First operation of the Chalk River superconducting cyclotron

Nuclear Instruments and Methods in Physics Research A254 (1987) 237-251 North-Holland, Amsterdam

FIRST OPERATION OF THE CHALK RIVER SUPERCONDUCTING

237

CYCLOTRON

C. Bruce B I G H A M , W a l t e r G. D A V I E S , E d w a r d A. H E I G H W A Y , J. D u n c a n H E P B U R N , C l a r e n c e R.J. H O F F M A N N , J o h n A. H U L B E R T , J o h n H. O R M R O D a n d H a r v e y R. S C H N E I D E R Atomic Energy of Canada Limited, Research Company, Chalk River Nuclear Laboratories, Chalk Rioer, Ontario, Canada KOJ 1JO

Received 18 September 1986

Successful development of the first beam from the Chalk River superconducting cyclotron has verified the design and construction of all elements of the cyclotron.

1. Introduction The Chalk River superconducting cyclotron is a booster accelerator for the laboratory's MP Tandem. The TASCC (Tandem Accelerator SuperConducting Cyclotron) combined facility is capable of producing particle beams of all nuclear species at energies in excess of that required to overcome the Coulomb bartier in nucleus-nucleus interactions. Injected from the Tandem, the cyclotron will accelerate ions, from lithium to 50 MeV/u, up to uranium to 2.S

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10 MeV/u. Fig. 1 shows the range of accelerated ions in the mass-energy plane. The boundaries inside the range delineate the subranges for the three possible combinations of cyclotron harmonic and accelerating cavity rf modes. The choice for the commissioning ion, 127I, 10 MeV/u, is shown at the centre of the range. The superconducting cyclotron concept, in which the use of a superconducting excitation winding for the main field results in a compact machine by virtue of the high current densities attainable in superconductors, was suggested by Bigham and Schneider [1]. This configuration was selected for the Tandem booster from among several options because it had the best economics coupled with reliance on well-established technology. The development of the cyclotron began in I974 with the design and construction of full size "models" of the magnet and rf systems. Each "model" was demonstrated to fulfill the required specifications independently. Then in 1982 the inner wall of the main magnet coil cryostat was modified to accommodate the rf accelerating structure and the two major components of the cyclotron operated together. In the final stage of development, the complete superconducting magnetic extraction channel was tested to full current, but in zero background field, in a specially designed cryostat, and then assembled between the coil vessels of the main magnet coil cryostat. The complete cyclotron system was then transported to its shielded vault in the TASCC facility for final assembly and tests, and commissioning with beam. This paper describes the overall performance of the cyclotron during the development of the first beam. Details of the subsystems have either been described in the literature or will be the subject of future publications.

C.B. Bigham et al. / The Chalk River superconducting cyclotron

238

nels 1, 2A and 2B) was installed in the cryostat during final assembly [2]. The hill lenses and iron bars to compensate perturbations from the hill lenses and Channel I were installed on the cryostat inner wall. The superconducting magnetic channel consists of a series of short linear modules [3] separated by short drift spaces. Channel 1 consists of 4 modules. Three of them are identical and have iron bars to generate a fixed radial focusing gradient and superconducting coils to provide variable steering. The fourth module, located at the entrance to Channel 2, has only iron bars for focusing and no steering coils. Channel 2 contains six identical modules that use superconducting coils to generate both variable steering and variable radial focusing fields. The steering windings have been grouped into 2 independently driven sets. The six sets of gradient windings are driven in series from a single power supply. The channel has four independently adjustable currents: three for steering and one for radial focusing,

2. Final assembly 2.1 Layout

The location of the cyclotron in TASCC [2] is shown in fig. 2. The injection beam line from the Tandem provides a high quality bunched beam at the cyclotron midplane for injection by stripping. After extraction, the beam line transports the beam via a Y magnet to the experimental area. A by-pass line for the Tandem beam to the experimental area is also shown. Cyclotron services are in the room over the injection and by-pass beam lines. The liquid helium transfer lines from the liquefier to cryostat can be seen in the photograph, fig. 3. The coaxial transmission line for the rf drive to the dees and numerous cooling water lines are also evident. 2.2. Magnetic extraction channel

2.3 Injection and extraction steerers

Fig. 4 is a midplane layout of the cyclotron and shows the components of the extraction system. The superconducting magnetic channel (displayed as Chan-

Two horizontal and two vertical steering magnets were also installed during final assembly for steering the

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C.B. Bigham et al. / The Chalk River superconductingcyclotron

239

Fig. 3. Photograph of the cyclotron in final position.

injected b e a m to the correct position a n d direction at the stripper foil. Two horizontal a n d one vertical steering magnets were installed after the extraction c h a n n e l

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to steer the b e a m from the superconducting channel into the b e a m line to the target. The m a g n e t positions are s h o w n in fig. 4 a n d details of the magnets axe listed in table 1 with the settings used o n the last day of the commissioning experiments. N o n e of the settings app r o a c h e d the m a x i m u m design currents.

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3. Magnetic field After the poles h a d b e e n reinstalled a n d the cryostat cooled d o w n [2], the field profile (fig. 5) required for the 127123+ b e a m was set up a n d test mapped. The solid dots show the measured field a n d curve the calculated field. Initially the trim rod drives were n o t installed a n d n o a d j u s t m e n t s were m a d e to the trim rods except for specific individual r o d tests in which the trim rods were o p e r a t e d manually. Later the trim rod drives were installed a n d some tests carried out. These were mostly o n the effects of the outer rods o n the last few turns. T h e field m a p p i n g a p p a r a t u s was also used to measure the first h a r m o n i c p e r t u r b a t i o n fields generated b y

C.B. Bigham et a L / The Chalk River superconducting cyclotron

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the magnetic channel superconducting windings. Field maps were made for each of the four independently driven winding sets. In each case the winding current was 100 A. The main magnet generated a midplane field of 3.8 T using inner and outer coil currents of 1700 and 800 A respectively. Channel 1 first harmonic field was about 50% higher than the calculated value. At a radius of 609 mm the measured first harmonic amplitude was -- 0.1 mT. The measured gradient winding perturbation at the same radius was 0.17 mT and the results generally agree with calculated values. The perturbation from Channel 2 steering windings were measured and found to be negligibly small, as calculated. The first harmonic perturbations can be corrected with the trim rods.

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structure

After the field mapping was complete, the accelerating structure dee system [4] was installed. Fig. 6 is a

photograph of the lower section of the structure on the lower pole in place in the cryostat. The accelerating dees in Valleys 23 and 41 (see fig. 4) are mounted on the upper resonator. (Valley n m is defined as being between hill n and hill m.) The dees in Valleys 12 and 34 are mounted on the lower resonator. The upper resonator is driven by a coupling capacitor below Dee 23 on a coaxial line through the lower pole from the power amplifier in the service room. The lower resonator is driven by capacitive coupling in the centre region from the upper resonator. The only mechanical connection between the upper and lower section is around the inner wall of the cryostat which is copper sheathed to complete the liner of the resonant cavity. The system has two resonances, 0-mode (all dees in phase) and ~r-mode (upper mounted dees 180 ° out of phase from lower mounted dees). The range of operations is shown in fig. 1 with the 1.27 GeV 127123+ point indicated at 42.6995 MHz. The midplane vacuum is maintained by two cryopumps [12] in the upper parts of Valleys 34 and 41 (see fig. 4). A reasonable vacuum is first established by two 100 1/s turbo pumps. One pumps the midplaneinto the

C.B. Bigham et al. / The Chalk River superconducting cyclotron

241

Fig. 6. A photograph of the lower pole in the cryostat with the lower dees in place.

manifold connecting the spaces between the upper and lower liners on the poles, the second pumps on this manifold maintaining it at about 10 mPa. The base pressure in the midplane is less than 0.07 mPa with the cryopumps operating. At this pressure there was no significant stripping beam loss for the ]271 beam. The radial probe system vacuums are maintained by individual 100 1/s turbo pumps. The sliding seals on the dee tuners have guard vacuums maintained at less than 10 Pa by a mechanical roughing pump. The adjacent beam line vacuum is maintained at better than 0.1 mPa by ion pumps.

5. 70.9 MeV Iz717+ beam from tandem

&l. Injecaon beam~ne The injection beam line to the cyclotron has six functions. They are (1) to transport the ion beam from the MP Tandem to the cyclotron, (2) to provide mass and charge state selection, (3) to provide precise voltage control of the tandem,

(4) to provide phase control of the low-energy buncher with respect to the high energy rebuncher, (5) to match the linear and angular dispersion of the cyclotron such that achromatic beam transport occurs from the low energy buncher to the cyclotron stripper, (6) to match the transverse emittance of the beam coming from the tandem to the acceptance of the cyclotron. A diagram of the beam line showing these functions is given in fig. 7. The detailed steps required to produce a "matched" beam at the cyclotron are given in ref. [5]. Except for a problem with the calibration of 3 of the 7 types of quadrupole lenses the initial beam setup went very smoothly, and the procedure was found to work well. Initial values of the 23 parameters that do not scale with Bp were calculated. Semiempirical estimates of the beam emittance out of the tandem were used. Beam diagnostic measurents were then made at diagnostic stations I and 3 (see fig. 7) and the programs described in ref. [5] were used to calculate new settings for the lens QI1, QI7 and QI8. (The settings for QI1 changed by less than 1%.) The programs converged to solutions that were within the errors of measurement in

242

C.B. Bigham et aL

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one iteration. Measurements at diagnostics stations 4 and 5 showed that the match was satisfactory but not perfect. The positions of the horizontal waists at stations 4 and 5 were displayed several cm from their correct positions and the size of the waist at the cyclotron center is about a factor of two too large. The axial resolution of the cyclotron probes (station 5) was not sufficient to allow a calculation of the position or the size of the axial waist. However the amplitudes of the incoherent radial and axial betatron oscillations were small, which provides further confirmation that the match is reasonably good. No attempt has been made to directly measure the degree of dispersion matching, but

the fact that the phase width of the beam in the cyclotron was within a factor of two of the optimum values indicates that the dispersion matching was quite good. It is likely that much of the residual error in matching the beam comes from uncertainties in the quadrupole calibrations.

5.2. Bunching The continuous beam from the ion source is bunched for injection into the cyclotron by a low energy buncher (LEB) before the Tandem and a high energy buncher (HEB) (rebuncher) after BI-2 in fig. 7. The LEB has 1

C.B. Bigham et al. / The Chalk River superconducting cyclotron

mm gridded gap operating at the cyclotron rf frequency, with second harmonic superimposed to approximate the ideal sawtooth wave form. Up to 2 kV peak voltage is required [6]. The HEB has a 65 mm drift tube mounted on a quarter-wave resonator operating at either twice or four times the cyclotron frequency for fix matching [7]. Up to about 20 kV peak voltage is required. The LEB is set to provide a time focus at the Tandem stripper to minimize the effects of straggling in the stripper; the HEB then provides a new time focus at the cyclotron stripper. Bunch length measurements were made with the beam pulse detectors (BPD) shown in fig. 7 after BI-2 and BI-6 [8]. Bunch timing measurements were made by a vector voltmeter on the capacitive phase probe (CPP) after BI-1 or by dc measurements of the beam current intercepted by the "phase" slits S1-4, after BI-3. Initial experiments [8] gave bunch lengths of 1.6 ns (25 °) fwhm measured at the first BPD and showed that Tandem transit time jitter observed to be _+3 ° in phase could easily be reduced to _+1/2 ° by the stabilization circuit. This circuit corrected the LEB rf drive phase by using an error signal either from a phase comparison between reference and CPP or the difference in beam current on the phase slits. In later experiments, however, Tandem jitter was usually much larger and phase stabilization was essential to obtain reasonable bunching. Fig. 8 shows a recorder trace of the time variation of the vector voltmeter measurement of the bunch phase relative to the reference phase for unstabilized, CPP stabilized and slit stabilized beams. Slit stabilization has been used throughout and reduces the jitter to better than + 2 °. This is adequate for present measurements but will require improvement to attain the specified energy stability. Fig. 9 shows BPD spectra for a typical beam. The

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Fig. 9. Beam pulse detector time spectra for low energy buncher (LEB) alone (a), high energy buncher (HEB) alone (b) and with both (c). The HEB operates at twice LEB frequency and, because of the phase slits removing the wings, there are two bunched and two debunched peaks per LEB cycle. The lower trace shows the intermediate bunched peak from the HEB bunching of the 'dark' current. spectrum is the number of ions arriving per unit time against time. The upper trace shows the shape of two bunches, one per rf cycle, measured at the first BPD in front of the HEB. The second trace shows the bunching effect of the HEB alone at the second BPD. There is a narrow and a broad peak, bunching and debunching, for each HEB cycle. The third trace is with both bunchers and shows beside the main bunches, inter-

244

C.B. Bigham et al. / The Chalk Riuer superconducting cyclotron

are interfaced with the CAMAC control system. The foil changer has operated successfully without requiring magnetic shielding for the stepping motors. Enclosing each foil in a copper sleeve, which makes contact to the dee via beryllium-copper fingerstock when a foil is positioned to strip beam, appears to be an effective method of shielding foils from rf fields. The stripping distribution observed with the foil at a radial position of 153 mm by doing a "radial" scan with Probe 2 is shown in fig. 10. The expected positions for the charge states are indicated on the figure and show that the 23 + state to be accelerated has the largest current. (The identification of the 23 + state was verified from the accelerated beam patterns measured later. See below - section 7.2) The peak positions are within 2 mm of the expected values and the relative intensities are in agreement with earlier data [10].

mediate bunches from HEB modulation of the "dark" current. The HEB reduced the 1.8 ns bunch widths produced at the HEB by the LEB to 1.2-1.4 ns typically. However, the radial beam widths observed in the cyclotron indicate that the bunches arriving at the cyclotron stripper foil are significantly shorter, about 0.5 ns long (8°). This is not unexpected since the position of the second BPD is not an isochronous point in the beam line. So far, the buncher voltages have been optimized using the second BPD and this also seems to give minimum radial beam width in the cyclotron.

6. Beam stripping injection (to tzTIZ~+) In the first beam injection into the cyclotron the unbunched 71 MeV 12717+ beam position was measured with the two radial probes with the stripper foil moved out of the beam. The beam was within 1 mm of the expected position at both probes. The foil changer system accurately positions carbon stripping foils in a dee to intercept and charge strip the injected beam [9]. The foils are mounted on stainless steel frames which fit into copper sleeves mounted on a stainless steel roller chain for transport down into the dee. Mechanisms at the top of the cyclotron remove spent foils and replace them with new ones from a magazine. Stepping motors provide mechanical drive and a 12-bit absolute optical shaft encoder senses foil position. The stepping motors and other components

7. Beam acceleration to 1.27 GeV 7.1. Acceleration without bunching

Early in the commissioning of the dee system, the joints between the pole liners and the tuner outer conductors failed. This is difficult joint and consisted of rf contacts and O-ring vacuum seal. The joints had been refitted after the earlier model tests mentioned above but both failed at low power probably because of a deterioration in the vacuum seal first and subsequent rf

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C.B. Bigham et aL / The Chalk River superconducting cyclotron

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7.2. Acceleration with bunching

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The bunching system was set up as described in section 5.2. The correct rf phase setting of the dee system to obtain acceleration of the bunches was t h e n found empirically. First the "phase acceptance" for accelerated beam was measured. Radial probe 2 was set at a fixed radius, 544 mm, and the total beam current observed as the adjustable delay line in the dee phase control circuit was varied. The result is shown in fig. 11. Ideally this phase acceptance curve should approach 180 ° in phase but here it is less than 90 ° . The reduced phase acceptance is in part due to a beam loss in the first few turns and is discussed in more detail below. The optimum phase (delay) setting was found carrying out radial scans with probes and maximizing the radius of a given turn as a function of delay. The optimum delay setting turned out to be near 5 ns as indicated on fig. 11. This is not at the maximum indicating that at the optimum phase for acceleration, some beam is being lost. Radial probe scans showed separated turns over the full radius. Fig. 12 shows the turn patterns measured with Probe 1 and Probe 2. Probe 1 is in Hill 1 (see fig. 4) about 45 ° in azimuth after the stripper foil in VaUey 41 and Probe 2 is in Hill 2, 90 ° from Probe 1. Each probe has five "fingers", 3 m m in height, mounted on a 4 mm pitch vertically and with a 1 m m radial length exposed to the beam. The probe steps in radial increments (0.5 mm for this trace) and the current picked up on each finger is plotted on vertically displayed traces [11]. The traces in fig. 12 show that the beam is mostly on the central finger with a vertical oscillation about five turns in period, in agreement with the vertical betatron period expected for this beam. The amplitude is less than + 4 mm and does not cause beam loss beyond the second turn (see below). The radial orbit spacing is quite regular indicating no large centering error. The radial width of the beam is noticeably greater on

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damage to both rf and vacuum seals. The problem was solved by removing the O-ring and contacts and brazing of the joint with cooling water circulating to keep adjacent components cool. The dee structure was then reassembled and beam commissioning continued. The first acceleration tests were carried out without the bunching system in operation. The injected beam, 71 MeV 12717+, was steered on to the stripper foil to produce the 127123+ beam for acceleration. The accelerated beam intensity was then measured as a function of radius with the two radial probes. Initial tests showed accelerated beam out to 430 m m only. This radial limit was eventually found to be an artifact of the probe system and that beam had been reaching the full energy at the calculated magnetic field. Shadowing Probe 2 with Probe 1 showed that the observed beam was at the correct radius.

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246

C.B. Bigham et aL

/ The Chalk River superconducting cyclotron

Probe 2 than on Probe 1 in the inner orbits indicating a small mismatch in beam phase space at the stripper foil. Beyond turn # 5 0 , Probe 1 peaks are wider, i.e., the beam pattern precesses by 90 ° in about 50 turns, as expected for the radial betatron frequency of approximately 1.01. The last few turns show radial displacements caused by a small first harmonic component in the magnetic field. These perturbations depend in detail on the exact positioning of the orbits and therefore on the particular accelerating voltage. The last seven turns are shown in more detail in fig. 13 for a slightly different dee voltage. These show the effect of a first harmonic perturbation which provides, by chance, a larger than normal orbit separation for the last full turn. An increase of 0.5% in dee voltage would move this orbit to the 662 mm radius required for the beam to enter the electrostatic deflector. Peaks from charge states other than the accelerated 23 + beam also appear on the traces with the first five turns. This is shown in more detail on fig. 14. The 23 + peaks are labeled 0, 1,... ,5. The beam widths are indicated on the turn 4 peaks as 1.2 m m on Probe 1 and 1.9 mm on Probe 2. The 1.2 m m is close to the probe resolution of about 1 mm. Turn 0 peaks of other charge states from 22 + , 21 + and 20 + also show on Probe 2. Only 22 + is accelerated through one turn with significant intensity and shows again on both Probe 1 and Probe 2. These identifications were made by varying the dee volts and observing changes in orbit position. The vertical displacement of the 23 + accelerated orbits helped in the identification.

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Fig. 14. Expanded turn patterns at injection showing the accelerated charge state 23 + labeled 0, 1.... 5 and other charge states. Of these, only the 22 + state completes one orbit with any intensity. (The states were identified by observing their position with different accelerating voltages.) The plots show the individual traces for each of the five fingers on the probe and show the vertical deflection which causes the beam loss indicated in fig. 11. The turn 3 current on finger 1 of Probe 2 is probably lost on the upper stripper foil chain guide located 7 mm above the midplane.

This vertical displacement depends on the rf phase of the injected beam and causes the cutoff in the phase acceptance at 4 ns as shown in fig. 11. Note that turn 1 is on the centre finger (finger 3) on Probe 1 and has moved up at Probe 2 to partly intercept finger 2. The second turn is on finger 2 on Probe 1 and beginning to appear on finger 1 on Probe 2. Turn three is almost equally split between fingers 1 and 2 but does not appear on finger I of Probe 1. This is because Probe I is shielded by the stripper foil chain rf shield which is 7 m m above the midplane. The beam hitting the chain guide is of course lost and is completely lost at delay line settings below 4 ns in fig. 11 and partly lost at the 5 ns setting for optimum energy gain. This vertical steering was reduced by adjusting the dee resonator tuning balance to increase the lower resonator (Dees 12 and 34) voltage relative to that of the

247

C B. Bigham et aL / The Chalk River superconducting cyclotron

upper resonator. Fig. 15 shows the phase acceptance curve measured with the dee balance or ratio changed by 20% and injection vertical steering adjusted. The curve is not symmetric about the optimum phase at 4.2 ns but there is no loss of beam. Other settings of vertical steering drove the beam down onto the lower foil chain rf shield at dee phase delays above 6.5 ns indicating some symmetry in the effect. There was however no corresponding shift on the 3 ns side of the curve.

The cutoff at 8.3 ns in fig. 15 corresponds to the minimum 4 mm orbit spacing required for the first turn to miss the stripper foil shroud. This would limit the phase acceptance to +4.1 ns delay or 128 ° for these conditions. Adjusting the magnetic field by adding a Br perturbation from vertical adjustments of no. 1 trim rods did perturb the vertical position of the orbits but did not alone improve the phase acceptance. Perhaps suitable adjustments of dee balance, trim rods and vertical injection steering could result in a symmetric phase acceptance curve and ensure that there is no beam loss in the first few turns. Further experience with other beams will be required to clarify the situation. Note that the condition shown in fig. 11 has the effect of decreasing the bunch length by scraping off the leading edge of the bunch. The energy gain per turn vs turn number obtained from the orbit spacings is shown in fig. 16. The total number of turns, 110, gives an average energy gain per turn of 11 MeV or Vo cos O = 60 kV. Auxiliary measurements of turn patterns near injection and near extraction indicated that 0 for optimum acceleration was only 8* different and thus the acceleration was close to being isochronous over the full range. The variation in energy gain per turn is then a reasonable measure of the effective dee voltage at that radius. The

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Fig. 16. Energy gain vs turn number from turn spacing. Other measurements indicated that cos 0 = 1 over the full range so the radial variation is a good indication of the radial variation in dee voltage. The expected variation from X-ray end point measurements is indicated by the dashed line.

dashed line indicates the expected variation obtained from X-ray end point measurement at three radial positions [12]. Acceleration during the first few turns is 15% less than expected and during the last few turns slightly more than expected. The magnetic field optimum for these conditions was the same as that set up from measured field maps (see fig. 5) except that the midplane induction was increased by 0.13%, to compensate minor inconsistencies between calibrations of the cyclotron magnet and the beam analyzer achromat, BI 1 and 2. 7.3. Trim rods

During the field mapping checks, the rods were set to the calculated positions for the isochronous field for 127I, 10 MeV/u, and were left in these positions for the commissioning of the iodine beam, after the resulting field distribution had been confirmed. To ensure that the rods would not be inadvertently moved the motor drives were not fitted. Owing to a contact fault in the probe readout, there was at first an apparent beam loss at around 400 mm radius. Review of the trim rod zero records suggested that two rods # 6 had been incorrectly set and the position correction of 1.5 mm was made on each. This correction was not amended when the probe fault was detected. In an attempt to correct the axial offset of turn # 2 , trim rods # 1 , 2 and 3 were moved, in upper and lower pairs in the same direction, in order to raise or lower the local magnetic midplane. It was found that the effects were too small to correct the axial offset, but that the radial field components induced considerably increased the axial betatron amplitudes. These could be corrected for one set of rods by compensating with the set at the next radial position. During this exercise it was noted

248

C.B. Bigham et a L / The Chalk River superconducting cyclotron

that the cumulative effects of rod displacement on the turn pattern were simply additive. Later, when it was found that the beam left the extraction channel well below the midplane elevation, an attempt was made to steer the beam back to the midplane by inducing a radial field component with the outer trim rod set. It was found possible to produce a partial correction, moving the beam up 3-4 mm without noticeably disrupting the turn pattern.

SUM

20-BEAM 15 CURRENT ON STUB (nA) I0 5 0

I0

8. Beam extraction

8.1. Electrostatic deflector During development of the electrostatic deflector the electrode configuration underwent only minor revisions [13], however, the method of feeding high voltage to the deflector electrode changed markedly. The deflector is located in a dee, subtends 31 ° at the machine centre and has a beam aperture of 7 ram. The septum and sparking plates are tungsten and the deflector electrode is stainless steel. High voltage is supplied to the deflector electrode through a standard high voltage cable and a series isolating resistor [14]. This resistor is a short column (20 mm long) of high resistivity, flowing water and is located in the body of a coaxial support insulator for the deflector electrode. The high voltage cable is contained within a coaxial teflon tube, which in turn fits inside a copper tube. This latter tube is the vacuum envelope for the high voltage system. Water for the resistor is supplied from a header at the top of the cyclotron and flows in the two annular regions between the tubes and the cable. At present, operation is limited to 70% of design voltage (100 kV) because of leakage current through the water circuit (1 mA is the maximum current available from the high voltage power supply). This current has been demonstrated to be dependent on water quality. A small, dedicated deionized water system is being assembled, which is expected to remove this limit. The steering effect of the deflector measured on the stub probe is illustrated in fig. 17. The readout from the probe [11] gives the sum of the beam currents on the left and right sectors and the difference between the two currents. These are plotted as a function of deflector voltage setting and show the beam moving as expected and becoming centred at about 60 kV.

8.2. Beam through the magnetic channel The current settings for superconducting extraction channel windings were first set from calculations [15] and usually with these settings, a small beam current would be observed on the first Faraday cup outside the cyclotron. This current was then maximized by "tuning"

20 30 40 50 60 DEFLECTOR VOLTS (KV)

70

Fig. 17. The stub probe beam current vs deflector voltage showing the sum current on the left-fight sectors and the difference current. The beam was centred at about 60 kV.

channel currents, deflector voltage, dee voltage, first harmonic perturbation and injection steering. The details of this system are beyond the scope of this paper and will be discussed more fully in future publications. A typical beam profile (fig. 18) taken with radial Probe 2 shows that the turn pattern for good extraction was very regular indicating that the first harmonic perturbations are small. The trim rod settings used then just cancel the residual error caused by extraction channel elements. The extracted beam profile measured with radial Probe 1 where it crosses the extraction beam line (see fig. 4) is shown in fig. 19. The beam width corrected for the oblique angle of the probe is shown as 6 mm fwhm. The beam is all on finger 4 rather than being centred on finger 3 and is therefore about 4 mm below the midplane and less than 5 mm high.

ii PROBE 2

_

5 620

I

I

I

I

630

640

650

660

_

670

PROBE RADIAL POSITION (mm) Fig. 18. A radial beam profile near extraction taken by radial Probe 2. The radial width and spacing is shown. The pattern is regular indicating first harmonic perturbations of the last few turns are small.

CB. Bigham et al. / The Chalk Ricer superconducting cyclotron

I 2

/

3

4

1680

I 1690

6 mm

L

~ I IA 1700 1710 1720 PROBE POSITION (rnm)

1730

Fig. 19. A radial profile of the beam in the extraction channel where radial Probe 1 crosses the beam line. The width indicated is corrected for the oblique angle of the probe.

9. Beam to target The four element lens QE1 matches the emittance of the beam from the cyclotron to the extraction beam line by producing a double waist ( x - y ) at the aperture SE1 (fig. 7). The lens has the ability to adjust the horizontal ( x ) and vertical ( y ) magnification over about a 2 to 1 range. All other elements scale with Bp. Two beam profile monitors (BPM) and the adjustable aperture can be used to provide data to calculate [5] the beam phase ellipses at SE1 in a similar manner to the calculation used at diagnostic stations 1 and 3. To date, the stability and intensity of the beam from the

249

cyclotron have not been sufficiently good to allow meaningful measurements. The x and y phase ellipse parameters at SE1 and the desired beam parameters are used [5] to obtain the best solution, in the least squares sense, of the strengths of the four elements of QE1. In the present case, theoretical estimates of the phase-ellipses were used which proved to be quite adequate and within our ability to measure the result. In fact the beam sizes observed at SE1 were nearly as predicted, indicating that the theoretical estimates were very good indeed. The beam from the extraction channel was typically low at probe I by 4 - 5 mm as shown in fig. 19 but on a rising trajectory through QE1 and it could he centered at the object slits of BE1. A n auxiliary vertical steering magnet was installed after the object slits to steer the beam through BE1. The reason for this steering problem is not yet understood. The size of the beam at the image slits of BE1 was typically 2 m m vertically by 3 m m horizontally (fwhm). The beam spot observed on fluorescent quartz at the target location showed time dependent horizontal wings in addition to the central spot. Observations of the time dependence of the beam current on the last cup show an intensity modulation which varies in form with the setting of several cyclotron parameters. There is steering of the beam on a millisecond time scale which modulates the intensity by steering the beam off the correct trajectory. Some of the modulation is correlated with the 60 Hz power line but most of it appears not to be. The form of the modulation indicates that it could account for most of the extraction losses from the cyclotron.

Table 2 127I 10 MeV/u beam transmission Charge From ion source

- 1

Energy 200 keV

Beam current (hA)

(pnA)

1700

1700

Transmission 2.7%

From tandem (buncher off)

7+

70.9 MeV

320

46

From tandem (buncher on)

7+

70.9 MeV

160

23

After stripper in cyclotron

23 +

> 70.9 MeV

At entrance to electrostatic deflector Through electrostatic deflector Through extraction channel

23 +

1.27 GeV

80

3.5

23 +

1.27 GeV

60

2.6

23 +

1.27 GeV

56

2.4

On cup before target

23 +

1.27 GeV

46

2.0

50% 15% (80) a)

3.5 "100%" 74% 92% 83%

a) Actually not measured at this time but is entered here to indicate that in other measurements with normal vacuum, there is no measurable beam loss between turn number 6 and the last turn before extraction.

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CB. Bigham et aL / The Chalk Ricer superconducting cyclotron

10. System transmission The observed transmission through the system for the best cyclotron extraction efficiency is summarized in table 2. The expected stripping yield in the cyclotron (7 + to 23 + ) is 20% and in the Tandem (1- to 7 + ) is 10%. The reduction in average beam current when the bunchers are turned on is typically 50%, i.e., half the current is intercepted by the "phase" slits after BI3. The "buncher ON" current includes some current (dark curren0 between bunches which will not be accelerated in the cyclotron and will therefore appear as injection losses. The BPD spectrum shown at the bottom of fig. 8 indicates that this loss could be 20% and perhaps account for the observed 15% against the expected 20%. Radial probe scans show that within the accuracy of the measurement, there is no beam loss between turn number 6 and the turn before extraction when there is a normal midplane vacuum (0.07 mPa or better). This is indicated by the "100%" in the transmission column of table 2.

adjustable to provide centred beams. Operation of the stripper foil in the dee where rf field levels are substantial was shown to be feasible. The radial beam probe system successfully mapped turn patterns allowing precise adjustment of parameters. The turn patterns were regular, showing no significant anomalies in magnetic or rf fields. The electrostatic deflector operated successfully in a dee with a special water resistor high voltage cable system to cope with transients and rf heating. The unique superconducting extraction channel has performed as expected. Successful beam extraction has demonstrated that the fringe fields of the magnet were very accurately modeled. The computer control system is not yet interfaced to all cyclotron systems, but proved very successful in operation of all the systems for which it could be used. The quality and intensity of the 1.27 GeV 127I beam and reliability of the cyclotron were adequate for the first experiment. Future work will involve completion of some control systems and commissioning of beams over the full mass energy range.

11. First experiment The first experimental run was scheduled for two shifts a day operation which allowing for start up and shut down could provide up to 12 h beam time per day. The beam intensity was in the range 0.5-2 pnA. The five day run was successfully completed with no major equipment failures.

12. Conclusions The Chalk River superconducting cyclotron has been commissioned with first beam thereby demonstrating the successful operation of all subsystems. Some of these subsystems are unique and others required significant adaptation from those used on other cyclotrons. The magnet has proved to be accurately resettable, both in field level and in trimming by the trim rods. The trim rod system is unique to this cyclotron and demonstrates the effectiveness of using saturated iron for magnet field shaping. The coupled resonator dee system has proved to be satisfactory and easy to operate. The midplane cryopump vacuum system works well providing a vacuum in which beam stripping losses are negligible. The injected beam from the Tandem was satisfactory both in emittance matching and in bunch structure and allowed single turn extraction. The phase feedback system was able to cope with occasional large transit time jitter through the Tandem. The injection system including the stripper foil changer/positioner and steering magnets were readily

Acknowledgements It is a pleasure to acknowledge the extensive and excellent technical support provided by dedicated people who worked on this project from Accelerator Physics, Mechanical Services, Nuclear Physics, Plant Design, TASCC Operations branches and the machine, welding and sheet metal shops at CRNL during the development, installation and commissioning phases.

References [1] C.B. Big,ham, J.S. Fraser and H.R. Schneider, Phys. Canada 29 (1973) 29. [2] J.A. Hulbert, C.B. Bigham, E.A. Heighway, J.D. Hepburn, C.R. Hoffman, J.H. Ormrod and H.R. Schneider, IEEE Trans. Nucl. Sci. NS-32 (1985) 2757. [3] C.R. Hoffmann, J.F. Mouris and D.R. Proulx, Proc. 10th Int. Conf. on Cyclotrons and their Applications, East Lansing, Michigan (1984) 222 (IEEE Cat. No. 84CH19963). [4] C.B. Bigham, IEEE Trans. Nuel. Sci. NS-26 (1979) 2142. [5] W.G. Davies, IEEE Trans. Nucl. Sci. NS-32 (1985) 2757. [6] E.A. Heighway, C.B. Bigham and J.E. McGregor, IEEE Trans. Nucl. Sci. NS-30 (1983) 2809. [7] C.B. Big,ham, A. Perujo, E.A. Heighway and J.E. McGregor, Proc. 10th Int. Conf. on Cyclotrons and their Applications, East Lansing, Michigan, (1984) 169 (IEEE Cat. No. 84CH1996-3). [8] C.R Bigham, T.K. Alexander, R.J. Burton, E.A. Heighway, J.E. McGregor and E.P. Stock, IEEE Trans. Nucl. Sci. NS-32 (1985) 2763.

C.B. Bigham et a L / The Chalk River superconducting cyclotron [9] C.R. Hoffmann, R.I. Kilbom, J.F. Mouris, D.R. Proulx and J.F. Weaver, IEEE Trans. Nucl. Sci. NS-32 (1985) 2986. [10] S. Datz, Atomic Data 2 (1971) 273. [11] J.D. Hepburn, J.D. Walsh and E.H. Williams, IEEE Trans. Nucl. Sci. NS-32 (1985) 1880. [12] J.H. Ormrod, C.B. Bigham, E.A. Heighway, J.D. Hepburn, C.R. Hoffmann, J.A. Hulbert and H.R. Schneider, Proc. 10th Int. Conf. on Cyclotrons and their Applications, East

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Lansing, Michigan (1984) 245 (IEEE Cat. No. 84CH19963). [13] C.R. Hoffmann, IEEE Trans. Nucl. Sci. NS-24 (1977) 1470. [14] S.W. Mosko, private communication (1982). [15] E.A. Heighway and C.R. Hoffmann, Proc. 10th Int. Conf. on Cyclotrons and their Applications, East Lansing, Michigan (1984) 141 (IEEE Cat. No. 84CH1996-3).