Bock, R

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Particle Accelerators, 1992, Vols. 37-38, pp. 13-22Reprints available directly from the publisherPhotocopying permitted by license only© 1992 Gordon and Breach Science Publishers, S.A.Printed in the United KingdomTHE GERMAN HEAVY ION INERTIALFUSION ACTIVITIESR. BOCKGSI, Darmstadt, Germany.(Received 15 January 1991)Problems of heavy-ion inertial fusion are investigated in the framework of a basic research program atGerman research laboratories and universities. The SIS/ESR facility at GSI became operational recentlyand will provide excellent conditions for studies on beam dynamics and beam-target interaction. Incollaboration with other European laboratories, advanced accelerator scenarios are investigated whichinclude concepts of non-Liouvillean beam handling techniques and indirect drive.1 INTRODUCTIONThis is a brief overview of the German research activities in Heavy Ion Inertial Fusionand their objectives, recent achievements, and near-term goals. Some more-detailedinformation is given in these proceedings.After a decade of research on issues of accelerator and target physics, two aspectswill strongly influence the direction of our future research: (1) the commissioning ofthe new accelerator facility SISjESR at GSI, and (2) the consideration of non-Liouvillean techniques for future concepts of driver or test facilities.1) The new two-ring facility at GSI, a heavy-ion synchrotron and a storage andcooler ri-ng, which recently became fully operational, will provide excellent and uniqueconditions for systematic accelerator and target investigations. Key issues of thedynamics of high-intensity beams at high phase-space density, in particular the limitsfor beam instabilities, will be addressed in an experimental research program at thisfacility, as well as problems of beam-target interaction and the physics of denseplasmas. In addition, the future high-current feature of SISjESR for very heavy ionswill provide an excellent basis for advanced technological developments.2) The inclusion of non-Liouvillean techniques in the concept of a heavy ion driver,as proposed by Rubbia 1 some years ago, will considerably improve the performanceof such a facility with respect to final focussing and final bunching. Even in case ofindirect drive, non-Liouvillean techniques still seem necessary to provide sufficientbeam quality. Stimulated by these observations, other non-Liouvillean concepts havebeen considered and will be an important aspect of our forthcoming activities.Research presented in this paper was carried out by a number of groups at someGerman research institutes (GSI, Max-Planck-Institute for Quantum Optics Garch-ing, Fraunhofer Institute Aachen) and universities (Darmstadt, Frankfurt, Giessen,1314 R. BOCKMunchen and Stuttgart). It is funded by the German Federal Ministry for Researchand Technology. In addition, collaborations exist for some years already in someareas with a French group of Orsay/Orleans and a group from ITEP in Moscow.It is obvious that the future program needs a stronger European effort. A newItalian-German collaboration will be active in research on target and driver issues,predominantly those emerging from proposed new driver concepts. After somemeetings in Varenna 1989 and 1990 arranged by the Italian Physical Society andwith an international participation, joint study groups have been established and aregoing to discuss new scenarios and the new framework of activities.2 OBJECTIVES AND SOME RECENT RESULTSThe main objectives of the program are the generation and investigation of highintensity heavy ion beams with high phase-space density, and the investigation ofthe interaction of these beams with matter. It is a basic physics- and technology-oriented research program 2 and can be subdivided into the following topics:• Experiments and theory on dynamics of space-charge dominated beams• Experiments on beam plasma interaction• Generation of dense plasmas, and radiative and hydrodynamic response• Development of special accelerator structures and components• Driver-relevant atomic-physics issues• Development of equipment and diagnostics• Systems studies and future accelerator conceptsMuch work done since the last symposium in 1988 has been devoted to preparationsfor the research at SIS/ESR. In this section we concentrate on some selected results,mainly achieved in collaborations with university groups.Stopping of Ions in Plasma and Plasma Lens Focusing (D. H. H. Hoffmann et al. 3 ).The experiments on the interaction of heavy ions with plasma were extended tohigher plasma density and to higher beam energy with an improved Z-pinch device.Previous results of roughly 3 times enhanced stopping power in plasma wereconfirmed. In addition, measurements on the charge state distribution of the beamions showed for the first time the shift to higher charge states in the plasma. Theseresults are in excellent agreement with a novel theory of Peter and Meyer-ter-Vehn4 ,which predicts that the high charge state remains until the end of the range, wherethe ion gets suddenly neutralized. Furthermore it shows the influence of dielectricrecombination on stopping. With the same device, focusing of heavy ions by theZ-pinch plasma has been demonstrated for the first time.First Heavy-Ion-Generated Plasnlas (Jacoby et al. 5 ). The high-current RFQ,MAXILAC, was used for the development of techniques and for preliminary targetGERMAN HIF ACTIVITIES 15experiments. For the penetration of a high-intensity Kr beam in a Xe gas target, thegas was heated up to the eV region and the hydrodynamic response was measured.Good agreement with simulations6 was observed.Electron Capture Cross Section (A. Muller et al. 7 ). An experiment has beenassembled to measure atomic cross sections for the interaction of a heavy ion beamwith an electron beam at very low relative velocity using merging-beam techniques.Measured cross sections for radiative capture are found to be much higher than thosepredicted by theory. With this new device, an "electron target", atomic processesrelevant to heavy ion inertial fusion can be measured.Ion-ion Charge Exchange Cross Sections (Salzborn et al. 8 ). Intra-beam scatteringis a critical issue for beam loss. Figure 1 shows a compilation of measured crosssections, including the recently measured case of Cs + + Cs +, a closed-shell ion. Theeffect of closed shells is not so dramatic as was expected; the loss of beam intensityin storage rings of a HIBALL scenario still remains in the region of 10/0.Accelerator Development (H. Klein et al. 9 ). Systematic work on RFQ structureshas been continued at Frankfurt University. A new prototype of a 27-MHz high-current injector for UNILAC has been designed and will be delivered in June 19913.94 . 1Q-17cm 2 • EO.621.12 ' 10- 11cm 2 • EI.OIS 't. I( .......E~c:C) '11.:·6 '10-'0(UV)~~C)(-u10-15 r--_ --------------~~_-___.'1005020E(In (I~eV)10'1 0-'1" '"---a.--I----L---L-..l -L__---L-_--L-_..l---l---L--l-~5FIGURE 1 Results of a crossed-beam experiment: charge-change cross sections for Bi + + Bi +,Cs + + Cs + and Xe + + Xe + as a function of velocity. The velocity range corresponds to the relativevelocity of ions in a beam bunch in storage rings. The loss rate by charge-changing intrabeam scatteringas determined from these cross sections is in the order of 1% for a storage time of 4 ms. The case Cs + + Cs +(closed shells) is by a factor of 2-3 better than Bi + + Bi +. The measurements were made by Salzbornet al. 8 at Giessen University.16 R. BOCKSpiral-RFQ:T = 2.5 -20 keYluf = 27.1 MHzqlA ~ 2/238U Q = 180 kVamin= 6.5 mmm :s: 1.35L cav. = 4mTlim = 40 rnAFIGURE 2 Spiral RFQ structure for the 27-MHz high-current injector for UNILAC (developed atFrankfurt University 9).(Figure 2). Wark on some crucial issues is making progress-in particular studies offunneling, of neutralization of ion beams, and of diagnostics for measuring beamprofiles.Theoretical Work is going on at MPQ Garching on beam target interaction anda number of topics concerning indirect-drive targets; in Frankfurt on target hydro-dynamics and on radiation efects; and in Darmstadt on stability limits of ICF targets,Langmuir waves, implosion and burn of indirect drive targets and on targets for SISexperiments. Recent progress concerns in particular the stopping of ions in plasma,hydrodynamic response of beam-heated plasmas, and the conversion of beam energyin radiation.3 THE SISjESR FACILITY AND THE FUTURE EXPERIMENTALPROGRAMThe synchrotron SIS has been in operation since autumn 1989. The storage ring ESRhad its first cooled beam in May 1990, as shown in Figure 3. The high-currentinjector-an integral part of the whole facility-is delayed by two years. As mentionedbefore, it will be a 27-MHz RFQ structure developed at Frankfurt University. A setof performance parameters for the whole facility and its expected realization is givenin Table 1.The first experiments will be devoted to heating of material of various densitiesup to solid-state density with heavy ion beams. Problems of interest are• the energy deposition in plasma• the hydrodynamic response of matter at high energy densities• thermal radiation processes• equation-of-state physicsGERMAN HIF ACTIVITIES 17~'" Q'a te '27 .Ju1. ·90 Time 09: 27: 32V Rel.tev- Harker -92.19 dBm100 %-72 00 dam 14 55952 MHzRes.Bw 100.0 Hz [3dB]CF.Stp 1.500 kHzVid .B\~ 30 HzRF .Att 10 dBUnit [dBml90 : I-----t---+---+---f---+---+---+----~-___jr__-~80 : J.------+--.----+---+--+----+--t-------t'---t----.,70 X~--+----J.--.I----.....-_H--_+_-__+--+__-__t_-___16..r:60 :50 X40 :30 ~20 %10 %~~ .1 '....U~J ~ .It L :1L~:;r,~ 'I'" 1"J'·TI~I".1 r I ·ll~' W,...I It:r:: 1· 10~o%Start14.5520 MHzSp.an15.000 kHzcenter14.5595 MHz Sween Stop15.0 s 14.5670 MHzFIGURE 3 First results of electron cooling at SISjESR 14. The measurement shows the Schottky noisespectrum of the cooled and uncooled beam. The momentum spread is improved by cooling by a factorof 10 to 100.The focusing of the heavy ion beam down to a diameter of about 0.2 mm shouldresult in a specific energy density up to about 10 TW/g, resulting in temperatures ofabout 20 eV at solid-state density. In diluted matter it might even be higher. Shockwaves of up to 4 times solid density are expected. Concerning the thermal processes,radiation cooling and the conversion efficiency into soft X-rays is of great interest.The time schedule for the experimental investigation of these issues very muchdepends on the achieved beam intensity, e.g., on the schedule for full commissioningof the facility, including the high current injector.TABLE IBeam performance with SISjESR for accelerator and target experiments within the next 3 years(Hofmann 12)Ion Ne 3 + Xe44 + Xe44 Upgradedsingle turn 25 turns coolingno cooling cooling + high-currentN 1.6 x 109 3.2 X 1010 1.2 X 1011 1.2 X 1011Energy (MeVju) 300 300 1100 1100Pulse Length (ns) 100 35 or > 70 10Total Energy (J) 1.5 30 2500 2500Specific Power (TWjg) 0.003 0.12 or > 8 60Temperature (eV) 1.4 20 5018 R. BOCKThe first experiments on the dynamics of space-charge-dominated beams willconcentrate on issues like:• space-charge effects in multi-turn injection• longitudinal micro-wave instability• fast bunching and resonance crossingAs a result of recent experiments 14 the Keil-Schnelllimit for microwave instabilitieshas been overcome by more than a factor of 6, good news for the lifetime of storedbeams. The results of these experiments, some of them have already started, willprovide the basis for further considerations on driver concepts.4 PERSPECTIVESIn our present understanding, the primary uncertainties of heavy ion inertial fusionare:for the pellet• the radial symmetry of implosion drive• hydrodynamic instabilitiesfor the accelerator• longitudinal microwave instabilities• final focusing• final bunchingTheoretical investigations about the necessary symmetry of implosion drive indicatethat, with a small number of heavy ion beams, sufficient symmetry might not beachieved. Taking this for granted, indirect drive would be a necessary ingredient ofa pellet implosion concept. In this case two more target issues are important: theconversion efficiency and the radiation confinement.Various types of indirect-drive pellets have been investigated 10; an example isshown in Figure 4. In order to achieve high conversion efficiency the requireddeposition power needs to be about 1016 Wig, as indicated in the inset of Figure 4.Indirect drive would impose much more severe conditions upon final focusing andfinal bunching.Table 2 shows a comparison of parameters for direct and indirect drive 11 . Thenecessary increase of the specific deposition power is more than an order ofmagnitude. At this point non-Liouvillean techniques are getting to be an essentialingredient of a driver concept.Since Rubbia suggested non-Liouvillean injection in 1988, a number of differentinjection schemes and accelerator scenarios (both driver and test facilities) have beenconsidered. The compilation of Table 3 shows several basic possibilities 13. Someexamples should be mentioned, including non-Liouvillean bunch compression which"l(an.(",)so....-,/ ,/I ,/ ,lase r ,Ion BeamGERMAN HIF ACTIVITIESHohlraum - Radiation\19L.J'--_,.L~ 1:-:-,--L-~,~'i';It,----;I~ 16I (W/cm')FIGURE 4 Configuration of an indirect-drive target for heavy-ion inertial fusion. The kinetic energy ofthe beam is converted into soft X-rays which implode the inner shell filled with DT fuel. The conversionefficiency depends strongly on the specific deposition power, as plotted in the insert. For comparison alsothe conversion efficiency of a short wave-length laser beam (). = 0.35 ,urn) is given. The radiationconfinement is achieved by the high-Z casing (Meyer-ter-Vehn 10).TABLE IIBeam parameters for direct vs. indirect drive (Hofmann 12).Beam powerAbsorber massSpot diameterFinal pulse durationSpecific powerDirect drive (HIBALL)250 TW400 mg8 mm20 ns6 * 10 14 WigIndirect drive500TW45 mg2.7 mm10 ns1016 WigTABLE IIIVarious possibilities for non-Liouvillean beam handling techniques. 13Candidate reactions for non-Liouvillian beam manipulation"In-beam chemistry" Examples Problems/remarks/statusMolecular dissociationPhotoionizationStimulated recombinationCluster or macromoleculeformationMultiple-photon ionizationof negative ionsHI + + hv -+ HO + 1+I; + hv -+ 21+ + e-Bi + + ~hv -+ Bi + + + e-Low dispersionUnknown branching (1°?)Space-charge problemwith Bi+ +To be studiedideal candidate if it worksIdeally good dispersion;best available data;best known overall case20 R. BOCKtorgetinduction bunchersectionsfrom injector 1 inac photonbeemFELFIGURE 5 Advanced driver scenario, including non-Liouvillean bunch compression using photo-ionization with a free electron laser (I. Hofmann 12).was recently suggested by I. Hofmann 12 for a driver accelerator) in which short piecesof a coasting beam are cut off by short laser pulses. The short heavy-ion bunchesare deposited on top of each other in a compression ring without increasing themomentum width (Figure 5). Instead of changing the charge state at injection, themass can be changed by dissociating a molecular ion at injection 13 (Figure 6). Somepreliminary results look quite encouraging. These possibilities have to be investigatedin much more detail.5 CONCLUSIONSThe rf linac with storage rings provides a challenging driver option for inertialconfinement fusion. Also, for the enhanced requirements of indirect drive, theLinacoBoosterGERMAN HIF ACTIVITIESLaserTarget2 GeV/'nuc,l.1000 TmSynchrotron(HIt21FIGURE 6 Suggested scenario for a test facility using non-Liouvillean stacking by photodissociationof a molecular beam (Arnold and Miiller 13).heavy-ion accelerator continues to be the best driver candidate if non-Liouvilleanstacking is included. The goals for a near-term research program are three fold:1) There is a serious lack of experimental data on many key issues, in particularin beam dynamics and in the physics of dense plasmas. The new 2-ring acceleratorSISjESR will be a unique facility for such investigations. Related theoretical workneeds to be continued as well. All possibilities for the investigation of non-Liouvilleanbeam manipulation and research on all related techniques, e.g., FEL development,have to be pursued with emphasis (time schedule ~ 5 years).2) A new conceptual design study, replacing HIBALL, has to be elaborated,including the new concepts of indirect drive and non-Liouvillean techniques asessentially new ingredients (time schedule ~ 1 year).3) A strategy for building an HIF Demonstration Accelerator has to be developed.It should enable• significant beam-target experiments• a feasibility prove of accelerator technology• non-Liouvillean stackingEither a dedicated test facility (i.e. with low repetition rate) or the first stage of alarger facility might be considered; it should be based on the new technology (timeschedule ~ 3-4 years).22 R. BOCKObviously the scope of such activities will exceed the framework of nationalprograms. Stimulated by Carlo Rubbia and Renato Ricci, a series of meetings havebeen arranged in Varenna under the sponsorship of the Italian Physical Society andof ENEA, in order to discuss goals for an Italian-German collaboration. Meanwhileworking groups have been established to analyse existing experience and to co-ordinate future activities. Some more collaborations exist with ITEP in Moscow andwith OrsayjOrleans. In the long run it needs to be an integrated European effort.Recently Fusion Committees in the US have recommended relaxing classificationin this field in the United States, which, if realized, would greatly improve interna-tional collaboration.ACKNOWLEDGMENTSDiscussions with R. Arnold, D. H. H. Hoffmann, I. Hofmann, R. W. Miiller, and J.Meyer-ter-Vehn are gratefully acknowledged.REFERENCES1. C. Rubbia, Nucl. Instrum. Meth. A278 253 (1989).2. Annual Reports GSI 88-17, 89-21 and 90-15. R. Bock, Laser and Particle Beams 8 563 (1990).3. D. H. H. Hoffmann et al. Laser and Particle Beams 8 583 (1990).4. Th. Peter and 1. Meyer-ter-Vehn, Phys. Rev. A43 1998 (1991) and Phys. Rev. A43 2015 (1991).5. 1. Jacoby et al., Nucl. Instrum. Meth. A278 82 (1989).6. V. Schneider and J. Maruhn, Laser and Particle Beams 7 807 (1989).7. A. Muller et al., private communication.8. E. Salzborn, F. Melchert, I. Hofmann, R. W. Muller and V. P. Shevelko, Nucl. Instrum Meth. A278(1989) 65 and private communiation.9. H. Klein et al., Annual Report GSI-90-15, (1990) 21-27.10. 1. Meyer-ter-Vehn, Nucl. Instrum Meth. A278 25 (1989).11. I. Hofmann, private communication.12. I. Hofmann, Laser and Particle Beam 8 527 (1990).13. R. Arnold and R. W. Muller, private communication.14. B. Franzke, Annual Report GSI-91-27 and private communication.

The German heavy ion inertial fusion activities

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