Eylon, S

Fessenden, T J

Garvey, T

Henestroza, E

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Particle Accelerators, 1992, Vols. 37-38, pp. 241-246Reprints available directly from the publisherPhotocopying permitted by license only© 1992 Gordon and Breach Science Publishers, S.A.Printed in the United KingdomBEAM ACCELERATION EXPERIMENTS ON AHEAVY ION LINEAR INDUCTION ACCELERATORT. GARVEY, S. EYLON, T. J. FESSENDEN and E. HENESTROZALawrence Berkeley Laboratory, University of California, Berkeley, CA 94720(Received 3 December 1990)The evolution of the noramlized rms transverse emittance of heavy ion beams in induction linacs dependson many factors, including envelope matching, details of the acceleration waveforms, and beam misalign-ments within the focusing system. Present acceleration experiments on MBE-4 confirm the difficulty offully resolving the explicit dependence of the emittance on these and other parameters. In this paper wediscuss the most recent acceleration experiments on MBE-4. These experiments utilize a beam of smallercurrent (5 rnA) and smaller emittance (0.03 mm-mrad normalized K-V) than was used in previous MBE-4studies. As before, we find that acceleration is accompanied by more transverse emittance growth thanpredicted by our theoretical models.1 INTRODUCTIONThe induction linac approach to inertial confinement fusion requires a currentamplification of the accelerated beam while minimizing growth of the normalizedrms transverse emittance. Current amplification is achieved by a process of differentialacceleration between the head and tail of the non-relativistic ion bunch, leading tolongitudinal compression of the bunch. The Multiple Beam Experiment (MBE-4)was built as a proof-of-principle device to demonstrate longitudinal compression andcontrol of such beams!. Previous experiments on MBE-4 have resulted in a demon-stration of current amplification during acceleration; however, this was accompaniedby a doubling of the transverse emittance2 . This was in contrast to the drifting-beamresults, which showed constant emittance. Several possible causes of the emittancegrowth have been advanced, although, taken together, they account for no more than30% of the measured growth. Prominent among the causes of emittance growth isthe effect of longitudinal compression, which leads to a change in the self-fieldelectrostatic energy of the beam, resulting in transverse heating3 . In this paper wediscuss the most recent studies of the transverse emittance growth for one ofthe MBE-4 beams.2 MBE-4 Source CharacteristicsReference 1 describes the MBE-4 apparatus, so we limit ourselves here to a discussionof an important change in the source geometry between the experiments describedin this paper and those reported previously. Early experiments on MBE-4 were241242 T. GARVEY et al.performed with a 10-mA beam extracted from a thermionic source. It was apparent,however, that the diode optics contained aberrations, which resulted in overfocusingof the outermost rays of the beam. This resulted in a hollow current densitydistribution and filamentation of the phase space profile. In order to remove theoutermost rays of the beam, a current-limiting aperture stop was installed in theexperiment just downstream of the extraction region. This results in a more uniformcurrent distribution and greatly improved beam distribution in phase space. Thetransmitted current, however, is reduced from 10 rnA to nominally 5 rnA. Thisreduction in current is accompanied by a normalized emittance reduction in bothplanes from 0.08 mm-mrad to 0.03 mm-mmrad, resulting in an overall increase inbeam brightness and tune depression ((Jo/(J == 10, as opposed to 5 previously). Themaximum beam envelope radius is also reduced from 14 mm to 10 mm and so thefraction of the quadrupole aperture occupied by the beam is halved (27 mm boreradius). Consequently the beam is less likely to experience the nonlinear field regionsof the quadrupoles. Before proceeding with acceleration we investigated the emittanceof the drifting 5 rnA beam. This work is described in reference 4; to summarize, wefind that if the beam is properly matched to the MBE-4 lattice and if beam centroiddisplacements are kept small (< 1.5 mm), then the measured emittance along the linacis constant to within ±10%. However, if the beam centroid has a large coherentbetatron oscillation amplitude, then the emittance varies greatly (± 25 %). Theseemittance fluctuations in displaced beams are especially pronounced for beams withlarge tune depression4 .3 Longitudinal DynamicsDue to the lower .beam current now employed in MBE-4, the acceleration schedule(i.e. the timings and amplitudes) of accelerating waveforms used previously is nolonger appropriate to compensate for the beam space-charge forces. To determine anew schedule we employed our existing longitudinal dynamics code, SLID, and anupgraded version, SLIDEs. The longitudinal space-charge field is given by Ez ==gA'/41t80, where A' is the gradient of the line charge density and g is a geometry-dependent shielding factor which also has to be determined for the 5 rnA beam. Wedetermined the value of g appropriate to the 5 rnA beam empirically by runningSLIDE calculations for a drift beam using various values of g and comparingthe output with measured beam parameters6 . Figure 1 shows the measured temporalvariation of current and energy at the input to the linac. These were used as inputdata to SLIDE. Figure 2 shows both the measured and calculated current and energydistributions after allowing the beam to drift to the end of the machine. Althoughthe simulation results are insensitive to variations in g between 2.5 and 4.0, we foundthe best agreement between the calculation and measurements is for g == 2.8In determining a new acceleration schedule we have not attempted to produce newpulser waveforms; rather, we have used the existing waveforms to provide currentamplification while maintaining a reasonably flat, sharp-edged current pulse profilethroughout the machine. In this respect we are using SLIDE as a simulation code rather20019016518017016015087.5SLIDEg=2.875.5 6 6.5time (Jls).- energy54.5Initial distribution of beam current and energy at the entrance to the linac.oBEAM ACCELERATION EXPERIMENTS 2437 19061855m180 :;,<- (1)4 ~oS co'<...-;:c (1)G) 3... 175 S...:;,0217004FIGURE 18765<'oS 4...c~...::s 30235343 130 32 33time (Jls)FIGURE 2 Comparison of the measured and computed current and energy of the beam after driftingto the end of the linac. Note the erosion of the beam head and tail due to space-charge effects.244 T. GARVEY et alethan as a design tool. SLIDE is more appropriate than SLID for this work as it allowsparticle overtaking to occur. Such overtaking is inevitable due to acceleration errorsresulting from real waveforms which depart from those ideally required, and thismay represent another contribution to emittance growth in MBE-4 experiments.4 Transverse EmittanceThe intrinsic normalised emittance Gn of a thermionic ion source of temperature Tand radius b is given by, Gn = 0.65b(T/A)1/2, where b is in centimeters, T is inelectron-volts, A is the ion atomic mass and Gn is in mm-;mrad. For the MBE-4 sourcethis gives an intrinsic emittance of 0.03 mm-mrad. Although in practice the achievedemittance is larger (by about a factor of 2) than the intrinsic value, the measuredemittance of that part of the MBE-4 beam which passes the current-limiting apertureand the beam envelope matching section is close to the intrinsic source value.In order to prevent unwanted emittance growth, it is necessary to maintain thebeam on-axis. To achieve this we have installed an array of steering elements at theinput to the linac to control injection errors. In addition, an extensive survey of thealignment of the focusing elements shows that they are aligned to within ±0.'13 mm.Smith and Hahn 7 give the maximum betatron oscillation ,amplitude Am for a beamdrifting through N lattice periods with randomly misaligned elements of peakdisplacement 1\ to beAm :=: 1\N1/ 2 F(ao, 1])where F(ao, '1) is a function of the zero-current tune and the lattice occupancy. Forthe 30 lattice periods of MBE-4 we expect a maximum displacement of 2 mm, whichis consistent with our observation of Am = 1.5 mm for the drifting MBE-4 beam.Recently we have measured the emittance along M-BE-4 of a 3.8 rnA beamaccelerated with our new schedule. The reduced current is due to a source problemand results in mismatch errors at injection (MBE-4 has. a ,matching section betweenthe source and the main linac that is currently optimized for 5 rnA beams). Thevariation of the- ,measured current at the diagnostic stations and the correspond-ing computed energies (using SLID) are shown in Table 1. The current amplification upto lattice period 20 is 4.5 which is intermediate to the values used in the early MBE-4TABLE IVariation of the Beam Current and Energy at theDiagnostic Stations.Lattice period Current (rnA) Energy (keV)0 3.8 1855 4.4 20810 6.3 27415 9.6 35220 16.9 444BEAM ACCELERATION EXPERIMENTS 245experiments where emittance growth was observed. The emittance variation alongthe linac is shown in Figure 3, where the measured values quoted correspond to 800/0of the full current. This percentage is chosen to exclude electrical noise signalsoriginating from the acceleration pulsers (see Reference 2 for a discussion of thedata-aquisition and reduction procedures). The full curve shows the emittance of thedrifted 3.8 rnA beam and is seen to be constant up to lattice period 20. The acceleratedbeam, however, begins to show some growth at lattice period 15, and by the timethe beam has reached lattice period 20 the normalized emittance has increasedby a factor of 2.5. Beyond lattice period 20 no data is shown, as it is found that eventhe drift-beam emittance begins to increase (in contrast to our earlier 5 rnA results),presumably due to the effects of the injection mismatch.5 DISCUSSIONIn order to resolve the contributions of mismatch and compression towards emittancegrowth, further measurem.ents are required with more-careful matching of the beamfrom the injector to the main linac. The emittance growth observed during accelera-tion in the experiments discussed here is greater than that seen in previous MBE-4studies. However one should note that these beams are extremely "cold," withinitial emittances as much as 3 to 7 times smaller than in earlier studies2 ,6. This isconsistent with the results of our drift beam studies which have shown us thatlow-emittance (high-tune-depression) beams are inherently more susceptible togrowth 3 . Indeed, we have recently obtained some tentative evidence that the 3.8 rnAbeam can be accelerated, using the same acceleration schedule (and hence currentamplification), with only '" 200/0 emittance growth when the emittance is abruptlydoubled at lattice period 5 by external means. Further experiments are required toconfirm this finding.20Drift dataAcceleration data-e----,e;--..-.. 0.06"'0~....e 0.05Ie! 0.04QJ(J=~ 0.03.w.w-8~ 0.02"'0QJ-~ 0.01c;e....Q 0.00:z 0 10Lattice PeriodFIGURE 3 Measured emittance for drifting and accelerating beams at the various diagnostic stations.The observed growth in the emittance of the accelerated beam is due to mismatch errors at injection.246 T. GARVEY et al.6 CONCLUSIONSWe have improved the quality of the beam extracted from our thermionic sourceand we have succeeded in controlling the beam centroid motion as it travels throughthe linac. Despite this we continue to observe a growth in the transverse emittanceof accelerated beams in MBE-4. It is not possible to fully resolve the contributionof compression to emittance growth until we have.eliminated the detrimental effects ofinjection mismatch. Definite conclusions about emittance growth must await theoutcome of further experiments to support the preliminary data presented above.Forthcoming experiments on MBE-4 will continue to address these issues.REFERENCES1. R. T. Avery et al., IEEE Trans. Nuc/. Sci., NS-32, 3187-3189 (1985).2. H. Meuth et al., Nuc/. Instrum. Meth, A278, 153-162 (1988).3. K. Hahn, Lawrence Berekely Laboratory report LBL-26167 (1988).4. S. Eylon et al., these Proceedings.5. E. Henestroza, Lawrence Berkeley Laboratory report LBL-27230 (1989).6. T. 1. Fessenden et al., AlP Conference Proceedings No. 152, 145-155 (1986).7. L. Smith and K. Hahn, Nuc/. Instrum. Meth, A278, 191-193 (1988).ACKNOWLEDGEMENTSWe are indebted to L. Smith, A. Faltens, W.M. Fawley and K. Hahn for valuable comments. Thanks arealso due to C. Lionberger, M. Gross (software support), W. Chupp (ion source fabrication), and W. Tiffany(technical support) and to R. Johnson for his assistance in making measurements.Work supported by the Director, Office of Basic Energy Research, Office of Basic Energy Sciences,Advanced Energy Projects Division, U.S. Department of Energy under Contract No. DE-AC03-76SF00098.

Beam acceleration experiments on a heavy ion linear induction accelerator

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