggler in the storage ring has been successful. No trim coil compensation on the magnet is required to adjust the electron beam orbit. Furthermore, the electron beams exhibit no loss and remain highly stable after the superconducting wiggler has been quenched. r 2005 Elsevier B.V. All rights reserved. PACS: 41.60.Ap; 41.85.Lc; 41.60.Cr Keywords: Superconducting wiggler; Magnet field measurement; Heat load analysis; Magnet cryostat; Cryogenic plant
1. Introduction The protein crystallography and advanced material science is growing up in Taiwan. However, no available straight section can be used to install any insertion device in Taiwan Light Source (TLS). Hence, a short length of 146 cm at the downstream of the superconducting RF cavity (SRF) is selected to accommodate an insertion device. A 3.2 T superconducting wiggler (SW6) [1,2] with a period of 6 cm, has been designed and constructed to fulfill the requirement. Table 1 lists the main specifications of the SW6 and Fig. 1 compares the spectral fluxes of the different insertion devices in the TLS 1.5 GeV storage ring. The X-rays flux, between 2 and 28 keV, from the SW6 exceeded that from other insertion devices. The SW6 was Corresponding author. Tel.: +886 35780281; fax: +886 35783890.
E-mail address: [email protected] (C.-S. Hwang). 0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.10.116
installed in the middle of January 2004 and was tested successfully at the end of April 2004. It was also tested with SRF successfully at the end of December 2004. The SW6 provides three protein crystallography beamlines with energies from 5 to 20 keV. Currently, the SW6 is operated routinely and the beamlines are open to general users. The SW6 was designed with an even number of poles, so the small integral field strength can be easily obtained and the electron trajectory offset adjusted almost to zero, by optimizing the end pole design. This SW6, which has a semi-cold beam duct, has 32 pairs of racetrack NbTi superconducting coils. The semi-cold beam duct is an ultrahigh vacuum (UHV) chamber so a pressure of 109 torr can easily be achieved. The temperature of the beam duct is distributed from 100 to 300 K. Table 1 shows the specifications and parameters of the constructed magnet. Meanwhile, in the magnet, a pair of vapor-cooled copper leads and a gas-electrode insulation array are used to
ARTICLE IN PRESS 608
C.-S. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 607–615
first and second-field integrals are quite close to the designed values. The results of magnet testing show that 13 quenches are required to yield a nominal field of 3.2 T at an excitation current of 285 A. The real boil-off rates of liquid helium and liquid nitrogen are around 2.5 and 1.1 L/ h, respectively. Following the commissioning of the magnet in the storage ring, the electron beams pass through the SW6 and remain in a stable orbit without any trim coil compensation. Hence, this work elucidates the design concept and the performance of the SW6, the semi-cold UHV beam duct, the magnetic field measurement and the results of commissioning the magnet in the TLS storage ring.
Table 1 Specifications of the SW6 magnet in the TLS 1.5 GeV storage ring Number of period Physical length (cm) Magnet period (cm) Magnet gap g (cm) Magnet total dimension L W H (cm) Horizontal (vertical) aperture of beam duct (cm) LHe boiling off (l/h) Beam duct temperature (K) Peak filed (T) Average excitation rate (A/s) Deflection parameter K ¼ 0.934 Bl Radiation angle (mrad.) (68% of flux at 0 mrad & 15 keV) Electron beam size and divergence sx (sy), sx0 (sy0 ) (mm), (mrad) Photon beam size and divergence srx (sry), srx0 (sry0 )(mm), (mrad) at 15 keV Photon flux (brilliance) at 15 keV (0.5 A) Total power (kW) at 500 mA
16 140.6 6.04 1.8 141 120 208 8 (1.1) 2.5 300–100 X3.2 X0.5 X19.1 76 (73) 0.46(0.065), 0.044(0.022) 0.01(0.04), 0.86(0.17) 13
15
6.5 10 (1.2 10 ) 6.4
2. Construction and performance of the magnet The SW6 was fabricated by Wang NMR Inc. (Livermore, USA) and the National Synchrotron Radiation Research Center (NSRRC). The construction of the magnet involves coils, iron poles, return yokes, quench protection components, coil impregnation with an aluminum block, cryogenic components at 4.2, 80 and 300 K, and the semi-cold UHV beam duct. The number of poles in this magnet is even. An aluminum block is impregnated with poles and coils in each up and down magnet array. A magnet gap separator made of aluminum bars, maintains a gap of exactly18 mm between the up and down magnetic array. The construction and performance of the magnet are discussed below. 2.1. Design and construction of magnet
Fig. 1. Intensity spectra of light from the X-ray source at the NSRRC.
recover the warm helium gas and reduce the consumption of liquid helium. Six pairs of back-to-back cold diodes are used for quench protection. Eight suspensions were used to fix the magnet on 300 K vessel. Additionally, a cryoplant [3] was built to provide liquid helium and nitrogen for the superconducting wiggler. Meanwhile, the cryoplant can be switched to provide LHe for the SRF cavity, to support backup in case of emergency or maintenance of the SRF cryoplant. Nowadays, the cryogenic system is used to provide liquid helium and nitrogen to the SW6 and SRF for normal operation. Highly accurate Hall probe [4] and stretch wire systems [5] were employed to measure the roll-off of the central pole, the multipole components and the field distribution along the longitudinal axis. The measurements are compared to the field calculations, and indicate that the
The RADIA code [6] of the 3D model was utilized to design the magnet circuit and optimize the end poles. Table 2 presents the optimal parameters and the dimensions of the flux return yoke, the iron pole and the coil cross-section. Table 3 lists the characteristics of the NbTi superconducting wire. The racetrack coil was constructed by winding a continuous (uncut) superconducting wire over the 32-pole. Accordingly, just a few welded joints are present between the superconducting wires. However, the overlap between the two superconducting wires is 30 cm, guaranteeing that the contact resistance is about 0.01 mO. The 32-pole SW6 has 28 central poles, two-first (1st) end poles (the outside poles) and two-second (2nd) end poles (near the outside poles). Fig. 2 schematically depicts the cross-section of the coils and the poles. Table 2 presents the dimensions. Fig. 3 presents the optimized magnetic field roll-off, DB/B, of the central pole along the x-axis. The DB/Bp0.15% of the central pole is about 720 mm. This range is sufficient to accommodate some misalignments and prevent the accumulation of quadrupole and octupole components. The integral field distribution on the transverse axis always remains at zero, because of the number of poles is even. Therefore, the first integral strength and the integral multipole components are only from errors in the construction of the magnet. Meanwhile, optimizing the end
ARTICLE IN PRESS C.-S. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 607–615
609
Table 2 Principle parameters that govern the design and construction of the magnet
No. of turn/layer Total layers Design current, I0 (A) Coil peak field Bm (T) Pole field strength (T) Current density (A/mm2) Lcx (mm) Rmax (mm) Rmin (mm) Rpole (mm) d (mm) h (mm) t (mm)
Main pole
1st end pole
2nd end pole
26 14 278 4.9 3.2 680 90 14.9 7.62 7.52 0.1 20.323 20.523
26 2 278 2.0 0.8 680 90 8.76 7.72 7.62 0.1 20.323 20.523
26 8 278 4.5 2.3 680 90 14.9 10.74 10.64 0.1 20.323 20.523
Fig. 3. Calculated magnetic field roll-off, DB/B, of the central pole along the x-axis.
Table 3 Specifications of conductor in magnet Cu/SC ratio Twist pitch (mm) Peak field of short sample at 380 A (T) Bare dimensions (mm2) Dimensions including insulation (mm2) Filament size (mm) Number of filaments R.R.R.
1.35 2672 5 0.46 0.72 0.52 0.78 54 54 70
Fig. 4. Calculated and measured (a) magnetic field and (b) second integral field distributions in the longitudinal direction (in the 1.5 GeV storage ring).
Fig. 2. Schematic drawing of design coil, pole and return yoke.
pole structure yields a small second integral strength. Fig. 4 depicts the magnetic field and the second integral field distributions in the longitudinal direction after the end pole has been optimized. It demonstrates that the calculated first and second integral fields are closed to zero. Fig. 5
plots the stored energy and inductance versus the excitation current. The figure shows that the energy and inductance at a nominal field of 3.2 T are 29.2 kJ and 0.75 H, respectively. This information is useful for designing the power supply, the current charging rate, and the quench protection components. Fig. 6 shows in three dimensions the overall structure of the designed magnet array, cryostat, current leads, service tower, and the adjustable support of the SW6. Eight temperature sensors monitor the status of the magnet. The temperature sensors, T1–T4, on the beam duct and T5–T8 on the magnet are PT-100 and Cernox RTDs, respectively.
ARTICLE IN PRESS 610
C.-S. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 607–615
Fig. 7. Cross-section of designed aluminum UHV beam duct and 4.2 K vessel aperture duct (unit: mm).
Fig. 5. Stored energy and inductance versus excitation current.
electric resistance of the 64 coils was 8.370.15 O. These data reveal that the mechanical and winding performance was quite good. Fig. 7 depicts the cross-sections of the designed aperture of the semi-cold beam duct and the liquid helium vessel duct (4.2 K duct). The vertical width and the thickness of the aluminum beam duct in the central region are 11 and 1.2 mm, respectively. A 1.2 mm gap between the Al beam duct and the 4.2 K duct prevents the former from touching the latter. Sixteen G10 rod bumpers with a diameter of 1 mm and a length of 3.5 mm were fixed on the UHV beam duct. The gap between the bumper and the 4.2 K duct was approximately 0.6 mm, to ensure that the only point of contact between the beam duct and the 4.2 K duct was the G10 rods. This design reduces the heat conducted on the coil when the Al beam duct touches the 4.2 K stainless steel duct. The Al beam duct was supported and fixed at both ends of the 4.2 K aperture duct, using a G10 sheet with a thickness of exactly 1.15 mm. The Al beam duct was thermally intersected at 100 K by two pieces of flexible copper plates connected to the liquid nitrogen (LN2) vessel, at both outside of the 4.2 K vessel. 2.2. Design and construction of UHV beam duct
Fig. 6. Overall 3D structure of the SW6, including the magnet array, cryostat, current leads, service tower and adjustable support.
The poles and coils are impregnated with aluminum block [2]. The flatness of the pole and coil in the longitudinal direction, the pole tilt in the transverse direction, the variation of the magnet gap and the periodic length were mechanically measured before the magnetic array was assembled. The measured data indicate that the rms variation of the periodic length was around 20 mm. The maximum variation of the magnet gap was about 40 mm. The maximum pole tilt was 0.1 mrad. The variation in the
Fig. 8 shows that the semi-cold UHV beam duct that the 304 stainless steel taper is connected to the A6061T5 aluminum alloys chamber, by welding the SS/Al bimetal pieces. The location of temperature monitor on beam duct is shown in Fig. 8(b). Temperature sensor of T1 is fixed at center of the Al beam duct and T2 and T3 locate at the two sides of the 80 K thermal intersection. The temperature sensor T4 is fixed at the upstream stainless steel taper. Fig. 8(a) also displays the dimensions of the bimetal and the thickness of the Al chamber. Fig. 8(b) shows a bellow between the flange at 300 K and the beam duct that is used to absorb the thermal contraction of the beam duct. The aperture of the semi-cold bore in vertical and horizontal is 11 and 80 mm, respectively. The aluminum chamber is used to improve the thermal conductivity and the electronic resistance of the beam duct. However, the stainless steel taper is used to reduce the thermal conductivity. Therefore, this beam duct can solve the heat load problem from the image current and the conduction of heat from the room
ARTICLE IN PRESS C.-S. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 607–615
611
operate at temperatures between 300 and 100 K. The length over which the temperature exhibits a gradient from 80 to 300 K on both sides of the beam duct taper is only 100 mm, which is just about to suppress the flow of heat from the duct at room temperature. Finally, the measured temperature of the beam duct is 100 K, which exceeds 80 K. After the SW6 had been installed in the electron storage ring, the UHV beam duct was maintained at room temperature for several weeks to clean the beam duct; cleaning the interior surface of the beam duct reduced the yield of PSD prior to cooling. Accordingly, the number of molecules was reduced and the molecules were absorbed on the cryogenic surface when the magnet was cooled. 2.3. Cryostat design and heat load analysis
Fig. 8. Semi-cold UHV beam duct: (a) dimensions of the Al chamber, Al/ SS bimetal and the stainless steel, (b) overall structure, showing Al chamber, Al/SS bimetal and stainless steel taper with 300 K flange. T1, T2 and T4 locate on beam duct at which temperature are measured.
temperature. The pressure inside the cryogenic beam duct below 20 K [7] is increased by the re-absorption of the accumulated gas molecules that originally resided on the inner surface of the chamber, and their stimulated desorption by scattered synchrotron light. The dominant residual gases in the NSRRC electron storage ring are H2 and CO. Therefore, when the beam duct is kept above 100 K, the number of re-absorbed molecules is small and H2 and CO are not then frozen on the surface but are pumped out. The beam duct is designed to have sufficient space to make an insulating vacuum barrier between the beam duct and the 4.2 K vessel duct. Fig. 7 shows the cross-section of the UHV beam duct and the 4.2 K duct. The Al beam duct is formed by extrusion and the outside surfaces are then machined to yield a distortion of smaller than 0.2 mm. CNC-machining is applied to the external surfaces of the Al beam duct in an ethanol-sprayed environment to prevent contamination of the oil. The beam duct is chemically cleaned thoroughly before TIG welding. The TIG welding of the Al beam duct is performed in the clean room, in which the amount of dust and the humidity are carefully controlled. The beam duct transition tapers of the end section, at between 100 and 300 K, are joined to the Al beam duct by welding the Al/SS bimetal pieces. The temperature of the UHV beam duct was designed to
Fig. 6 schematically depicts the SW6 magnet cryostat. An 80 K aluminum thermal shielding surrounds the 4.4 K cold mass, which consists of the pole pieces, the coils, the aluminum block, the return yoke, the 240 L liquid helium reservoir and a 50 L liquid nitrogen aluminum reservoir. One service tower on the top of the cryostat has a copper current lead feedthrough port, the pumping port, the warm helium gas return port, a vent port with a burst disk, a pressure gauge, a safety valve, and the electrical feedthrough port. A total of 60 (30) super-insulation layers cover the 80 K (4.4 K) thermal radiation shield with a vacuum gap of 25.4 mm (12.7 mm). The 4.4 K vessel, with a cold mass assembly, is suspended by four suspension links near the top of vacuum chamber at 300 K, and by four near the bottom. The suspension links point downward 301 from the vertical direction. The eight suspension links point horizontally away from the center to facilitate the rotational alignment of the 4.4 K cold mass. The links are thermally intercepted at 80 K and can be adjusted from outside the vacuum insulation chamber through eight vacuum ports. All suspension links are made of unidirectional fiberglass and have a racetrack shape. One pair of vapor-cooled leads inside the 4.2 K vessel is used to remove the heat load by counter-flowing cold helium gas. The cryogenic-ceramic breaks with 9 mm diameter hole were welded to the tubular array for passing through cold helium gas to form a gas-electrode insulation array. The gas-electrode insulation array electronically insulates between the power current lead and the whole magnet was connected to a VCR tube that is double-layered suction line. The helium gas was returned to the compressor of the cryogenic system via the suction line. The calculated heat load includes heat from conduction, thermal radiation, synchrotron radiation, heat from the vapor-cooled current lead and the image current. Table 4 presents the estimated heat load, including the extra heat load produced by the image current. The estimated total heat loads at 80 K and 4.2 K are 12.3 W (0.3 L/h) and 1.46 W (2 L/h), respectively. However, the boiling-off rate of the cryostat is 1.1 and 2.5 L/h under normal operating conditions, respectively. The formula for the heat load
ARTICLE IN PRESS C.-S. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 607–615
612
determined from the image current under the narrow gap condition is [8] PðW Þ ¼
128ðrho1=2 ÞT 0 L I 2 . M b Sig z3=2
Under the TLS conditions (E ¼ 1.5 GeV, I ¼ 200 mA, Al resistivity rho ¼ 1.7 108 Om at 100 K, chamber length L ¼ 1 m, period of revolution T0 ¼ 0.4 ms, number of bunches M ¼ 140, beam aperture b ¼ 0.0055 m and bunch length Sig z ¼ 0.006 m), the heat load estimated from the image current is 0.6 W. Furthermore, the heat load from the synchrotron radiation and the scattered light is about 0.17 W. The ANSYS calculation showed that the UHV beam duct temperature is a function of the heat load on the beam duct. The calculations reveal that the beam duct temperature increases by 4 K/1 W on the beam duct [9]. The heat load on beam duct raises the temperature of the beam duct, generating the irradiative heat load on the coil. The ANSYS calculation shows that the coil temperature will increase by 0.05 K/1 W on the beam duct [9]. Therefore, the total heat load on the beam duct is 0.77 W (Table 4), which increases the beam duct temperature by 3.1 K. Fig. 9 plots the measured variation in temperature of the beam duct at different electron beam currents. The temperatures T1 of the beam duct with and without the electron beam are 101.2 and 97.3 K, respectively. The measured results are strongly consistent with the results of ANSYS. 2.4. Quenching protection and power supply Three bipolar power supplies are needed to charge and discharge the magnet. One main power supply with an output 350 A and 15 V is connected in series with all 64 coils to generate the nominal peak field. Hence, one pair of vapor-cooled leads through which is passed 350 A is connected between the superconducting wire at 4.2 K and the normal copper current leads at room temperature. The length and diameter of the vapor-cooled leads are 35 and 0.95 cm, respectively. Additionally, two trim power supplies, each with an output of 20 V and 20 A, are connected in series to the two first end side coils, respectively, to nullify the first and second field integrals. Two pairs of
Table 4 Budgeted cryogenic heat loads of magnet Source of heat
At 4.2 K
At 80 K
Thermal radiation (W) Penetration tubes (W) Suspension support (W) Beam duct conduction (W) Vapor-cooled leads (W) Image current (W) Synchrotron radiation (W) Total heat load (W)
0.01 0.3 0.13 0.01 0.91 0 0.1 1.46
2.1 6.6 1.5 1.3 0 0.6 0.2 12.3
Fig. 9. Variation of beam duct temperature with electron beam current: (a) electron beam current, (b) T4 of stainless steel beam duct taper and (c) T1 of Al beam duct in the central region, and temperatures T2 and T3 at the thermal intercept locations of the Al beam duct.
copper leads with a diameter of 1 mm are used for the trim superconducting wire. If quenching occurs on the superconducting coil, the quenching-protection circuit is designed to reduce the coil voltage and the coil temperature, and to minimize the induced current. The stability of the main power supply should be better than 0.001%/h and that of trim power supply should be better than 0.01%/h. The required resolution of the power supply is about 1 mA, to the first and second integral strengths to be adjusted to zero. Six pairs of back-to-back R620 cold diodes with 5 mO stainless steel resistors are installed. Each pair of back-to-back diode arrays is connected across the 14 coils to form the hardware quench-protection circuit [11]. The diodes do not conduct during normal operation. If a magnet coil is quenched and become resistive, then the voltage across this coil increases, causing the diode to conduct and providing an alternative path through which the current bypasses the resistive coil. This limits the voltage and power dissipated in the coil. The resistors in series with the diodes assist in dissipating the storage energy and limit the build-up of current in the other coils. This quench protection circuit is active when the voltage across any coil exceeds 4 V (forward voltage drop across each cold diode is 0.7 V). Any quenching of the magnet will be detected by comparing the voltage drops. Eight voltage and three temperature signals are also available for use in software quench protection against the coil voltage and temperature over 300 V and 200 K, respectively.
ARTICLE IN PRESS C.-S. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 607–615
2.5. Field measurement Several field measuring methods [4,5], including the Hall probe and the stretch wire method, have been developed to characterize the quality of the field. Before the magnet was assembled in the magnet cryostat, the magnet was put in a test dewar and a cryogenic Hall probe (Lake Shore MCA3160-WN) with an automatic driving and data acquisition measurement system [4] was used to measure the magnetic field along the longitudinal axis at x ¼ 0 mm and that parallel to the axis at x ¼ 20 mm. The measured field in the test dewar is quite consistent with the calculated field. The measured magnetic field roll-off, DB/B, at x ¼ 0 and 20 mm is similar to the designed value. After the magnet was assembled with the magnet cryostat, a room temperature Hall probe [4] was used to measure the field distribution of the magnet at room temperature. A 1 mmthick flat warm duct with an aperture that was 57 mm wide and 5.9 mm high was employed to isolate the vacuum from the atmosphere. The Hall probe was inserted into the flat warm duct to measure the magnetic field at room temperature [4]. Fig. 4 presents the measured field on the on-axis and the distributions of the second field integrals along the longitudinal axis. The trim current of the end pole does not apply to correct the measured field. The measured field strength is almost the same as the designed field strength. However, the second integral field distribution exhibits an orbit offset of 18 mm. This orbit offset can be corrected by the end pole trim current. Accordingly, the construction performance is quite consistent with that of the ideal design. The measured field strength as a function of the current is used to determine the transfer function of the peak field and to establish an integral field look-up table for use during the commissioning of the magnet. 3. Cryogenic system A helium cryogenic system that is based on the modified Claude cycle was built up to supply liquid helium and nitrogen for SRF and SW6. The cryogenic system [3] includes one 315 kW compressor, one 45 kW recovery compressor, one 10 kW refrigerator, one 2000 L dewar and two 100 m3 helium gaseous buffer tanks. Inside the cold box, two expansion turbines connected in series provide most of the cooling power for the helium gas stream. The cold box is equipped with two 80 K absorbers and one 20 K absorber. The two 80 K absorbers are fully automatically switched and regenerated without disturbing the cold box. Two analyzers monitor the impurity of the flowing helium. One analyzer monitors the oxygen and humidity levels on the side of the cold box; the other monitors the oil aerosol and the nitrogen level on the side of the compressor. The designed thermal dynamic state and preliminary test results concerning this helium system can be found in Refs. [3,10]. The system provides a maximum liquefaction rate of 134 L/h when liquid nitrogen is used for pre-cooling and a liquefaction rate of 52 L/h without liquid nitrogen. The
613
cooling power of this cryogenic system has been measured and the data shows that the cooling power in refrigeration mode with and without liquid nitrogen for pre-cooling is 469.5 and 327.2 W, respectively. The stand-alone operation of the cryogenic system has the following features. The interlock logic protects the cryogenic system from utility failure and the recovery compressor returns the helium to the storage tanks, maintaining a suction line pressure of 1.1 bar. The emergency power ensures the continuous operation of the control system and the recovery compressor minimizes helium loss in the event of an electrical failure. As the dewar level approaches the setting value (80%), the turbines slow down and the heater in the dewar is turned on to prevent an over-level event. Liquid helium and liquid nitrogen are supplied to the SW6 through a multi-channel transfer line. The cold gases from the dewar and the multi-channel transfer line are returned to the cold box. However, the cold gas from the SW6 cryostat is warmed up by a passive warmer before returning to the compressor. The operating pressures of the SW6 and the dewar are 1.08 and 1.4 bar, respectively. 4. Magnet testing and commissioning The commissioning of the SW6 with the cryogenic system has been successful. A special LHe transfer line was designed to be operated during both the pre-cooling and the normal operation stages. The SW6 can be filled with liquid helium and nitrogen in the PID control mode (continuous-filling mode) and the on/off-filling mode. However, the control valve box, the multi-channel line, the flexible transfer line and the connectors, are estimated to lose heat at a total rate of 30 W. Filling the SW6 cryostat in the PID control mode consumed liquid helium at a rate of over 60 L/h. Hence, the on/off-filling mode is selected to keep the LHe level at (75710)% to reduce the rate of consumption. Consequently, the mean value of the liquid helium consumption is reduced from 60 to be 4 L/h. For the magnet alone (without liquid filling), the boil-off rates of LHe and LN2 are around 2.5 and 1.1 L/h, respectively, at a magnet excitation current of 285 A. Magnet quenching vaporized a maximum of 10 L of liquid helium within 15 s. The stored energy in the coil is 29 kJ, some of which is dumped to the flywheel diode. During the period of training of SW6, the recovery compressor was kept running to recycle the large amount of the gaseous helium from the SW6 cryostat. The operating temperature (T5) of the magnet is about 4.3 K. Moreover, two sensors, T6 and T7, on the two protection diodes detect the temperature of the diodes when the magnet is quenched. The heat load from the synchrotron light, the scattering light and the image current increases the temperature of the beam duct. The temperatures of the beam duct with (without) a 200 mA electron beam are around T1 ¼ 101 K (97 K), T2 ¼ 102 K (100 K), T3 ¼ 101 K (99 K) and T4 ¼ 233 K (229 K). The temperature of the beam duct is a function of the electron
ARTICLE IN PRESS 614
C.-S. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 607–615
beam current and the magnet will be quenched when the temperature of the beam duct (T1) exceeds 136 K at the LHe vessel pressure of 15 psi. Cold helium gas is used to cool the vapor-cooled current lead. Thus, the measured temperature of the gas-electrode insulation pipe is between 60 and 200 K. The cold helium gas is warmed using a warmer that consists of a multi-channel aluminum pipe, and the warm helium gas is recycled to the recovery compressor. The testing of the magnet in the storage ring was successful. Thirteen training events of SW6 yielded a field of 3.2 T at the nominal current of 285 A. SW6 quenching only weakly affects the cryogenic system; the pressure fluctuates by 72 mbar in the suction line and by 73 mbar in the main dewar. The temperature of the cold gas returned from the valve box is increased by 0.3 K, and the frequency of the cold turbine fluctuates by 710 Hz. The vacuum pressures of the SW6 beam duct change only a little with different temperature on the beam duct (Fig. 10). The pressure of the UHV beam duct is maintained at an almost constant value (0.23–0.19 nTorr), regardless of whether the beam duct is warm or cold, or of whether the beam current (of up to 200 mA) is stored. Fig. 10 plots the pressure of the UHV beam duct in the various electron beam current and temperatures. Trim coils on the SW6 magnet can be used to compensate for the first and second field integrals, but this option is not exploited because the field quality is already quite consistent with the designed
Fig. 10. Vacuum pressure of the UHV beam duct with and without an electron beam current of 200 mA: (a) electron beam current, (b) T1 of the beam duct and (c) vacuum pressure of the SW6 Al UHV beam duct.
value. The commissioning results indicate that the tune shift effect of the SW6 is very consistent with the value predicted by the model, and the magnetic field causes no
Fig. 11. Horizontal and vertical tune shifts versus the magnetic flux density of the SW6.
Fig. 12. Deviation of (a) electron beam current, (b) electron beam size, (c) lifetime and normalized photon flux and (d) electron orbit, due to SW6 quenching.
ARTICLE IN PRESS C.-S. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 607–615
unexpected horizontal tune shift. Fig. 11 presents the measured horizontal and vertical tune shifts. When the magnet is quenched or the power supply is tripped off, the electron beam current remains without beam loss, as displayed in Fig. 12(a) and the orbit of the electron beam does not detectably change (Fig. 12(d)). However, the horizontal and vertical beam sizes changed in steps of 6 mm (Fig. 12(b)) and the lifetime (Fig. (12(c)) changes a little when the magnet is quenched. Although, the photon flux deviation ratio (DI/I0) (where I0 is measured using the beam line photon monitor) changes by around 2.5% when the magnet is quenched, the photon flux deviation ratio recovers to 0.04% after 10 min and the photon beam is kept stable again (Fig. 12(c)). The even-pole design of the SW6 is very effective for preventing the loss of electron beams when the magnet is quenched.
5. Conclusions An aluminum chamber is an effective UHV electron beam duct when the temperature is maintained at 100 K. The beam duct does not need to be coated with Cu to solve the heat load problem. The results of commissioning the magnet in the storage ring confirm that magnet performs well and the real magnetic field is close to the ideal value. Additionally, the even-pole design of a superconducting wiggler and undulator is preferred, especially for use in high-field insertion devices. The authors believe that the even-pole design helps to prevent loss of the electron beam current and to keep the beam when the magnet is quenched. The pumping and purging of the magnet system involves liquid transfer and return gas lines to maintain at a low dew point; purify impure gases and reduce the number of particles, to prevent a fault of the cryogenic system. This process prevents damage to the refrigerator of the cryogenic system.
615
Acknowledgements The authors would like to thank the National Science Council of Taiwan for partially supporting this research under Contract No. NSC 94-2112-M-213-018. Mr. C. J Lin and Dr. D. J. Wang are appreciated for their technical support as well as Ms. W. P. Li, and P. H. Lin for processing the measured data. References [1] C.S. Hwang, B. Wang, et al., Design of a superconducting multipole wiggler for synchrotron radiation, IEEE Trans. Appl. Supercond. 13 (2) (2003) 1209. [2] C. S. Hwang, B. Wang, et al., Construction and performance of a superconducting multipole wiggler, in: Eighth International Conference on Synchrotron Radiation Instrumentation, AIP, CPC05, 2004, p. 199. [3] Hsiao, F. Z., et al., The liquid helium cryogenic system for the superconducting cavity in SRRC, in: PAC2001, Chicago, Illinois, 2001. p. 1604. [4] T. C. Fan, F. Y. Lin, M.H. Huang, C.H. Chang, C. S. Hwang, Magnetic field measurement on superconducting multipole wiggler with narrow duct, in: PAC’2003, Oregon, Portland, 2003, p. 1047. [5] C.S. Hwang, F.Y. Lin, T.C. Fan, Integral magnetic field measurement using an automatic fast long-loop-flip coil system, IEEE Trans. Instrum. Meas. 52 (3) (2003) 865. [6] P. Elleaume, O. Chubar, J. Chavanne, Computing 3D magnetic field from insertion devices, in: PAC1997, Vancouver, Canada, 1997, p. 3509. [7] V. Baglin, I.R. Collins, O. Gro¨bner, C. Gru¨nhagel, B. Jenninger, Molecular desorption by synchrotron radiation and sticking coefficient at cryogenic temperatures for H2, CH4, CO and CO2, Vacuum 67 (2002) 421. [8] Private communication with Dr. Alex Chao. [9] H.H. Chen, C.S. Hwang, et al, Heat load analysis of superconducting wiggler with Semi-cold UHV beam duct, IEEE Trans. Appl. Supercond, in press (2006). [10] F.Z. Hsiao, et al., the pilot-runs of the helium crygoneic system for the TLS superconducting cavity, in: PAC2003, Oregon, Portland, 2003, 2402. [11] C.S. Hwang, B. Wang, R. Wahrer, H.H. Chen, C.H. Chang, F.Y. Lin, T.C. Fan, C.T. Chen, Construction and performance of a compact cryogen-free superconducting wavelength shifter, IEEE Trans. Appl. Supercond. 12 (1) (2002) 686.