SUPERCONDUCTING HEAVY ION CYCLOTRON

Transcription

1 Atomic Energy of Canada Limited SUPERCONDUCTING HEAVY ION CYCLOTRON by C.B. BIGHAM, J.S. FRASER and H.R. SCHNEIDER Chalk River Nuclear Laboratories Chalk River, Ontario November 1973 AECL-4654

2 - 1 - SUPERCONDUCTING HEAVY ION CYCLOTRON by C.B. 3igham, J.S. Praser and H.R. Schneider We have been working on a conceptual design of a superconducting heavy ion cyclotron to go with the MP Tandem at Chalk River. Fig. 1 shows the tandem and the cyclotron approximately to scale. The small size of the cyclotron is a consequence of using a magnetic field about three times the kg possible with an iron core. We can now do this economically with an air core superconducting magnet because of the rapid development of superconducting magnet technology in the last few years. There are also obvious savings in building costs. The beam from the negative ion source passes through a buncher cavity and then into the Tandem Van de Graaff. It is stripped, once in the 13 MV terminal, forming multiply charged positive ions, and again on injection, at the inner orbit of the cyclotron. The output energy range as a function of ion mass number A, is shown in Fig. 2. The tuning range of the accelerating structure defines both the low energy limit for heavy ions and the high energy limit for light ions. Maximum energy of intermediate A ions is limited by available focussing. The bending limit of the magnet occurs at higher energies, e.g. 127 MeV/A for fully stripped light ions and progressively less for heavier ions, as the most probable charge state to mass ratio, after stripping, decreases with increasing A. Fig. 3 shows a section through the cyclotron. The superconducting coils provide an average 5T field out to the 65 cm extraction radius. The "normal" trim coils make about 0.1% adjust-

3 - 2 - ments for isochronism. The NbTi superconducting coils are mounted in cryostats immersed in liquid helium at 4.5 K. The overall dimensions are about 3 m diameter by 2.8 m high. The beam from the Tandem enters on the midplane, is stripped at the inner orbit radius, accelerated in about 100 turns and extracted by electrostatic deflectors so that it leaves on the midplane. The iron shovm is in four sectors and provides a four sector flutter focussing field. Fig. 4 is a photograph of a model of the accelerating structure and flutter poles. There are four "hot" sectors and four grounded sectors. The hot sectors are connected alternately to the upper and lower coaxial X/4 resonators, with adjustable shorting discs for tuning. The grounded sectors contain the iron flutter pole pieces. Fig. 5 shows an azimuthal section with the flutter pole geometry above and azimuthal variation in midplane field below. This is for an average 5T field. In the range 3-5 T for which saturation magnetization is ensured, the increase between the poles remains approximately constant at 1.6 T. The flutter factor depends on the fractional field change and therefore increases with decreasing average field. This allows us to go to higher specific energies for light ions than for heavy ions. One edge of the iron is spiralled to obtain sufficient focussing. A current-sheet model of the poles was constructed and the magnetic fields measured. The results were combined with a constant field to simulate the coil fields and used in an equilibrium orbit calculation. Fig. 6 shows the radial variation of the radial and vertical betatron oscillation frequencies for U ions with 4 and 10 MeV/A output energy. The values are well behaved except near the extraction radius at 65 cm. We hope to establish the flutter pole geometry more exactly using current sheet models.

4 - 3 - Fig. 7 shows the radiofrequency model separated into two halves so one can see the alternate sectors attached to top and bottom resonators. There are eight accelerating gaps. The two resonators form a coupled system with two modes - the o-mode with resonators in phase and the 7r-mode with resonators out of phase or in "push-pull". in the ir-mode the ion velocity at extraction is twice that in the O-rnode. Fig. 8 shows the frequency against shorting disc position as measured for the model. in full scale the frequencies are 1/10th of those shown here so that the frequency range required is MHz. The 3-12 Mev/A range is covered in the O-mode and the Mev/A range in the ir-mode. Going back to the superconducting magnet. Table I lists the parameters for the main coils. The coils are constructed of 76 pancake windings each with 130 turns of 1000 A conductor. The superconducting KlbTi is in the form of fine filaments embedded in copper and twisted for stabilization against eddy currents. Sufficient copper conductor and cooling surface is allowed for complete cryostatic stabilization. This means that the coil could recover from any possible thermal transient. A stainless steel ribbon is wound in with the conductor to keep the hoop stress below the yield point. The axial force is substantial so that a strong support is required between the coils. The field in the coil is well below the critical value for NbTi. The current density is in the range that has been used in some large magnets. This is not really a very large magnet - the Big European Bubble chamber magnet (BEBC) at CERN has a stored energy of 800 MJ.

5 - 4 - Fig. 9 shows the midplane injection geometry. Most of the desired ions can be injected between the two orbits shown. This requires the foil stripper to be adjustable radially by about 10 cm and the beam to be steerable by about 5 outside the cryostat. The orbits will be curved in passing through the coils and will have varying radii in passing through the hills and valleys. There will also be some out of phase accelerations by the rf gaps crossed on the way in. These effects will have to be taken into account but will not perturb the orbit very much. A double drift harmonic buncher at the ion source end of the Tandem will bunch the dc beam to a phase width of 4 with 50% efficiency. Most of the beam will be lost in the unused charge states in the two strippings. There may also be significant losses because of the scattering in the strippers. With the cyclotron tuned for the beam quality desired, AE/E ^ 10, the contribution of the cyclotron to beam emittance will be small. The transmission of the cyclotron will then depend primarily on the fraction of the beam scattered by the stripping foil that can be focussed to a radial width of ~ 1 mm at the extraction orbit. Single turn extraction is then possible with the > 3 mm orbit separation available. Two electrostatic deflectors in adjacent grounded sectors deflect the beam into an escaping orbit. This orbit depends on the fringing field details which we have not yet established. It looks as if a satisfactory beam channel can be arranged without active magnetic elements that would perturb the final orbits in the cyclotron.

6 - 5 - in summary, we have a conceptual design for a heavy ion cyclotron that looks promising. Superconducting magnet technology is now at the stage where little development is required for the magnet. The rf system is also within the "state of the art" but may require some development to attain the beam quality desired. The orbit dynamics of isochronous cyclotrons is well understood and several powerful computer programs are available for their study. The midplane i»ijaction does not look difficult and has been used successfully at Orsay and Dubna. A reliable foil changing mechanism will have to be developed to cope with the short life expected for the stripper foils. Extraction of the beam will require some careful optimization of the fringing fields of the coils and the flutter pole pieces. Extraction is probably the most difficult problem remaining. We have had a lot of help in putting this conceptual design together. At chalk River H.R. Andrews, R.K. Elliott, C.R. Hoffmann, J.A. Hulbert, A.B. MacDonald, P.R. Tunnicliffe and C.H. Westcott have contributed. We have also had helpful discussions with M.M. Gordon and H.G. Blosser at Michigan State University, J.R. Purcell at Argonne National Laboratory and R. Pollock at Indiana University.

7 - 6 - TABLE I SUPERCONDUCTING MAGNET - MAIN COILS Mechanical inside diameter 1.84 metres Cross section (square) 0.46 metres Spacing metres Turns (both) 9880 Weight (both) tonnes Average Hoop Stress 7250 psi Axial Force 3400 tonnes Electrical Maximum Midplane Field Maximum Field at Conductor Conductor current Overall Current Density Charging Time at 10 Volts (0-5T) Stored Energy 5 Tssla 6.2 Tesla 1000 A A/cm 3.5 hours 64 MJ

8 - 7 - T 10 METRES 1 SUPERCONDUCTING CYCLOTRON HEAVY IONS It) MeV/A LIGHT IONS 50 MeV/A GAS OR FOIL STRIPPER MP TANDEM 13.5 MV BUNCHER NEGATIVE ION SOURCE FIGURE 1: Superconducting cyclotron with MP Tandem injector. The accelerator sizes are to scale but not their separation.

9 - 8 - MeV A RF FREQUENCY LIMIT INJECTION LIMIT I" \ N ^INSUFFICIENT»^FOCUSING s \ \ \ ( \ \ MAGNETIC ^_ FIELD / LIMIT 10 \ 3 - TANDEM OUTPUT ^N RF FREQUENCY LIMIT N N MASS NUMBER (A) FIGURE 2; Operating limits of the superconducting cyclotron.

10 2. 8m SUPERCONDUCTING CYCLOTRON Xfff / s / X s s' S S / 7 NORMAL TRIM COILS h FIGURE 3; Vertical elevation of flutter poles, accelerating structure, coils and cryostats.

11 FIGURE 4«Accelerating structure model.

12 90 SECTOR FLUTTER POLES CROSS SECTION AT 51cm RADIUS 6 5 FLUTTER FIELD 1.6 TESLA FIGURE 5: FLUTTER POLES AND THE AZIMUTHAL VARIATION OF THE MIDPLANE FIELD

13 h- V 4 MeV/A 10 MeV/A UJ CJ or u. CD.3 sh o 1X1 a 2T. M - ' L 4 MeV/A. 10 MeV/A 0 10 Z FIGURE 6: ORBIT RADIUS cm N O R M A L I Z E D F O C U S S I N G F R E Q U E N C I E S V Z, V R F O R U, AS A F U N C T I O N OF O R B I T R A D I U S

14 FIGURE 7: Model of flutter poles and acceleration structure; scale 1/10 I

15 MeV/A MeV/A MeV/A TT-MODE MeV/A 100 I L i i i i. I i L 5 10 SHORT POSITION (cm) FIGURE 8; Tuning range for 1/10 scale four-sector rf structure model.

16 STRIPPER FOIL Q R o =65 cms FIGURE 9; Midplane injection geometry ions with the desired range of charge state ratios, Q o /Q.-. can be injected between the two orb"its illustrated. STEERING MAGNET

17 Additional copies of this document may be obtained from Scientific Document Distribution Office Atomic Energy of Canada Limited Chalk River, Ontario, Canada KOJ 1J0 Price - 50<J per copy