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Pulse Line Ion Accelerator

Pulse Line Ion Accelerator. Dick Briggs SAIC (consultant to LBNL & IFE VNL) Paper B1-7, RPIA 2006 March 7, 2006. The Pulse Line Ion Accelerator - an “air core” induction linac. Outline of talk HEDP/WDM motivation, beam parameter requirements Helical pulse line concept

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Pulse Line Ion Accelerator

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  1. Pulse Line Ion Accelerator Dick Briggs SAIC (consultant to LBNL & IFE VNL) Paper B1-7, RPIA 2006 March 7, 2006

  2. The Pulse Line Ion Accelerator - an “air core” induction linac Outline of talk • HEDP/WDM motivation, beam parameter requirements • Helical pulse line concept • Acceleration scenarios • Transformer coupling • HV demonstration model

  3. AcknowledgementsCollaborators and contributors include: Will Waldron, Lou Reginato, Prabir Roy, Josh Coleman, Frank Bieniosek, Matthaeus Leitner, Enrique Henestroza, Dave Grote , Simon Yu, Grant Logan (LBNL/VNL) Alex Friedman, John Barnard, George Caporaso, Scott Nelson (LLNL/VNL)

  4. The motivation for the Pulse Line Ion Accelerator concept came from ion beam requirements for High Energy Density Physics • Uniform volumetric heating at Bragg peak yields < 5% target temperature variation with 75% of the ion energy deposited in target • Requirement: accelerate ~ 0.1 microcolumbs of medium mass ions (Na+1) to an energy slightly above the Bragg peak (~ 24 MeV), in as short a pulse as possible • The pulse power driven Pulse Line Ion Accelerator (PLIA) inserted into a large bore superconducting solenoid (~ 5-10 T for focusing) appears to be a very inexpensive route to meet these requirements

  5. System concept for HEDP/WDM ion beam facility 1 eV target heating with > 0.1 μC of Na+1accelerated to 24 MeV to utilize the Bragg peak Neutralized Compression PLIA Acceleration Solenoid Focusing Short Pulse Injector Final focus

  6. Straw man beam parameters -0.1 micro coulomb Na+1 example In the PLIA section: • Accelerate 20 cm long beam pulse to 24 MeV, keeping its length constant (duration goes from ~ 100ns to ~ 20 ns) • Radial focusing by solenoid (5T, 2 cm beam radius) • Axial focusing by ~ 10% “tilt” to accelerating E field In the neutralized drift compression section: • Extra velocity “tilt” at accelerator output compresses pulse to ~ 1 -2 cm length (~ 10 x)

  7. A pulse power driven traveling wave pulse on a helical pulse line accelerates the ion bunch Low HV Voltage Oil Model Helix

  8. Zero order model is simple dispersionless transmission line (assumes ka < 1) Fourier spectrum of pulse must be contained in ka<1

  9. A ramped voltage pulse at the helix input launches a traveling wave that accelerates the ion bunch distance time

  10. The axial electric field is the gradient of the helix voltage (Ex: 3 MV/m with Vo = 450kV, 30 cm ramp length) “tilt” in Ez

  11. Significant energy gains are possible in a section with constant helix velocity

  12. With a finite beam length, the ion bunch can slide only part way “up and down the hill” – increasing the ramp length gives more energy gain but lowers the gradient Example -To double the ion energy we pick:

  13. Transformer coupling is an attractive option; it significantly reduces the feed-through voltage Transformer strap on Transformer coupler low voltage air model on high voltage oil (~ 5:1 step up demonstrated) demonstration helix

  14. A demonstration oil-dielectric helix has been used for initial tests of K ion acceleration Main design parameters a = 8.1 cm b = 11.75 cm helix pitch = 1.6 turns/cm one meter helix length helical termination resistor

  15. A low voltage pulse was used to measure the wave speed in the oil helix with a movable B-dot loop (4” intervals shown) Measured velocity = 2.8x10(6) m/sec design = 2.9x10(6) m/sec

  16. K ion acceleration was demonstrated in the LBNL NDCX facility (P. Roy et al)

  17. Waveform at the helix output: a 33 kV peak topeak ramp accelerated a low current K+ beam

  18. The ion energy gain was 150kV with a 15 KV cap charge voltage switched onto a 2 turn primary With a helix voltage ramp from –21 kV to +12kV, the ion energy gain (loss) varies from + 150 kV to – 80 kV. The measured longitudinal phase space agrees with WARP3D simulations. 350 keV

  19. Fundamental limitations to the acceleration gradient will come from several factors 1. Axial voltage stress along the vacuum insulator (3-5 MeV/m is a reasonable goal considering the “inductive grading” of the helix voltage and ~ 100ns typical pulse duration) 2. Radial voltage holding ( ) 3. Limits on ramp length( implies ) #2 + #3 implies ~ 3-5 MeV/m for in the oil or epoxy dielectric

  20. Axial electric fields that continuously accelerate and focus a slowly moving ion bunch, with lots of “knobs” Significant energy gains in untapered helical lines with pulse power voltages in the 200 – 600 keV range (simplicity, broad range of operating modes, etc) Many features of the helix wave generation and propagation can be (have been) tested on full scale models at low voltage Transport of high line charge density ion beams in commercially-available large bore superconducting solenoid magnets (spin-off from $B NMR/MRI markets) However --- many practical problems like control of ion losses, stray electrons, etc., need to be faced and resolved to realize its potential The Pulse Line Ion Accelerator has a number of attractive features

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