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Design and commissioning of the Beam Test Facility at the Spallation Neutron Source
Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162826
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Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Design and commissioning of the Beam Test Facility at the Spallation Neutron Source Z. Zhang a ,∗, S. Cousineau a,b , A. Aleksandrov b , A. Menshov b , A. Zhukov b a b
Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37966, USA Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
ARTICLE
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Keywords: SNS Beam test facility High intensity hadron beam Beam halo High dynamic range beam instrumentation
ABSTRACT The Beam Test Facility (BTF) originally built to verify the spare RFQs at the Spallation Neutron Source (SNS) possesses the additional capability to carry out high intensity beam experiments with unique capabilities. These capabilities include a full 6D phase space measurement system, and a beam line extension for studying the propagate beam dynamics including halo development under different match conditions. A phase space density plot method that characterizes beam halo is utilized in the design of the beamline. Simulations demonstrate that the beamline design meets the requirements for conducting beam halo research. Installation and commissioning of the BTF has been completed in stages, with the first stage demonstrating that the RFQ achieves the design parameters. Commissioning of the beamline extension has begun, and a high transmission efficiency has been achieved. More work, e.g. analysis of beam halo formation under different match conditions, will be done in the future.
1. Introduction
2. Beam test facility at SNS
In the first decade of operation the SNS encountered issues with the transmission and reliability of the original RFQ, which stimulated the procurement of a spare of RFQ with the same physics design but improved mechanical and thermal properties [1]. To commission the RFQ, the SNS constructed the Beam Test Facility (BTF), a functional duplicate of the SNS front end systems. The similarity ensures results of beam measurement at BTF are applicable to SNS operation. While the original purpose of the BTF is to verify the operation of spare RFQs with full power beam, it also serves as a general platform for testing and developing new equipment, e.g. ion source technology, accelerator physics software and controls, and high dynamic range diagnostic devices. A subsequent upgrade of the diagnostics suite and expansion of the beam line has also enabled the BTF to serve as a facility for R&D in high intensity hadron beam physics. Fig. 1 shows a photograph of the final BTF beam line. This paper describes the design and capabilities of the BTF, and is organized as follows: First, the configuration of the original BTF is described; second, the design of the FODO extension beam line is presented; third, results of simulation of the whole beam line are shown; fourth, the main subsystems and top-level parameters for the BTF are given; and fifth, the commissioning results of the BTF are shown; finally, conclusions and future work are discussed.
Fig. 2 shows the configuration of the entire BTF facility including the FODO line. The original BTF consists of the sub-portion including the 65 keV H− injector, the 2.5 MeV RFQ, the medium energy beam transport (MEBT) line and the 7.5 kW water cooled beam dump. The extension includes everything downstream of the first dipole. In this section the major subcomponents of the BTF are described. 2.1. H− injector The H− injector is comprised of a RF-driven, multi-cusp, Cs enhanced H− ion source with internal or external antenna [2,3] and a two-lens electrostatic low energy beam transport (LEBT) with chopping capability. It provides the RFQ with 1 ms beam pulses of 65 keV at 60 Hz repetition rate with up to 60 mA peak current. 2.2. RFQ The SNS RFQ accelerates H− beam of up to 50 mA from 65 keV to 2.5 MeV with a transmission efficiency equal to or greater than 90% [4]. The total length and the peak RF power of the cavity are 3.72 m and 600 kW, respectively. Due to the high frequency of 402.5 MHz, a 4-vane structure is adopted for the cavity. Different spare RFQs are used in the BTF.
∗ Corresponding author. E-mail address: [email protected] (Z. Zhang).
https://doi.org/10.1016/j.nima.2019.162826 Received 7 June 2019; Received in revised form 4 September 2019; Accepted 19 September 2019 Available online 23 September 2019 0168-9002/© 2019 Elsevier B.V. All rights reserved.
Z. Zhang, S. Cousineau, A. Aleksandrov et al.
Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162826
Fig. 1. A photograph of the final BTF beam line at SNS.
Fig. 2. Layout of beam test facility at SNS.
2.3. MEBT
3. Design of the BTF extension beam line
The BTF MEBT is built to transport beam without loss from RFQ to the beam dump, and to provide space for beam instrumentation equipment. The BTF MEBT is equipped with six quadrupole magnets to provide transverse beam focusing. The two quadrupoles closest to the RFQ are outfitted with dipole correctors for beam trajectory correction. There are no rebunchers in the BTF MEBT due to no requirement of keeping the bunch length short for injection in a linac. The BTF MEBT contains a variety of diagnostic devices for the purpose of R&D experiments which include the full 6D phase space measurement capability. The diagnostics will be introduced in detail in Section 5.2.
Beam loss caused by beam halo is a major challenge for high intensity and high power accelerators, because it can damage accelerator components and lead to unwanted radioactivity. Although modern multiparticle tracking codes are sophisticated enough to reproduce the measured RMS beam parameters, they failed to precisely predict growth rates for halo amplitude and beam emittance in comparison with experiments [5,6]. The presumed reason is insufficient knowledge of the initial particle distribution [7,8]. The 6D measurement capability of the BTF provides an opportunity to solve this problem. An extension beam line after the MEBT together with the full 6D phase space distribution will allow for exploring the beam dynamics of beam halo development under different lattice matching conditions with complete knowledge of the initial distribution. The design criterion for the extension beam line included:
2.4. Beam dump
• Capable of generating matched and mismatched beams; • Sufficient number of betatron cycles to allow beam halo to develop; • Zero dispersion in the beamline, which is connected to the BTF by a bending section due to the limited available space;
The 7.5 kW-rated water cooled beam dump made of Titanium– Zirconium–Molybdenum (TZM) alloy is shielded to avoid neutrons and gamma radiation. It was used during the commissioning phase of the RFQ for high power beam test. The vast majority of beam operation for physics purposes will be at 10 Hz or less and 50 μs or less. 2
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Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162826
Fig. 3. Transformation of a 2D phase space distribution into phase space density plot. Figure (a): a 2D distribution in x–xp phase space; Figure (b): the 2D distribution of Figure (a) with normalized coordinates; Figure (c): particle number 𝑑𝑁𝑟 in 𝑑𝑟 interval; Figure (d): normalized particle number 𝑛 in 𝑑𝑟 interval with log scale. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Configuration of one FODO period.
• Large dynamic range measurement capability at the end of the FODO line. Based on the above criterions the extension beam line consists of an achromatic bending section, a matching section, a periodic FODO lattice and a large dynamic range measurement station. 3.1. Characterization of beam halo
Fig. 5. Phase space density curves of vertical particle distributions of different FODO length (‘Input’: input distribution; ‘C’: FODO cycle or period).
For the purpose of designing the BTF extension beam line, a definition of beam halo is required. A quantitative, universally-accepted definition of beam halo is not available, though many attempts have been made [9–12]. In this paper beam halo is defined as the portion of the beam distribution that is far from the beam core and has a low particle density in the range of 10−4 to 10−6 relative to the peak density. These particles have large oscillation amplitudes and can reach the aperture of beam pipe causing uncontrolled beam loss. For this work, a normalized phase space density plot is used to characterize the beam halo. The procedures used to transform a 2D phase space distribution into a phase space density plot is as follows
c. Plot normalized particle numbers 𝑛 (𝑟) = scale.
𝑑𝑁𝑟 2𝜋𝑟𝑑𝑟
vs. 𝑟 with log
Fig. 3 shows the transformation steps. The black curve in plot (d) is the phase space density curve of the distribution in plot (a). The method to identify whether halo develops during beam transport is to compare the phase space density curves of the output distribution with the input distribution. The discrepancy between the curves indicates halo has developed during the transport. From the transformation procedures we can see that the phase space density plot is independent of beam energy and distribution location along beam line. Therefore, it can be used for comparing general distribution properties of different accelerators and even different particle species.
a. Transform particle coordinates 𝑥, 𝑥′ to normalized coordinates according to the following formula: 𝑥 𝑥𝑛 = √ 𝛽𝑟𝑚𝑠 √ 𝛼𝑟𝑚𝑠 ⋅ 𝑥 𝑥′𝑛 = √ + 𝑥′ 𝛽𝑟𝑚𝑠 𝛽𝑟𝑚𝑠
3.2. Design of FODO lattice
where 𝛼𝑟𝑚𝑠 , 𝛽𝑟𝑚𝑠√are the RMS Twiss parameters. b. Calculate 𝑟 = 𝑥2𝑛 + 𝑥′2 𝑛 , and count number of particles 𝑑𝑁𝑟 within 𝑑𝑟 intervals.
The FODO lattice is composed of multiple identical FODO periods. The configuration of one FODO period is shown in Fig. 4. In the figure, Ld, Lq and D denote the drift length, quadrupole length and quadrupole 3
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Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162826
Fig. 6. Phase space density plot of the vertical particle distributions at the end of a 9 FODO period lattice with 𝜎0 = 72.9◦ (left) and 𝜎0 = 112.4◦ (right) (‘Input’: input distribution; 1M: mismatch factor = 1; 2M: mismatch factor = 2; 3M: mismatch factor = 3).
Fig. 8. Quadrupoles installation on tracks in vacuum pipe.
Fig. 7. Quadrupole permanent magnet (left: 3D model; right: first sample).
FODO periods). The matched and mismatched beams will be provided by four quadrupoles (Q10, Q11, Q12 and Q13 as shown in Fig. 9) with adjustable gradients installed upstream of the FODO entrance. Since the footprint of the BTF is constraint by external factors, the matching region should have a short configuration to preserve space for the FODO line. The quadrupoles are required to have gradients less than 20 T/m in order to use the already existing quadrupoles at SNS, and real estate should be available for diagnostic devices such as a Faraday cup, etc. The final matching section design which meets these requirements is shown in Fig. 9.
spacing, respectively. These parameters together with quadrupole gradients decide the FODO period’s zero current phase advance 𝜎0 which influences oscillation of particles in halo [13]. The number of betatron cycles and total lattice length influence the halo evolution. Fig. 5 shows the phase space density plot of particle distributions at the end of a FODO line for different total FODO lengths. For each case, the mismatch factor is 2 (mismatch factor = 1 means matched beam), the beam current is 40 mA, and the zero current phase advance 𝜎0 = 81.6◦ . In Fig. 5, ‘‘Input’’ denotes the phase space density curve of the input distribution, and ‘‘C’’ refers to FODO cycle or period. Results indicate that halo develops when the FODO length is or is greater than 6 FODO (6C) periods, and the longer the FODO length is the broader the halo extends. Fig. 6 displays halo development at different mismatch conditions when 𝜎0 is 72.9◦ and 112.4◦ and the FODO length is 9 FODO periods. The final BTF FODO lattice consists of 19 quadrupoles comprising 9.5 FODO periods. For cost savings, all of the quadrupoles are identical permanent magnets (as shown in Fig. 7) of length 7.5 cm and integrated gradient 1.8 T. For future flexibility in the lattice configuration, the magnets are designed with three sandwiched pieces that can be separated to give a higher number of smaller gradient magnets. The quadrupoles are mounted on stainless steel tracks in a vacuum pipe (as shown in Fig. 8) whose mounting structure allows modification of FODO phase advance through different combinations of magnets and drift lengths.
3.4. Achromatic design of bending section The bending section has been designed to connect the FODO lattice and the original BTF. The section turns the beam 180.0◦ which is required for the BTF to fit within the available footprint. It is desirable to have a zero dispersion solution at the exit of this section. This section has two 90.0◦ bending magnets with quadrupoles in between to create the achromat. Assuming M is the transfer matrix through one section of beam line ⎡ 𝑀11 𝑀12 𝑀13 ⎤ ⎥. Eliminating containing bending magnets, 𝑀 = ⎢ 𝑀21 𝑀22 𝑀23 ⎥ ⎢ 0 1 ⎦ ⎣ 0 dispersion is achieved by making the dispersion term 𝑀13 and 𝑀23 equal to zero. One quadrupole can realize beam transport without dispersion when it is located at the symmetry axis of the two bending magnets. However, this solution does not leave enough real estate for the desired diagnostic devices to keep the beam size small. Two quadrupoles can also be used to achieve an achromat, however some particles would reach the vacuum pipe wall in the bending section due to the inadequate focusing. Therefore, three quadrupoles (Q07, Q08 and Q09 in Fig. 10) are used to eliminate dispersion and provide adequate
3.3. Design of the matching section The FODO lattice design shows that halo develops when the beam is mismatched and the FODO has a reasonable length (e.g. longer than 6 4
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Fig. 9. Configuration of matching section (unit: cm).
Fig. 10. Configuration of bending section with three quadrupoles (unit: cm). Table 1 Quadrupole parameters of the bending section. L1 L2 Q07 gradient Q08 gradient Q09 gradient
15 cm 35 cm 8.10 T/m −5.00 T/m 6.56 T/m
room for beam instrumentation. The configuration of the bending section is shown in Fig. 10. In the layout L1 and L2 are the drift lengths that need to be optimized to avoid beam loss and to minimize required quadrupole gradients. The final optimized values of L1, L2, and quadrupole gradients are listed in Table 1. Fig. 11 shows the dispersion function of the bending section when parameters in Table 1 are used. The figure indicates that the dispersion function is zero before and after the bending section (10 cm is extended before and after the bending section in the calculation), which confirms the achromatic design of the bending section.
Fig. 11. Dispersion function (D) of the bending section (10 cm is extended before and after the bending section in the calculation).
3.5. Beam instrumentation in the extension beamline The final beam is transported into a beam stop after the measurement station through a 90.0◦ bending magnet (B3 in Fig. 12), which prevents the influences of ions that scattered by the first slit on the dynamic range of the slit-based emittance measurement system.
A bunch shape monitor (BSM) will be installed between quadrupole Q08 and Q09 in the bending section to carry out the 6D phase space measurement. A Faraday cup is to be installed between Q12 and Q13 in the matching section to measure the beam current entering the FODO lattice and to serve as a beam stop. A large dynamic range measurement station (2 slit pairs) will be used to measure the 2D transverse particle distributions at the end of the FODO lattice with a high dynamic range of 10−5 –10−6 . The measurement station is 132.85 cm long without any beam focusing elements, therefore, additional quadrupoles are needed to ensure beam passes through without loss. It has been found that at least two quadrupoles are needed, and their positions have a significant influence on beam transmission. The final configuration of the two quadrupoles (Q14, Q15) can be seen in Fig. 12. The gradients of the two quadrupoles are adjustable (less than 20 T/m) depending on beam matched conditions.
4. Simulation of the whole beam line of the final BTF The final layout of the BTF is shown in Fig. 2. Simulations of the whole beam line with PyORBIT [14] from the RFQ exit to the final measurement station have been conducted based on Fig. 2 configuration. The beam distribution at RFQ exit, reconstructed by combining together measured 2D distributions of 40 mA at BTF [15], was used as the initial distribution for the simulation. It should be noted that, all the designs and simulation of the BTF beam line were done with the ‘Hard edge’ model of all the quadrupole fields, because the fringe field distributions of all the quadrupoles has not been available at present. Simulation with the fringe field model will be performed in future. 5
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Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162826
Fig. 12. Configuration of quadrupoles after FODO lattice (unit: cm).
Fig. 13. Phase space density plot of the particle distributions at measurement station (‘Input’: input distribution; 1M: mismatch factor = 1; 2M: mismatch factor = 2; 3M: mismatch factor = 3).
Fig. 14. Distribution comparison between different position under matched condition (‘Input’: input distribution; FODO_enter: distribution at the FODO entrance; MeasStation: distribution at measurement station).
Fig. 13 shows the phase space density plots of the particle distributions at the middle of measurement station, where the ‘‘input’’ curves are the phase space density curves of distributions at RFQ exit. It can be found that halo has grown obviously under mismatched conditions compared with the initial distributions, meanwhile, halo is also present in the distributions under the matched condition. Particle distributions at the FODO entrance are investigated to figure out the cause of the halo generation under the matched condition, and results are shown in Fig. 14. It can be seen that halo has already developed at the FODO entrance and is transported to the measurement station. It may be
desirable that the halo be scrapped off before entering the FODO lattice. This is left for future work. Fig. 15 displays the full beam size evolution along the whole BTF beam line when the beam is matched to the FODO lattice. Simulation shows that the transmission from FODO entrance to the measurement station is 100% under matched and mismatched conditions, and transmission efficiency along the whole beamline is 99.76%. However, with ideal Gaussian distribution or simulated RFQ output distribution as the input distribution for the simulation the transmission efficiency along the whole beamline is 100%. 6
Z. Zhang, S. Cousineau, A. Aleksandrov et al.
Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162826 Table 2 Main parameters of quadrupole magnets in the BTF.
Fig. 15. Full beam size evolution along the whole BTF beam line (beam matched to FODO lattice).
Quad number
Aperture / cm
Max integrated gradient / T
Magnetic length / cm
Q01 Q02 Q03 Q04 Q05 Q06 Q07 Q08 Q09 Q10 Q11 Q12 Q13 Q14 Q15
3.2 4.2 4.2 4.2 6.5 6.5 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2
2.5 1.9 1.7 1.7 0.75 0.75 1.7 2.5 1.7 1.7 2.5 2.5 2.5 2.5 1.7
6.1 6.6 9.6 9.6 10.6 10.6 9.6 14.6 9.6 9.6 14.6 14.6 14.6 14.6 9.6
supplies. Each magnet has independent horizontal and vertical dipole coils for beam steering. The maximum angular deflections of the dipole steerers is 3.2 mrad in the first quadruple and 2.6 mrad in the second quadrupole.
5. Subsystems after RFQ in the final BTF 5.1. Magnetic system
5.1.2. Air cooled low current quadrupole magnets The other 13 quadrupoles in the BTF are air cooled low current magnets, which have three combinations of magnetic length, magnetic strength, and aperture (see Table 2). Fig. 17 shows the pictures of the air cooled quadrupoles. The magnets are powered by independent 12 A power supplies.
Except for the 19 permanent magnet quadrupoles in the FODO lattice, the magnets in the final BTF are electromagnetic magnets including 15 quadrupoles, 4 dipole steerers and 3 big bending dipoles. The locations of the electromagnetic magnets and bending dipoles are shown in Fig. 2. The main parameters of the quadrupole magnets are summarized in Table 2.
5.1.3. Bending dipoles The three 90.0◦ bending dipoles are identical in design (as shown in Fig. 2). The aperture of the dipoles is 4.0 cm, and the bending radius is 35.56 cm, respectively. Fig. 18 shows the inner structure and a photo of the bending dipoles. All the three dipole magnets are connected in series and powered by a single 400 A power supply.
5.1.1. Water cooled high current quadrupole magnets The first two quadrupoles in the BTF MEBT are water cooled high current magnets. The mechanical structure of the two quadrupoles are the same (as shown in Fig. 16) but with different apertures (see Table 2). The magnets are powered using independent 400 A power
Fig. 16. Mechanical drawing of water cooled quadrupole magnets (see Table 2 for definitions).
Fig. 17. Mechanical drawing of air cooled quadrupole magnets (see Table 2 for definitions).
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Fig. 21. Plot of beam current at BCM and final beam stop. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 18. Bending dipole (top: inner structure; bottom: photo).
Fig. 22. Schematic drawing of movable BPM.
Fig. 19. Beam current attenuation system (top: overall view; bottom: attenuation foils).
Fig. 23. Bunch phase dependence on BPM displacement (beam velocity is linearly proportional to the curve slope).
reduce peak beam current while preserving the beam distributions in the transverse and longitudinal phase space. The system consists of 4 movable semitransparent Carbon–carbon composite grids with different transmission (9%, 23%, 45% and 72%). The aperture of each foil is 2.0 cm. The first bracket contains a thin carbon foil to convert the H− ions to proton for experiments with proton beam.
Fig. 20. Picture of BCM.
5.2. Diagnostics A heavy suite of diagnostic devices is included in the BTF. The positions of each device can be found in Fig. 2.
5.2.2. Beam current monitor (BCM) The BCM (as shown in Fig. 20) in the BTF MEBT is used for beam current measurement at the RFQ exit. A commercial clamp-on current transformer (PEARSON 5949) over a standard ceramic break is used. The transformer output is connected directly to a 50 Ohm input of a
5.2.1. Beam current attenuation system A beam current attenuation system (as shown in Fig. 19) is installed between the second and the third quadrupoles in the BTF to 8
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Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162826
Fig. 24. Picture of horizontal and vertical slit pair (left: overall view; right: zoomed in).
Fig. 25. Energy selection slit. Fig. 26. Camera and view screen installation for energy distribution measurement.
digital-to-analog converter. The aperture of the BCM is 5.0 cm. The yellow curve in Fig. 21 is an example of the BCM current signal. 5.2.3. Movable beam position and phase monitor (BPM) The movable BPM (as shown in Fig. 22) is employed for beam energy measurement by using the time-of-flight technique and for beam position measurement. The BPM uses the standard SNS MEBT strip-line pick-up with 42 mm aperture. The pick-up can be moved along the beam line within +∕−25.0 mm range. The standard 402.5 MHz SNS BPM electronics is used for signal processing [16]. Fig. 23 gives an example of beam energy measurement by the movable BPM.
Fig. 27. Energy distribution on the energy slit captured by camera.
5.2.6. Bunch shape monitor (BSM) The BSM is used to measure the bunch temporal profile. Fig. 28 illustrates the principle of BSM. The ion beam hits the Tungsten wire suspended in BSM producing secondary electrons with the same temporal distribution as the ion beam. The temporal distribution of the secondary electrons is then converted to transverse deflection by an RF deflector. The transverse distribution is measured using a luminescent screen and a digital camera [17]. The measurement accuracy of the BSM is about 1.0◦ of 402.5 MHz. Fig. 29 shows an example of the measured temporal profiles for different energy slices.
5.2.4. Slits Two pairs of orthogonal 200 μm wide carbon slits installed in the BTF MEBT allow for the beam distribution measurement in the transverse phase space. The horizontal and vertical slits in each pair are separated by 10 mm longitudinally in order to move simultaneously without collision. The distance between the two slit pairs is 0.94 m. All the slits can be scanned over the 40 mm aperture. The slits are water cooled with maximum average beam power of 63 W (2.5 MeV, 50 mA, 10 Hz, 50 μs). Fig. 24 shows the picture of one slit pair. An identical slit arrangement is installed at the end of the FODO for the large dynamic range measurement. The slit separation in this case is 0.78 m. The charge passing through the slits is measured by any of the downstream beam stops or Faraday Cups.
5.2.7. Faraday cups (FC) There are three Faraday cups in the BTF beam line. The first one is formed by the high power beam stop insulated from the ground; the second one is in the matching section and is mounted on an actuator as shown in Fig. 30; and the third one (as shown in Fig. 31) is fixed at the very end of the beam line. The two low power Faraday cups are made of carbon and rated for 63 W of beam power. The aperture of the FCs is 4.5 cm.
5.2.5. Energy slit/view screen Two viewscreens each with an imbedded 200 μm slit (as shown in Fig. 25) are installed on actuators after the first and the last 90.0◦ bending magnet. The slits are used for energy selection by utilizing the dispersion feature of the bending magnet. The screen is made of an alumina ceramic based luminescent material. This allows for the entire energy distribution to be measured in one shot by acquiring the screen image with a video camera, or for selecting a portion of the energy spectrum for temporal analysis downstream. The image on the view screen is recorded by a video camera installed on the other side of the bending magnet as shown in Fig. 26. Fig. 27 shows a measured energy distribution on the energy slit/view screen.
5.2.8. Measurement arrangements of beam distributions The two pairs of slits in the MEBT together with the energy slit and the BSM allows for the BTF to measure up to 6D beam distributions. By moving one slit of the two pairs of slits a 1D transverse beam profile can easily be obtained (as shown in Fig. 32). By moving the two vertical slits or two horizontal slits a 2D transverse beam distribution will be produced (an example is shown in Fig. 33). A 4D transverse distribution will be generated by moving all the slits of the two pairs sequentially. 9
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Fig. 31. 3D model (left) and mechanical drawings (right) of Faraday cup/beam stop.
Fig. 28. Schematic drawing of BSM.
Fig. 32. 1D transverse beam profile by scanning one transverse slit.
Fig. 29. Measured temporal profiles for different energy slices.
Fig. 33. Vertical beam emittance measured by two slits. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
5.2.9. Beam loss monitors (BLMs) There are 8 beam loss monitors uniformly distributed along the BTF beam line. The detectors used are of the standard SNS scintillator/PMT design [19]. A prototype of the new generation SNS BLM processing electronics is used for data acquisition. The BLM system is capable to detect losses at ∼1 W/m level. The system is connected to the Machine Protection System, which shuts off the beam if losses are above the safe level. 5.2.10. Diagnostics data acquisition system The diagnostics data acquisition system is based on NI PXI-1075 chassis that holds: PXIe-8133 Real Time controller, NI-5105 and NI4481 ADC cards, NI 7961R FPGA card for timing and NI 7330 Motion Controllers. Cameras are also connected to the same controller over dedicated Gigabit Ethernet network. Timing information is decoded by FPGA and triggers are generated on the chassis backplane to allow hardware synchronization for all ADCs and motion cards. This helps to implement continuous movement of actuators while running beam
Fig. 30. Movable Faraday cup (left: whole structure; right: cup).
The main outstanding feature of the BTF diagnostics is its capability of conducting a full 6D phase space measurement which has been introduced in Ref. [18]. 10
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Fig. 35. Measured vertical emittance with large dynamic range at the measurement station of BTF extension. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Fig. 34. Vacuum system diagram of the BTF beam line.
Commissioning of the FODO extension beamline began at the start of 2019. Preliminary results show the beam transmission efficiency from RFQ exit to the final beam stop (BS34) is about 98% when the beam current is 25 mA. Transverse emittances have also been measured with the measurement station. Fig. 35 shows an example of a measured vertical emittance when the beam current is 13 mA. Beam halo investigation has not yet been conducted.
at up to 60 Hz rate, instead of performing steps and waiting for a beam pulse in every position. Two ADCs allow wider dynamic range vs time resolution. NI-5105 samples at 10–60 MS/s but the range is 12-bit only. Conversely, the NI-4481 provides 24-bit range but at 1.25 MS/s only. The system is indifferent to camera type, so any GigE camera can be used with minor tweaking. The data acquisition is fully interfaced with EPICS control system to provide connectivity to external software. The system is programmed in NI LabVIEW RT including SNS-made EPICS Channel Access.
7. Conclusions and future work The combination of SNS-equivalent hardware and beam parameters, the extensive and novel beam instrumentation suite, and the dedicated FODO line make the BTF at SNS a unique facility for R&D in high intensity hadron beam physics. Commissioning of the BTF shows the measured high beam transmission efficiency from RFQ to the end of the extension beam line has verified the design of the extension line. Up to now, only the commissioning and some preliminary measurements have been performed. There is a large program of planned work, including:
5.3. Auxiliary systems The auxiliary systems of the BTF includes the vacuum system and the control system. 5.3.1. Vacuum Due to the limited conductance of the BTF beam pipe, a distributed pumping strategy is adopted to achieve the required vacuum (<10−7 Torr). Six 300 l/s Turbo molecular pumps are approximately uniformly distributed along the beam line. The vacuum system diagram is shown in Fig. 34.
• Increasing beam transmission efficiency along the whole beam line based on model; • Measuring beam distributions and analyzing beam halo formation under different match conditions; • Improving the 6D measurement which includes increasing scan speed and enhancing scan resolution; • Developing methods to analyze multi-dimensional data; • Benchmarking simulation results against measurements, verifying methods of generating initial distributions for simulation codes.
5.3.2. Controls The Integrated Control Systems (ICS) is designed to provide remote control of all the BTF subsystems, machine protection and personnel protection systems. The personnel protection system includes radiation monitors, which shut off beam when high levels of radiation are detected. The machine protection system limits the beam power to < 63 W when beam is not directed to the high-power beam stop. The control system allows full control of the BTF from the local control room and/or from the SNS central control room.
Acknowledgments This work has been partially supported by National Science Foundation, USA Accelerator Science grant 1535312. This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC0500OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for the United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). The authors would like to thank A. Shishlo at SNS for his help in use of the PyORBIT code and thank Huan Jia at IMP for useful discussion.
6. BTF operation The commissioning of the BTF occurred in several stages. First, the beam was commissioned in a straight line throughout the RFQ and MEBT to the high power beam stop to verify performance of the new RFQ [20]. Next the BTF was used to demonstrate the feasibility of 6D phase space measurements [21]. The H− ion source was operated at 60 Hz with pulse length 1 ms during commissioning, while the RFQ was operated at 10 Hz with pulse length 50 μs. Particles that cannot be accelerated were lost in RFQ. Following this, the original SNS RFQ was installed at the BTF and the beam line was extended to include the new FODO line and high dynamic range emittance station. 11
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References
[13] T.P. Wangler, K.R. Crandall, R. Ryne, T.S. Wang, Phys. Rev. ST-AB 1 (084201) (1998). [14] A. Shishlo, S. Cousineau, J. Holmes, et al., Procedia Comput. Sci. 5 (2015) 1272–1281. [15] Z. Zhang, S. Cousineau, A. Aleksandrovv, Reconstruction of particle distributions at RFQ exit at SNS beam test facility, in: Proc. of ICAP2018, SUPAF10, pp. 107–110. [16] J. Power, J. O’Hara, S. Kurennoy, et al., Beam position monitors for the SNS linac, in: Proc. of PAC01, Chicago, IL, USA, 2001, pp. 1375–1377.. [17] A.V. Feschenko, Methods and instrumentation for bunch shape measurements, in: Proc. Particle Accelerator Conf., PAC’01, Chicago, IL, USA, 2001, pp. 517–519. [18] V. Danilov, A. Aleksandrov, in: Proceedings of EPAC2004, (IEEE, Lucerne, 2004), pp. 1518–1520. [19] A. Zhukov, A. Assadi, Beam loss simulation of SNS Linac, in: Proc. of PAC07, Albuquerque, New Mexico, USA, 2001, pp. 4138–4140. [20] A. Aleksandrov, B. Cathey, S. Cousineau, et al., Commissioning of the new SNS RFQ and 2.5MeV beam test facility, in: Proc. Of IPAC2017, TUPVA145, pp. 2438–2440. [21] B. Cathey, S. Cousineau, A. Aleksandrov, et al., Phys. Rev. Lett. 121 (2018) 064804.
[1] M. Champion, A. Aleksandrov, M. Crofford, et al., Plans for an integrated frontend test stand at the spallation neutron source, in: Proc. of Linac2012, THPB044, pp. 954–956. [2] R. Keller, R. Thomae, M. Stockli, R. Welton, AIP Conf. Proc. 639 (2002) 47. [3] R.F. Welton, B. Han, S.N. Murray, et al., The status of the SNS external antenna source and spare RFQ test facility, in: Rev. Sci. Instrum, Vol. 87, 2016, 02B146. [4] A. Ratti, R. DiGennaro, R.A. Gough, et al., The design of a high current, high duty factor RFQ for the SNS, in: Proc. Of EPAC2000, Vienna, Austria, pp. 495–497. [5] C.K. Allen, et al., Phys. Rev. Lett. 89 (2002) 214802. [6] H. Jiang, Y. Zou, S. Fu, et al., Beam dynamics analysis in the beam halo experiments at IHEP, in Proc. of IPAC2014, THPRO112, pp. 3159–3161. [7] T.P. Wangler, et al., Nucl. Instrum. Methods A 519 (2004) 425. [8] J. Qiang, et al., Phys. Rev. Spec. Top. Accel. Beams 5 (2002) 124201. [9] T.P. Wangler, K.R. Crandall, in: Proc. of XX International Linac Conference, 2000. [10] C.K. Allen, T.P. Wangler, Phys. Rev. Spec. Top. Accel. Beams 5 (2002) 124202. [11] P.A.P. Nghiem, et al., in: Proc. Of HB2012, THO3A04, pp. 511–513. [12] M.A. Dorf, R.C. Davidson, E.A. Startsev, Phys. Plasmas 18 (2011) 043109, http://dx.doi.org/10.1063/1.3574665.
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Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Design and commissioning of the Beam Test Facility at the Spallation Neutron Source Z. Zhang a ,∗, S. Cousineau a,b , A. Aleksandrov b , A. Menshov b , A. Zhukov b a b
Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37966, USA Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
ARTICLE
INFO
Keywords: SNS Beam test facility High intensity hadron beam Beam halo High dynamic range beam instrumentation
ABSTRACT The Beam Test Facility (BTF) originally built to verify the spare RFQs at the Spallation Neutron Source (SNS) possesses the additional capability to carry out high intensity beam experiments with unique capabilities. These capabilities include a full 6D phase space measurement system, and a beam line extension for studying the propagate beam dynamics including halo development under different match conditions. A phase space density plot method that characterizes beam halo is utilized in the design of the beamline. Simulations demonstrate that the beamline design meets the requirements for conducting beam halo research. Installation and commissioning of the BTF has been completed in stages, with the first stage demonstrating that the RFQ achieves the design parameters. Commissioning of the beamline extension has begun, and a high transmission efficiency has been achieved. More work, e.g. analysis of beam halo formation under different match conditions, will be done in the future.
1. Introduction
2. Beam test facility at SNS
In the first decade of operation the SNS encountered issues with the transmission and reliability of the original RFQ, which stimulated the procurement of a spare of RFQ with the same physics design but improved mechanical and thermal properties [1]. To commission the RFQ, the SNS constructed the Beam Test Facility (BTF), a functional duplicate of the SNS front end systems. The similarity ensures results of beam measurement at BTF are applicable to SNS operation. While the original purpose of the BTF is to verify the operation of spare RFQs with full power beam, it also serves as a general platform for testing and developing new equipment, e.g. ion source technology, accelerator physics software and controls, and high dynamic range diagnostic devices. A subsequent upgrade of the diagnostics suite and expansion of the beam line has also enabled the BTF to serve as a facility for R&D in high intensity hadron beam physics. Fig. 1 shows a photograph of the final BTF beam line. This paper describes the design and capabilities of the BTF, and is organized as follows: First, the configuration of the original BTF is described; second, the design of the FODO extension beam line is presented; third, results of simulation of the whole beam line are shown; fourth, the main subsystems and top-level parameters for the BTF are given; and fifth, the commissioning results of the BTF are shown; finally, conclusions and future work are discussed.
Fig. 2 shows the configuration of the entire BTF facility including the FODO line. The original BTF consists of the sub-portion including the 65 keV H− injector, the 2.5 MeV RFQ, the medium energy beam transport (MEBT) line and the 7.5 kW water cooled beam dump. The extension includes everything downstream of the first dipole. In this section the major subcomponents of the BTF are described. 2.1. H− injector The H− injector is comprised of a RF-driven, multi-cusp, Cs enhanced H− ion source with internal or external antenna [2,3] and a two-lens electrostatic low energy beam transport (LEBT) with chopping capability. It provides the RFQ with 1 ms beam pulses of 65 keV at 60 Hz repetition rate with up to 60 mA peak current. 2.2. RFQ The SNS RFQ accelerates H− beam of up to 50 mA from 65 keV to 2.5 MeV with a transmission efficiency equal to or greater than 90% [4]. The total length and the peak RF power of the cavity are 3.72 m and 600 kW, respectively. Due to the high frequency of 402.5 MHz, a 4-vane structure is adopted for the cavity. Different spare RFQs are used in the BTF.
∗ Corresponding author. E-mail address: [email protected] (Z. Zhang).
https://doi.org/10.1016/j.nima.2019.162826 Received 7 June 2019; Received in revised form 4 September 2019; Accepted 19 September 2019 Available online 23 September 2019 0168-9002/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. A photograph of the final BTF beam line at SNS.
Fig. 2. Layout of beam test facility at SNS.
2.3. MEBT
3. Design of the BTF extension beam line
The BTF MEBT is built to transport beam without loss from RFQ to the beam dump, and to provide space for beam instrumentation equipment. The BTF MEBT is equipped with six quadrupole magnets to provide transverse beam focusing. The two quadrupoles closest to the RFQ are outfitted with dipole correctors for beam trajectory correction. There are no rebunchers in the BTF MEBT due to no requirement of keeping the bunch length short for injection in a linac. The BTF MEBT contains a variety of diagnostic devices for the purpose of R&D experiments which include the full 6D phase space measurement capability. The diagnostics will be introduced in detail in Section 5.2.
Beam loss caused by beam halo is a major challenge for high intensity and high power accelerators, because it can damage accelerator components and lead to unwanted radioactivity. Although modern multiparticle tracking codes are sophisticated enough to reproduce the measured RMS beam parameters, they failed to precisely predict growth rates for halo amplitude and beam emittance in comparison with experiments [5,6]. The presumed reason is insufficient knowledge of the initial particle distribution [7,8]. The 6D measurement capability of the BTF provides an opportunity to solve this problem. An extension beam line after the MEBT together with the full 6D phase space distribution will allow for exploring the beam dynamics of beam halo development under different lattice matching conditions with complete knowledge of the initial distribution. The design criterion for the extension beam line included:
2.4. Beam dump
• Capable of generating matched and mismatched beams; • Sufficient number of betatron cycles to allow beam halo to develop; • Zero dispersion in the beamline, which is connected to the BTF by a bending section due to the limited available space;
The 7.5 kW-rated water cooled beam dump made of Titanium– Zirconium–Molybdenum (TZM) alloy is shielded to avoid neutrons and gamma radiation. It was used during the commissioning phase of the RFQ for high power beam test. The vast majority of beam operation for physics purposes will be at 10 Hz or less and 50 μs or less. 2
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Fig. 3. Transformation of a 2D phase space distribution into phase space density plot. Figure (a): a 2D distribution in x–xp phase space; Figure (b): the 2D distribution of Figure (a) with normalized coordinates; Figure (c): particle number 𝑑𝑁𝑟 in 𝑑𝑟 interval; Figure (d): normalized particle number 𝑛 in 𝑑𝑟 interval with log scale. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Configuration of one FODO period.
• Large dynamic range measurement capability at the end of the FODO line. Based on the above criterions the extension beam line consists of an achromatic bending section, a matching section, a periodic FODO lattice and a large dynamic range measurement station. 3.1. Characterization of beam halo
Fig. 5. Phase space density curves of vertical particle distributions of different FODO length (‘Input’: input distribution; ‘C’: FODO cycle or period).
For the purpose of designing the BTF extension beam line, a definition of beam halo is required. A quantitative, universally-accepted definition of beam halo is not available, though many attempts have been made [9–12]. In this paper beam halo is defined as the portion of the beam distribution that is far from the beam core and has a low particle density in the range of 10−4 to 10−6 relative to the peak density. These particles have large oscillation amplitudes and can reach the aperture of beam pipe causing uncontrolled beam loss. For this work, a normalized phase space density plot is used to characterize the beam halo. The procedures used to transform a 2D phase space distribution into a phase space density plot is as follows
c. Plot normalized particle numbers 𝑛 (𝑟) = scale.
𝑑𝑁𝑟 2𝜋𝑟𝑑𝑟
vs. 𝑟 with log
Fig. 3 shows the transformation steps. The black curve in plot (d) is the phase space density curve of the distribution in plot (a). The method to identify whether halo develops during beam transport is to compare the phase space density curves of the output distribution with the input distribution. The discrepancy between the curves indicates halo has developed during the transport. From the transformation procedures we can see that the phase space density plot is independent of beam energy and distribution location along beam line. Therefore, it can be used for comparing general distribution properties of different accelerators and even different particle species.
a. Transform particle coordinates 𝑥, 𝑥′ to normalized coordinates according to the following formula: 𝑥 𝑥𝑛 = √ 𝛽𝑟𝑚𝑠 √ 𝛼𝑟𝑚𝑠 ⋅ 𝑥 𝑥′𝑛 = √ + 𝑥′ 𝛽𝑟𝑚𝑠 𝛽𝑟𝑚𝑠
3.2. Design of FODO lattice
where 𝛼𝑟𝑚𝑠 , 𝛽𝑟𝑚𝑠√are the RMS Twiss parameters. b. Calculate 𝑟 = 𝑥2𝑛 + 𝑥′2 𝑛 , and count number of particles 𝑑𝑁𝑟 within 𝑑𝑟 intervals.
The FODO lattice is composed of multiple identical FODO periods. The configuration of one FODO period is shown in Fig. 4. In the figure, Ld, Lq and D denote the drift length, quadrupole length and quadrupole 3
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Fig. 6. Phase space density plot of the vertical particle distributions at the end of a 9 FODO period lattice with 𝜎0 = 72.9◦ (left) and 𝜎0 = 112.4◦ (right) (‘Input’: input distribution; 1M: mismatch factor = 1; 2M: mismatch factor = 2; 3M: mismatch factor = 3).
Fig. 8. Quadrupoles installation on tracks in vacuum pipe.
Fig. 7. Quadrupole permanent magnet (left: 3D model; right: first sample).
FODO periods). The matched and mismatched beams will be provided by four quadrupoles (Q10, Q11, Q12 and Q13 as shown in Fig. 9) with adjustable gradients installed upstream of the FODO entrance. Since the footprint of the BTF is constraint by external factors, the matching region should have a short configuration to preserve space for the FODO line. The quadrupoles are required to have gradients less than 20 T/m in order to use the already existing quadrupoles at SNS, and real estate should be available for diagnostic devices such as a Faraday cup, etc. The final matching section design which meets these requirements is shown in Fig. 9.
spacing, respectively. These parameters together with quadrupole gradients decide the FODO period’s zero current phase advance 𝜎0 which influences oscillation of particles in halo [13]. The number of betatron cycles and total lattice length influence the halo evolution. Fig. 5 shows the phase space density plot of particle distributions at the end of a FODO line for different total FODO lengths. For each case, the mismatch factor is 2 (mismatch factor = 1 means matched beam), the beam current is 40 mA, and the zero current phase advance 𝜎0 = 81.6◦ . In Fig. 5, ‘‘Input’’ denotes the phase space density curve of the input distribution, and ‘‘C’’ refers to FODO cycle or period. Results indicate that halo develops when the FODO length is or is greater than 6 FODO (6C) periods, and the longer the FODO length is the broader the halo extends. Fig. 6 displays halo development at different mismatch conditions when 𝜎0 is 72.9◦ and 112.4◦ and the FODO length is 9 FODO periods. The final BTF FODO lattice consists of 19 quadrupoles comprising 9.5 FODO periods. For cost savings, all of the quadrupoles are identical permanent magnets (as shown in Fig. 7) of length 7.5 cm and integrated gradient 1.8 T. For future flexibility in the lattice configuration, the magnets are designed with three sandwiched pieces that can be separated to give a higher number of smaller gradient magnets. The quadrupoles are mounted on stainless steel tracks in a vacuum pipe (as shown in Fig. 8) whose mounting structure allows modification of FODO phase advance through different combinations of magnets and drift lengths.
3.4. Achromatic design of bending section The bending section has been designed to connect the FODO lattice and the original BTF. The section turns the beam 180.0◦ which is required for the BTF to fit within the available footprint. It is desirable to have a zero dispersion solution at the exit of this section. This section has two 90.0◦ bending magnets with quadrupoles in between to create the achromat. Assuming M is the transfer matrix through one section of beam line ⎡ 𝑀11 𝑀12 𝑀13 ⎤ ⎥. Eliminating containing bending magnets, 𝑀 = ⎢ 𝑀21 𝑀22 𝑀23 ⎥ ⎢ 0 1 ⎦ ⎣ 0 dispersion is achieved by making the dispersion term 𝑀13 and 𝑀23 equal to zero. One quadrupole can realize beam transport without dispersion when it is located at the symmetry axis of the two bending magnets. However, this solution does not leave enough real estate for the desired diagnostic devices to keep the beam size small. Two quadrupoles can also be used to achieve an achromat, however some particles would reach the vacuum pipe wall in the bending section due to the inadequate focusing. Therefore, three quadrupoles (Q07, Q08 and Q09 in Fig. 10) are used to eliminate dispersion and provide adequate
3.3. Design of the matching section The FODO lattice design shows that halo develops when the beam is mismatched and the FODO has a reasonable length (e.g. longer than 6 4
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Fig. 9. Configuration of matching section (unit: cm).
Fig. 10. Configuration of bending section with three quadrupoles (unit: cm). Table 1 Quadrupole parameters of the bending section. L1 L2 Q07 gradient Q08 gradient Q09 gradient
15 cm 35 cm 8.10 T/m −5.00 T/m 6.56 T/m
room for beam instrumentation. The configuration of the bending section is shown in Fig. 10. In the layout L1 and L2 are the drift lengths that need to be optimized to avoid beam loss and to minimize required quadrupole gradients. The final optimized values of L1, L2, and quadrupole gradients are listed in Table 1. Fig. 11 shows the dispersion function of the bending section when parameters in Table 1 are used. The figure indicates that the dispersion function is zero before and after the bending section (10 cm is extended before and after the bending section in the calculation), which confirms the achromatic design of the bending section.
Fig. 11. Dispersion function (D) of the bending section (10 cm is extended before and after the bending section in the calculation).
3.5. Beam instrumentation in the extension beamline The final beam is transported into a beam stop after the measurement station through a 90.0◦ bending magnet (B3 in Fig. 12), which prevents the influences of ions that scattered by the first slit on the dynamic range of the slit-based emittance measurement system.
A bunch shape monitor (BSM) will be installed between quadrupole Q08 and Q09 in the bending section to carry out the 6D phase space measurement. A Faraday cup is to be installed between Q12 and Q13 in the matching section to measure the beam current entering the FODO lattice and to serve as a beam stop. A large dynamic range measurement station (2 slit pairs) will be used to measure the 2D transverse particle distributions at the end of the FODO lattice with a high dynamic range of 10−5 –10−6 . The measurement station is 132.85 cm long without any beam focusing elements, therefore, additional quadrupoles are needed to ensure beam passes through without loss. It has been found that at least two quadrupoles are needed, and their positions have a significant influence on beam transmission. The final configuration of the two quadrupoles (Q14, Q15) can be seen in Fig. 12. The gradients of the two quadrupoles are adjustable (less than 20 T/m) depending on beam matched conditions.
4. Simulation of the whole beam line of the final BTF The final layout of the BTF is shown in Fig. 2. Simulations of the whole beam line with PyORBIT [14] from the RFQ exit to the final measurement station have been conducted based on Fig. 2 configuration. The beam distribution at RFQ exit, reconstructed by combining together measured 2D distributions of 40 mA at BTF [15], was used as the initial distribution for the simulation. It should be noted that, all the designs and simulation of the BTF beam line were done with the ‘Hard edge’ model of all the quadrupole fields, because the fringe field distributions of all the quadrupoles has not been available at present. Simulation with the fringe field model will be performed in future. 5
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Fig. 12. Configuration of quadrupoles after FODO lattice (unit: cm).
Fig. 13. Phase space density plot of the particle distributions at measurement station (‘Input’: input distribution; 1M: mismatch factor = 1; 2M: mismatch factor = 2; 3M: mismatch factor = 3).
Fig. 14. Distribution comparison between different position under matched condition (‘Input’: input distribution; FODO_enter: distribution at the FODO entrance; MeasStation: distribution at measurement station).
Fig. 13 shows the phase space density plots of the particle distributions at the middle of measurement station, where the ‘‘input’’ curves are the phase space density curves of distributions at RFQ exit. It can be found that halo has grown obviously under mismatched conditions compared with the initial distributions, meanwhile, halo is also present in the distributions under the matched condition. Particle distributions at the FODO entrance are investigated to figure out the cause of the halo generation under the matched condition, and results are shown in Fig. 14. It can be seen that halo has already developed at the FODO entrance and is transported to the measurement station. It may be
desirable that the halo be scrapped off before entering the FODO lattice. This is left for future work. Fig. 15 displays the full beam size evolution along the whole BTF beam line when the beam is matched to the FODO lattice. Simulation shows that the transmission from FODO entrance to the measurement station is 100% under matched and mismatched conditions, and transmission efficiency along the whole beamline is 99.76%. However, with ideal Gaussian distribution or simulated RFQ output distribution as the input distribution for the simulation the transmission efficiency along the whole beamline is 100%. 6
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Fig. 15. Full beam size evolution along the whole BTF beam line (beam matched to FODO lattice).
Quad number
Aperture / cm
Max integrated gradient / T
Magnetic length / cm
Q01 Q02 Q03 Q04 Q05 Q06 Q07 Q08 Q09 Q10 Q11 Q12 Q13 Q14 Q15
3.2 4.2 4.2 4.2 6.5 6.5 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2
2.5 1.9 1.7 1.7 0.75 0.75 1.7 2.5 1.7 1.7 2.5 2.5 2.5 2.5 1.7
6.1 6.6 9.6 9.6 10.6 10.6 9.6 14.6 9.6 9.6 14.6 14.6 14.6 14.6 9.6
supplies. Each magnet has independent horizontal and vertical dipole coils for beam steering. The maximum angular deflections of the dipole steerers is 3.2 mrad in the first quadruple and 2.6 mrad in the second quadrupole.
5. Subsystems after RFQ in the final BTF 5.1. Magnetic system
5.1.2. Air cooled low current quadrupole magnets The other 13 quadrupoles in the BTF are air cooled low current magnets, which have three combinations of magnetic length, magnetic strength, and aperture (see Table 2). Fig. 17 shows the pictures of the air cooled quadrupoles. The magnets are powered by independent 12 A power supplies.
Except for the 19 permanent magnet quadrupoles in the FODO lattice, the magnets in the final BTF are electromagnetic magnets including 15 quadrupoles, 4 dipole steerers and 3 big bending dipoles. The locations of the electromagnetic magnets and bending dipoles are shown in Fig. 2. The main parameters of the quadrupole magnets are summarized in Table 2.
5.1.3. Bending dipoles The three 90.0◦ bending dipoles are identical in design (as shown in Fig. 2). The aperture of the dipoles is 4.0 cm, and the bending radius is 35.56 cm, respectively. Fig. 18 shows the inner structure and a photo of the bending dipoles. All the three dipole magnets are connected in series and powered by a single 400 A power supply.
5.1.1. Water cooled high current quadrupole magnets The first two quadrupoles in the BTF MEBT are water cooled high current magnets. The mechanical structure of the two quadrupoles are the same (as shown in Fig. 16) but with different apertures (see Table 2). The magnets are powered using independent 400 A power
Fig. 16. Mechanical drawing of water cooled quadrupole magnets (see Table 2 for definitions).
Fig. 17. Mechanical drawing of air cooled quadrupole magnets (see Table 2 for definitions).
7
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Fig. 21. Plot of beam current at BCM and final beam stop. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 18. Bending dipole (top: inner structure; bottom: photo).
Fig. 22. Schematic drawing of movable BPM.
Fig. 19. Beam current attenuation system (top: overall view; bottom: attenuation foils).
Fig. 23. Bunch phase dependence on BPM displacement (beam velocity is linearly proportional to the curve slope).
reduce peak beam current while preserving the beam distributions in the transverse and longitudinal phase space. The system consists of 4 movable semitransparent Carbon–carbon composite grids with different transmission (9%, 23%, 45% and 72%). The aperture of each foil is 2.0 cm. The first bracket contains a thin carbon foil to convert the H− ions to proton for experiments with proton beam.
Fig. 20. Picture of BCM.
5.2. Diagnostics A heavy suite of diagnostic devices is included in the BTF. The positions of each device can be found in Fig. 2.
5.2.2. Beam current monitor (BCM) The BCM (as shown in Fig. 20) in the BTF MEBT is used for beam current measurement at the RFQ exit. A commercial clamp-on current transformer (PEARSON 5949) over a standard ceramic break is used. The transformer output is connected directly to a 50 Ohm input of a
5.2.1. Beam current attenuation system A beam current attenuation system (as shown in Fig. 19) is installed between the second and the third quadrupoles in the BTF to 8
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Fig. 24. Picture of horizontal and vertical slit pair (left: overall view; right: zoomed in).
Fig. 25. Energy selection slit. Fig. 26. Camera and view screen installation for energy distribution measurement.
digital-to-analog converter. The aperture of the BCM is 5.0 cm. The yellow curve in Fig. 21 is an example of the BCM current signal. 5.2.3. Movable beam position and phase monitor (BPM) The movable BPM (as shown in Fig. 22) is employed for beam energy measurement by using the time-of-flight technique and for beam position measurement. The BPM uses the standard SNS MEBT strip-line pick-up with 42 mm aperture. The pick-up can be moved along the beam line within +∕−25.0 mm range. The standard 402.5 MHz SNS BPM electronics is used for signal processing [16]. Fig. 23 gives an example of beam energy measurement by the movable BPM.
Fig. 27. Energy distribution on the energy slit captured by camera.
5.2.6. Bunch shape monitor (BSM) The BSM is used to measure the bunch temporal profile. Fig. 28 illustrates the principle of BSM. The ion beam hits the Tungsten wire suspended in BSM producing secondary electrons with the same temporal distribution as the ion beam. The temporal distribution of the secondary electrons is then converted to transverse deflection by an RF deflector. The transverse distribution is measured using a luminescent screen and a digital camera [17]. The measurement accuracy of the BSM is about 1.0◦ of 402.5 MHz. Fig. 29 shows an example of the measured temporal profiles for different energy slices.
5.2.4. Slits Two pairs of orthogonal 200 μm wide carbon slits installed in the BTF MEBT allow for the beam distribution measurement in the transverse phase space. The horizontal and vertical slits in each pair are separated by 10 mm longitudinally in order to move simultaneously without collision. The distance between the two slit pairs is 0.94 m. All the slits can be scanned over the 40 mm aperture. The slits are water cooled with maximum average beam power of 63 W (2.5 MeV, 50 mA, 10 Hz, 50 μs). Fig. 24 shows the picture of one slit pair. An identical slit arrangement is installed at the end of the FODO for the large dynamic range measurement. The slit separation in this case is 0.78 m. The charge passing through the slits is measured by any of the downstream beam stops or Faraday Cups.
5.2.7. Faraday cups (FC) There are three Faraday cups in the BTF beam line. The first one is formed by the high power beam stop insulated from the ground; the second one is in the matching section and is mounted on an actuator as shown in Fig. 30; and the third one (as shown in Fig. 31) is fixed at the very end of the beam line. The two low power Faraday cups are made of carbon and rated for 63 W of beam power. The aperture of the FCs is 4.5 cm.
5.2.5. Energy slit/view screen Two viewscreens each with an imbedded 200 μm slit (as shown in Fig. 25) are installed on actuators after the first and the last 90.0◦ bending magnet. The slits are used for energy selection by utilizing the dispersion feature of the bending magnet. The screen is made of an alumina ceramic based luminescent material. This allows for the entire energy distribution to be measured in one shot by acquiring the screen image with a video camera, or for selecting a portion of the energy spectrum for temporal analysis downstream. The image on the view screen is recorded by a video camera installed on the other side of the bending magnet as shown in Fig. 26. Fig. 27 shows a measured energy distribution on the energy slit/view screen.
5.2.8. Measurement arrangements of beam distributions The two pairs of slits in the MEBT together with the energy slit and the BSM allows for the BTF to measure up to 6D beam distributions. By moving one slit of the two pairs of slits a 1D transverse beam profile can easily be obtained (as shown in Fig. 32). By moving the two vertical slits or two horizontal slits a 2D transverse beam distribution will be produced (an example is shown in Fig. 33). A 4D transverse distribution will be generated by moving all the slits of the two pairs sequentially. 9
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Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162826
Fig. 31. 3D model (left) and mechanical drawings (right) of Faraday cup/beam stop.
Fig. 28. Schematic drawing of BSM.
Fig. 32. 1D transverse beam profile by scanning one transverse slit.
Fig. 29. Measured temporal profiles for different energy slices.
Fig. 33. Vertical beam emittance measured by two slits. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
5.2.9. Beam loss monitors (BLMs) There are 8 beam loss monitors uniformly distributed along the BTF beam line. The detectors used are of the standard SNS scintillator/PMT design [19]. A prototype of the new generation SNS BLM processing electronics is used for data acquisition. The BLM system is capable to detect losses at ∼1 W/m level. The system is connected to the Machine Protection System, which shuts off the beam if losses are above the safe level. 5.2.10. Diagnostics data acquisition system The diagnostics data acquisition system is based on NI PXI-1075 chassis that holds: PXIe-8133 Real Time controller, NI-5105 and NI4481 ADC cards, NI 7961R FPGA card for timing and NI 7330 Motion Controllers. Cameras are also connected to the same controller over dedicated Gigabit Ethernet network. Timing information is decoded by FPGA and triggers are generated on the chassis backplane to allow hardware synchronization for all ADCs and motion cards. This helps to implement continuous movement of actuators while running beam
Fig. 30. Movable Faraday cup (left: whole structure; right: cup).
The main outstanding feature of the BTF diagnostics is its capability of conducting a full 6D phase space measurement which has been introduced in Ref. [18]. 10
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Fig. 35. Measured vertical emittance with large dynamic range at the measurement station of BTF extension. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Fig. 34. Vacuum system diagram of the BTF beam line.
Commissioning of the FODO extension beamline began at the start of 2019. Preliminary results show the beam transmission efficiency from RFQ exit to the final beam stop (BS34) is about 98% when the beam current is 25 mA. Transverse emittances have also been measured with the measurement station. Fig. 35 shows an example of a measured vertical emittance when the beam current is 13 mA. Beam halo investigation has not yet been conducted.
at up to 60 Hz rate, instead of performing steps and waiting for a beam pulse in every position. Two ADCs allow wider dynamic range vs time resolution. NI-5105 samples at 10–60 MS/s but the range is 12-bit only. Conversely, the NI-4481 provides 24-bit range but at 1.25 MS/s only. The system is indifferent to camera type, so any GigE camera can be used with minor tweaking. The data acquisition is fully interfaced with EPICS control system to provide connectivity to external software. The system is programmed in NI LabVIEW RT including SNS-made EPICS Channel Access.
7. Conclusions and future work The combination of SNS-equivalent hardware and beam parameters, the extensive and novel beam instrumentation suite, and the dedicated FODO line make the BTF at SNS a unique facility for R&D in high intensity hadron beam physics. Commissioning of the BTF shows the measured high beam transmission efficiency from RFQ to the end of the extension beam line has verified the design of the extension line. Up to now, only the commissioning and some preliminary measurements have been performed. There is a large program of planned work, including:
5.3. Auxiliary systems The auxiliary systems of the BTF includes the vacuum system and the control system. 5.3.1. Vacuum Due to the limited conductance of the BTF beam pipe, a distributed pumping strategy is adopted to achieve the required vacuum (<10−7 Torr). Six 300 l/s Turbo molecular pumps are approximately uniformly distributed along the beam line. The vacuum system diagram is shown in Fig. 34.
• Increasing beam transmission efficiency along the whole beam line based on model; • Measuring beam distributions and analyzing beam halo formation under different match conditions; • Improving the 6D measurement which includes increasing scan speed and enhancing scan resolution; • Developing methods to analyze multi-dimensional data; • Benchmarking simulation results against measurements, verifying methods of generating initial distributions for simulation codes.
5.3.2. Controls The Integrated Control Systems (ICS) is designed to provide remote control of all the BTF subsystems, machine protection and personnel protection systems. The personnel protection system includes radiation monitors, which shut off beam when high levels of radiation are detected. The machine protection system limits the beam power to < 63 W when beam is not directed to the high-power beam stop. The control system allows full control of the BTF from the local control room and/or from the SNS central control room.
Acknowledgments This work has been partially supported by National Science Foundation, USA Accelerator Science grant 1535312. This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC0500OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for the United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). The authors would like to thank A. Shishlo at SNS for his help in use of the PyORBIT code and thank Huan Jia at IMP for useful discussion.
6. BTF operation The commissioning of the BTF occurred in several stages. First, the beam was commissioned in a straight line throughout the RFQ and MEBT to the high power beam stop to verify performance of the new RFQ [20]. Next the BTF was used to demonstrate the feasibility of 6D phase space measurements [21]. The H− ion source was operated at 60 Hz with pulse length 1 ms during commissioning, while the RFQ was operated at 10 Hz with pulse length 50 μs. Particles that cannot be accelerated were lost in RFQ. Following this, the original SNS RFQ was installed at the BTF and the beam line was extended to include the new FODO line and high dynamic range emittance station. 11
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References
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