Systems Analysis for Modular versus Multi-beam HIF Drivers *

1. Systems Analysis for Modular versus Multi-beam HIF Drivers* 15th International Symposium on Heavy Ion Inertial Fusion June 7-11, 2004 Princeton, NJ Wayne Meier – LLNL Grant Logan – LBNL * This work performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore and Lawrence Berkeley National Laboratories under contracts No. W-7405-Eng-48 and DE-AC03-76SF00098. The Heavy Ion Fusion Virtual National Laboratory

2. Outline • Introduction / Motivation for modular drivers • R&D advances needed • Design trades for all-solenoid modules • Number of modules • Ion mass • Solenoid/quadrupole hybrid options • Optimal transition energy • Potential improvements for multi-beam, quad-focus accelerator • Future work

3. Modular drivers have potential advantages but also present some new challenges • Primary motivation is to address development cost issue with conventional multi-beam linacs • Modularity is proven approach for lasers • Disadvantage for HI accelerator is need for induction cores for each beam • Circumvented by reducing number of beams, using lower mass ions (higher current per beam), and double pulsing each module on each shot • Solenoid magnets are best for large currents, especially at low ion energy

4. Solid state lasers have taken advantage of modular development Beamlet NIF The Beamlet laser was a single-beam, scientific prototype of the 192-beam National Ignition Facility (NIF).

5. We are considering a range of options for modular HI drivers • Single-beam solenoid accelerator, tens of accelerators for driver • Hybrids: Solenoids at front end feeding single-beam quad section, tens of accelerators • Solenoids feeding multi-beam quad section, tens of accelerators • All quads (multi-beam), tens of accelerators • A systems code is being developed for consistent comparisons

6. Key developments required for this approach • Large aperture source/injectors (~30 cm radius) • Double pulsing • Neutralized drift compression to pulse duration required by target (10’s of ns) • Larger spot size target (~5 mm radius) • Plasma channel (assisted pinch) or compensated neutralized ballistic focusing (See talks by Simon Yu and Ed Lee)

7. Hybrid target allows larger spot size beams ~ 5 mm radius Hohlraum Shine shield Beams Capsule

8. Example design point parameters illustrate the features of the modular design • Total driver energy = 6.7 MJ • Number of modules = 24 (12 per side) • Double pulsing (48 total beam pulses) • Energy per pulse = 140 kJ • Ion = Neon+1 (A = 20) • Final ion energy = 200 MeV • Core radial build = 0.62 m • Acceleration gradient = 0.28 – 2.4 MV/m • Accelerator length = 125 m • Accelerator efficiency = 33%

9. Example beam parameters for this case • Initial/final ion energy = 0.9 MeV / 200 MeV • Charge per pulse = 0.70 mC • Initial pulse duration = 20 ms • Pre-accel bunch compression = 8x  2.5 ms • Beam current into accelerator = 280 A • Pulse length = 7.2 m = constant • Line charge density = 97 mC/m • Final pulse duration = 0.17 ms • Beam current at exit of accelerator = 4.1 kA

10. Magnetic pulse compression, especially at higher ion energy is cost effective at 100 MeV 100 MeV 150 MeV Total Cost, $/m Cost, $/m Cost, $/m 50 MeV Magnetic comp Switching Pulse compression factor Pulse compression factor Pulse compression factor

11. Magnet bore is held constant; occupancy decreases due to increasing gap with higher accel gradient Solenoid spacing Occupancy fraction Meters Winding radius Pipe radius Beam radius Ion energy, MeV

12. Optimal initial pulse duration is ~ 20 ms Ed = 6.7 MJ 24 modules Total Total cost, $B Accelerator Injector Initial pulse duration, ms

13. A small number of modules would be best, but target requires ~24 for drive symmetry and pulse shaping Ed = 6.7 MJ Ne+ (A = 20) Tf = 200 MeV

14. Driver cost increases with increasing ion mass -A = 20 (Neon) is our base case Ed = 6.7 MJ 24 Modules Tf = 10A MeV

15. A transition to quad focusing at ~120 MeV has a slight benefit for single beam modules Total Solenoids Total cost, $B Injector Quads Ion energy for transition to quads, MeV

16. If beams could be split at transition, quads become attractive at lower ion energy 4 beams per module in quad section Total Solenoids Total cost, $B Injector Quads Ion energy for transition to quads, MeV

17. Neutralized drift compression and relaxed focusing requirements also benefit multi-beam, quad-focus drivers 1 accelerator Ne+1 Tf = 200 MeV 3.2 MJ/pulse Double pulsing (6.4 MJ total) Total Total cost, $B Front end (Injector + ESQ) Number of beams Electrostatic quads up to ~ 6 MeV Magnetic quads for remainder

18. Neutralized drift compression/focusing + hybrid targets may reduce costs by ~50 % for both conventional multiple-beam quadrupole and modular solenoid driver options for IFE Multiple-beam quad linac driver Modular solenoid linac driver 3000 “Robust Point Design” 2500

19. Findings are promising for modular drivers • Modular drivers are a potentially attractive option with: • Low mass ions (< 40 amu) • 10’s of modules (not 100’s) • Neutralized drift compression • Relaxed target spot size requirements • All-solenoid modules or solenoid-to-quad hybrid modules are comparable in cost • If feasible, beam splitting at transition to quads would be beneficial • Neutralized drift compression and larger spot size targets also benefit standard multi-beam, quad-focus linacs

20. More systems modeling work is needed • Improve injector model – dominates in some cases • Beam focusing models (including pulse shaping) are needed for new schemes • Determine target gain scaling with beam spot size • Compare high-current modular drivers using large spot size targets to low-current multi-beam linacs using smaller spot size targets