Status of Heavy Ion Fusion Research

1. Status of Heavy Ion Fusion Research Grant Logan Director Heavy Ion Fusion Virtual National Laboratory (LBNL, LLNL and PPPL HIF groups) Presented at Fusion Power Associates Symposium Washington, DC November 19-21, 2003

2. Outline Motivation for heavy ion fusion research Critical technical issues Status of current research Scientific goals for near term future research

3. Heavy ion fusion research motivation • World wide experience with high energy accelerators support inertial fusion energy driver prospects for efficiency, pulse-rate, and durability. • Focusing magnets may survive target radiation and debris for many years of operation. • Expected very good ion-target coupling efficiency (classical dE/dx) • Compatibility with indirect drive and thick liquid protected chambers. These attributes are good for both fusion and high energy density physics applications • Present heavy ion beam research emphasizes primary scientific issues: intense ion beam transport physics, beam-wall interactions, focusing, and beam-target plasma interactions. • Intense ion beam-wall interactions are a common area of accelerator science important to heavy-ion fusion and high energy and nuclear physics.

4. Heavy ions can apply to a variety of targets, chambers, and focusing schemes, but a key motivation is the desirability of using thick liquid-protected fusion chambers with much reduced materials development Approaches emphasized in the U.S. program (primary emphasis) (secondary)

5. Heavy ion beam requirements follow from the designs of accelerators, chambers and targets that work together A self-consistent HIF power plant study was recently published in Fusion Science and Technology, 44, p266-273 (Sept. 2003) Beam brightness Bnt > 4x106 A.s/(m2rad2) at target High gain targets that can be produced at low cost and injected Beams at high current and sufficient brightness to focus Long lasting, thick-liquid protected chambers for 300 MJ fusion pulses @ 5 Hz

6. The science of heavy-ion fusion is unique To drive inertial fusion energy or high energy density physics targets, heavy ion beams must be intense enough that beam space-charge forces (without plasma neutralization) dominate the ion particle thermal pressure due to emittance. This space-charge-dominated regimeand the associated collective phenomena distinguish much of heavy-ion fusion beam science from that of higher energy particle accelerators. The primary scientific challenges are to transport, compress and focus heavy ion beams onto targets. A few selected examples of the most important scientific issues follow.

7. Office of Fusion Energy Sciences - Targets and MeasuresTen Year Measures for Inertial Fusion Energy and High Energy Density Physics • Develop the fundamental understanding and predictability of high energy density plasmas for Inertial Fusion Energy (IFE). • Minimally Effective Outcome: Develop and apply physical theories and mathematical techniques to model the physical processes in high-energy density plasmas and intense beams for inertial fusion energy. • Successful Outcome: With the help of experimentally validated theoretical and computer models, determine the physics limits that constrain the use of IFE drivers in future key integrated experiments needed to resolve the scientific issues for inertial fusion energy and high energy density physics.

8. An important scientific question fundamental to future application of heavy-ion beams to both high energy density physics and inertial fusion energy: • Can accelerated bunches of heavy ions be compressed to sufficient intensity to create the high energy density conditions for warm dense matter and propagating fusion burn in the laboratory? • Some subsidiary science campaigns needed to address this top-level question are: • Determine how well high beam brightness can be preserved under transport and focusing of intense high current beams. • Understand how beam-plasma interactions affect transverse focusing. • Explore the shortest pulses achievable with longitudinal compression. • Measure how uniformly warm dense matter can be heated with accelerated and tailored ion beams.

9. How well can initially compact 6-D beam phase space density (~Ibt /enxenyenz ) be preserved through acceleration, compression, and focusing to the target? Hitting targets allows ~10 x lower brightness and ~10x higher Dp/p than at injection Current experiments Issues that can affect beam emittance e and brightness Bn = Ib/en2 Final focus Source & injector Accelerator Drift compression HCX PTSX STS NTX Random acceleration and focusing field errors Beam mismatches Aberrations, emittance growth, instabilities in plasma Beam loss-halos, gas desorption, neutralizing secondary electrons Dpz - momentum spread increase with drift compression

10. Example of critical physics issue: beam loss in high intensity accelerators -a current world research topic (GSI-SIS-18, LANL- PSR, SNS) • Gas desorption Gas desorbed by ions scraping the channel wall can limit average beam current. • Electron cloud effects Ingress of wall-secondary electrons from beam loss and from channel gas ionization. WARP (below) and BEST simulations indicate incipient halo formation and electron-ion two-steam effects begin with electron fractions of a few percent. Ion Halo Ion Beam (core) Electron Fraction (extreme case) 0 % 2  2% 10%  10% • Random focusing magnet errors Gradient and displacement errors can also create halos and beam loss.

11. Example of critical physics issue: drift compression of bunch length by factors of 10 to 30 Induction acceleration is most efficient at tpulse ~100 to 300 ns Target capsule implosion times require beam drive pulses ~ 10 ns Bunch tail has a few percent higher velocity than the head to allow compression in a drift line Final Focus Drift compression line Perveance The beam must be confined radially and compressed longitudinally against its space-charge forces • Physics issues that need more study and experiments: • Balance beam focusing and space-charge forces during compression. • Beam heating due to compression (conservation of longitudinal invariant) • Chromatic focus aberrations due to velocity spread

12. Example of critical physics issue: plasma neutralization of beam space charge in focusing chamber Example: simulations of time histories of a driver Xenon beam radius at selected points over a 6 meter focal length by plasma With by plasma No Target Without plasma in the chamber, the ion kinetic energy and linac voltage, length and cost would have to increase by 2 to 3 x to recover the 2 mm focal spot for the target

13. Status of heavy ion fusion research • Past research (prior to FY01) validated fundamental beam dynamics with low current (mA-scale) beams with correct energy/current ratios for relevant space-charge regimes. • Research since FY01 has completed initial phase of experiments on injection (STS), transport (HCX) and focusing (NTX) at higher currents (25 to 250 mA) where non-ideal effects can be studied, such as gas and electron effects, and neutralization of beam space charge with plasma. • More research is needed and planned (FY04-06 ) to complete high current experiments, and to study longitudinal physics, including drift compression. • An integrated beam experiment to study beam brightness evolution from the source through acceleration, drift compression and focusing to the target is the appropriate (proof-of-principle) next step.

14. Past research (prior to FY01) validated fundamental beam dynamics with low current (few mA scale) beams • Some examples: • Single-Beam Transport Experiment (SBTE) Verified simulations of transport over 86 electric quadrupoles with negligible emittance growth. Multiple-Beam Experiment with 4 beams (MBE-4)Studied 200-900 keV acceleration, >5 x current amplification in drift compression, longitudinal confinement, and multiple-beam transport Final-Focus Scaled Experimentshowed ballistic focusing at 1/10 scale, and neutralizing electrons from a hot filament could reduce the focal spot size

15. 0.5 m 1.0 0.5 0.0 0 5 10 15 20 Z (m) Z Source-Injector Test Stand (STS – operating at LLNL) (Recent paper submitted for publication in Review of Scientific Instruments. Simulation published Jan 2003 Phys. Rev Special Topics-Accelerators and Beams) Injector Brightness:source brightness, aberration control with apertures, beamlet merging effects Merging-beamlet simulation ex, ey (π-mm-mrad) Beamlet brightness measurement meets IFE requirement

16. High Current Experiment (HXC- operating at LBNL) • Low en ~ 0.5 p mm-mr (negligible growth as simulations predict) • Envelope parameters within tolereances for matched beam transport Marx ESQ injector Matching and diagnostics 10 ES quads (Recently submitted for publication in Physical Review Special Topics-Accelerators and Beams) End Diagnostics Propagation of longitudinal perturbation launched at t = 0. New Gas-Electron Source Diagnostic (GESD) shows secondary electrons per ion lost follows theory (red curve) Four magnetic quadrupoles and additional diagnostics have been recently added to study gas and secondary electron effects

17. Neutralized Transport Experiment (NTX- operating at LBNL) 400 kV Marx / injector Space charge blow-up causes large 1-2 cm focal spots without plasma. Focusing magnets Drift tube Pulsed arc plasma source Smaller 1 to 2 mm focal spot sizes with plasma are consistent with WARP/LSP PIC simulations. (Submitted for publication in Physical Review Special Topics- Accelerator and Beams) Scintillating glass . Envelope simulation of NTX focusing with and without plasma

18. Small-scale experiments are available to study long-path transport physics such as slow emittance growth Construction of the University of Maryland Electron Ring experiment (UMER) is nearing completion. UMER uses electrons to study HIF-beam physics with relevant dimensionless space charge intensity. The Paul Trap Simulator Experiment at PPPL uses oscillating electric quadrupole fields to confine ion bunches for 1000s of equivalent lattice periods (many kilometers).

19. Track beam ions consistently along entire system A key goal is an integrated, detailed, and benchmarked source-to-target beam simulation capability • Study instabilities, halo, electrons, ..., via coupled detailed models • Systems code IBEAM for synthesis, planning

20. Understanding how the beam distribution evolves passing sequentially through each region requires an integrated experiment The beam is collisionless, with a “long memory” Its distribution function --- and its focusablity --- integrate the effects of applied and space-charge forces along the entire system NOW NEXT A source-to-target integrated beam experiment (IBX) which sends a high current beam through injection, acceleration, drift compression, and final focus STS- injection Combine these elements and add acceleration and drift compression HCX- transport NTX-focusing

21. Ion accelerators provide a complementary tool to lasers for High Energy Density Physics • Intense accelerator beam physics is itself part of the broad field of high energy density physics. • Accelerator-produced ion beams can be tailored in velocity spread and at energies near the Bragg peak to provide a tool to control and improve deposition uniformity in thin foil targets. How much uniformity is possible and how much it improves equation-of-state measurement accuracy needs further exploration. Future accelerators could drive large volume targets. • Ion-driven high energy density physics benefits from the same accelerator and beam-plasma physics base needed for inertial fusion. • Laser–produced ion beams such as L’Oasis @ LBNL may also allow near-term studies of collective effects of intense ion beams in regimes relevant to heavy-ion fusion. • There are excellent opportunities for collaboration in ion-driven high energy density physics at GSI.

22. Two ion dE/dx regimes are available to obtain uniform ion energy deposition in 1 to few eV warm-dense matter targets Linacs with ~ 1 J of ions @ ~0.3 MeV/u would work best at heating thin foils near the Bragg peak where dE/dx~ 0.  ~3 % uniformity possible (Grisham, PPPL).Key-physics issue: can < 300 ps ion pulses to avoid hydro-motion be produced? dE/dx z ~3 mm ~3 mm Heavy ion beams of >300 MeV/u at GSI must heat thick targets with ions well above the Bragg peak kJ energies required @ <300 ns to achieve ~15% uniformity.

23. z=980 cm z=940 cm z=900 cm z=500 cm z=100 cm Key scientific issue for ion accelerator-driven HEDP: limits of beam compression, focusing and neutralization to achieve short (sub-nanosecond) ion pulses with tailored velocity distributions. Recent HIF-VNL simulations of neutralized drift compression of heavy-ions in IBX are encouraging: a 200 ns initial ion pulse compresses to ~300 ps with little emittance growth and collective effects in plasma. • Areas to explore to enable ion-driven HED physics: • Beam-plasma effects in neutralized drift compression. • Limits and control of incoherent momentum spread. • Alternative focusing methods for high current beams, such as plasma lens. • Foil heating (dE/dx measurements for low range ions < 10-3 g/cm2) and diagnostic development. (LSP simulations by Welch, Rose, Olson and Yu June 2003) Ion driven fast ignition possibility ?

24. Conclusions • Space-charge-dominated beam regimes and associated collective phenomenadistinguishes much of heavy-ion fusion beam science from that of higher energy particle accelerators, and poses the primary scientific challenges: transport, compress and focus heavy-ion beams onto targets. • High current experiments in injection (STS), transport (HCX) and focusing (NTX) are underway at higher currents ( 25 to 250 mA) where non-ideal effects can be studied, such as gas and electron effects, and neutralization of beam space charge by background plasma. • An integrated beam experiment to study beam brightness evolution from the source through acceleration, drift compression and focusing to the target is the appropriate (proof-of-principle) next step. • Accelerator-produced ion beams can be tailored in velocity spread and at energies near the Bragg peak to provide a tool to control and improve deposition uniformity in thin foil targets. How much uniformity is possible and how much it improves equation of state measurement accuracy need further exploration.