Beam Research Program Lawrence Livermore National Laboratory High

Beam Research Program Lawrence Livermore National Laboratory High Gradient Dielectric Wall Accelerators*, # Yu-Jiuan Chen Lawrence Livermore National Laboratory Muon Collider Design Workshop December 8 -12, 2008 Thomas Jefferson National Laboratory * This work performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC 52 -07 NA 27344. # Patents Pending

Compact Proton DWA Accelerator Team Beam Research Program Lawrence Livermore National Laboratory Principal Investigator: G. J. Caporaso LLNL Team: D. T. Blackfield, Y. -J. Chen, S. Falabella, G. Guethlein, J. R. Harris, S. A. Hawkins, C. L. Holmes, S. D. Nelson, A. C. Paul, B. R. Poole, M. A. Rhodes, R. Richardson, S. E. Sampayan, D. M. Sanders, J. S. Sullivan, L. -F. Wang, J. A. Watson, J. Weir 1 Tomo. Therapy Team: D. W. Pearson 1. Also Compact Particle Acceleration Corporation, USA High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 2

A new type of induction accelerator promises to provide a compact proton source for cancer therapy Beam Research Program Lawrence Livermore National Laboratory • Dielectric Wall Accelerator (DWA) for flash x-ray radiography • Important technologies for the DWA – High gradient insulator technology – Dielectric materials – Blumlein development – Solid-state switch development • Proton therapy concept • Source and system tests • Summary High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 3

DWA technology originated with a desire for more compact flash x-ray sources Beam Research Program Lawrence Livermore National Laboratory FXR 18 Me. V, 2 k. A DARHT-I 18 Me. V, 2 k. A DWA Existing LIA sources have gradients < 0. 5 MV/m High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 4

Dielectric Wall Accelerator (DWA) incorporates pulse forming lines into a high gradient cell with an insulating wall Beam Research Program Lawrence Livermore National Laboratory Novel Zero Integral Pulse (ZIP) Forming Line with potential for > 10 MV/m* Switched load (beam) ≈ 1 meter Switch E-field in gaps only Important elements for the DWA Z 0/2 * Patent Pending Z 0 State of the Art Electron Induction Accelerator ≈ 0. 3 - 0. 5 MV/meter Gradient • • High gradient insulators PFL architecture Switches Large size dielectrics with high dielectric constant and high bulk breakdown strength High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 5

High gradient insulators (HGIs) perform 2 - 5 x better than conventional insulators - field emitted electrons + Emitted electrons repeatedly bombard surface Lawrence Livermore National Laboratory HGI structure forms a periodic electrostatic focusing system for low energy electrons Closely spaced conductors inhibit the breakdown process Conventional Insulator High Gradient Insulator - Beam Research Program floating conductors Leopold, et. al. , IEEE Trans. Diel. and Elec. Ins. 12, (3) pg. 530 (2005) Kapton + Emitted electrons repelled from surface High Gradient Insulators Conventional Insulators 100 MV/M, 3 ns 3 mm HGI sample * U. S. Patent No. 6, 331, 194 High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 6

A Blumlein pulse generator is formed from two transmission lines In each transmission line: Beam Research Program Lawrence Livermore National Laboratory dielectric conductors Impedance: One way transit : E d w Current in the line with a gradient E: Output voltage - Example: E = 50 MV/m, w = 1 cm, r = 3 => 2. 3 k. A → inherently has low impedance → Has large current flowing in transmission lines for high gradients (k. A’s) + + Closing switch Matched load Beam Output voltage V 0 time High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 7

The Blumlein systems are more efficient with k. A’s beams for high gradient DWAs Beam Research Program Lawrence Livermore National Laboratory • System efficiency increases with Switch beam current up to a theoretical maximum of 50% for our “Blumlein” design radial ZIP line - vary dielectric “constant” in layers to make Z constant • Proton therapy system requires ~ 1 A maximum … virtually no beam loading – System efficiency on the order of 0. 01 % • Radial “ZIP line” architecture, with Switched load (beam) Z 0/2 • Switch resistive losses and use of Z 0 varying dielectric constant in layers to make Z constant, can theoretically deliver 100% of stored energy into the beam less than full accelerating pulsewidth will reduce this figure High gradient insulator High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 8

A new castable dielectric is one of the possible materials for a DWA* accelerator Beam Research Program Lawrence Livermore National Laboratory Cast dielectric has high bulk breakdown strength > 400 MV/m (small samples) and can have epsilons from ≈ 3 up to ≈ 50 for transmission lines r = 45, 1 meter length Epoxy Buried Ball Electrodes Breakdown Strength = V/gap average gap = 0. 287 mm 100 MV/m Cast Composite Dielectric Polished Steel Ball Nominal Gap = 11 mils Polished Steel Ball * Patent pending High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 9

Cast dielectric has been successfully used for a long pulse DWA Beam Research Program Lawrence Livermore National Laboratory r = 10, 1. 2 m long, cast ZIP line • Prototype single arm cell – 4 cast dielectric, zero integral pulse – – – generating lines producing 25 ns pulse 4 self-breaking oil switches Power coupled to beam through 4 high gradient insulators 3 MV/m gradient (600 k. V) across stack and HGI’s with 1 k. A electron beam load High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 10

Si. C photoconductive switch has demonstrated fast operation at 27. 5 MV/m average gradient* Beam Research Program Lawrence Livermore National Laboratory Si. C offers the possibility of high voltage, high current operation at elevated temperature with long lifetime and low jitter Wasted Optical Energy Injection Electrical Contacts Si. C 13 m. J, 1064 nm Nd: YAG laser * Patent pending High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 11

Control of the temporal laser profile potentially can compensate for beam loading in high-current accelerators Beam Research Program Lawrence Livermore National Laboratory • Photoconductive switch is really a light-controlled resistor • Control of the temporal laser profile can change the shape of the accelerating waveform and potentially compensate for beam loading Modulator/ Switching Region … How well this can work is TBD High Gain Amplifier Region (Idark~n. A) High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 12

DWA can be used in the single pulse “traveling wave" mode for any charge particle Beam Research Program Lawrence Livermore National Laboratory HGI characteristics imply that the highest gradients will be attained for the shortest pulses b u Super Gaussian Ez(axis)/Ez(wall) Longitudinal Electric Field Plot Uncharged transmission lines in this region are not shown Charged Blumleins A high on-axis gradient is maintained as long as q £ 0. 3 This implies pulses in the range of a Sech fraction to several ns Gaussian = full width at half maximum u = speed of wall excitation g = Lorentz factor *patent pending High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 13

Stacks of Blumleins with independent switch triggers implement the virtual traveling wave* accelerator Beam Research Program Lawrence Livermore National Laboratory Optical fiber distribution system Laser HGI “Blumlein” Proton source Focusing HGI Beam Si. C photoconductive switches Monitor Stack of “Blumleins” * Patents pending High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 14

Spark source, capable of providing large proton current, is a potential compact proton source Grid assembly Beam Research Program Lawrence Livermore National Laboratory Near Faraday cup Spark sources Time of flight between 2 Faraday cups indicates proton extraction Near Faraday cup High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 15

We are working with Tomotherapy, Incorporated and CPAC to develop a compact proton DWA Beam Research Program Lawrence Livermore National Laboratory • 200 Me. V protons in 2 meters • Energy, intensity and spot width variable pulse to pulse • Nanosec pulse lengths • At least 200 degrees of rotation • Up to 50 Hz pulse repetition rate • Less neutron dose (neutrons still produced in the patient) • System will provide CT-guided rotational IMPT • Tomo. Therapy has licensed the DWA technology from the Lawrence “artist's Livermore National Laboratory and rendition of a CPAC has a Cooperative Research and possible Development Agreement (CRADA) with proton therapy LLNL system” High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 16

We have designed a small integrated test of the accelerator Beam Research Program Lawrence Livermore National Laboratory • F. A. S. T. (First Article System Test) is designed to • • be taken apart and rebuilt many times to increase system performance by using improved components It incorporates the essential elements for the accelerator – HGI’s – Photoconductive switches – Solid dielectric Blumleins Progress is made by finding breakdowns, repairing them and increasing performance to find the next limiting factor, etc. High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 17

F. A. S. T. is designed for rapid iteration Beam Research Program Lawrence Livermore National Laboratory 7 Blumleins containing photoconductive switches F. A. S. T. mounted in an oil tank HGI Sensitive beam current monitor Laser line of sight High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 18

F. A. S. T. was initially operated with sub-standard switches • Breakdown of the low quality Beam Research Program Lawrence Livermore National Laboratory 1 mm X 1 mm corner of original substrate (5 X) dielectric in the Blumleins was expected, instead there were unexpected switch failures at low charge voltage • Original Si. C switches were defect free • Replacement material used for 1 mm X 1 mm corner of FAST substrate (5 X) FAST had structural defects and failed at stresses 20 - 30 x lower than original material • We now have a new (proprietary) source of Si. C High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 19

Near term plans: working towards an integrated test of proton source, HGI’s and Photoconductive switched Blumleins Beam Research Program Lawrence Livermore National Laboratory • A small length of accelerator sufficient to Proton injector • F. A. S. T. verify the accelerator architecture and HGI performance with photoconductive switches New photoconductive switches over next 5 months – Optimized dopant levels to lower “on” – resistance and improve quantum efficiency High voltage packaging • First Article System Test (F. A. S. T. ) – Integrate new photoconductive switches, solid dielectric, HGIs into a proof-ofprinciple device • Proton acceleration by 31 December, 2008 High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 20

12/31/08 deliverable requires a 300 -ke. V proton injector Beam Research Program Lawrence Livermore National Laboratory • F. A. S. T. pulsewidth of 3 ns = Proton injector crossing time of injected protons => 300 ke. V initial energy • Acceleration provided by induction F. A. S. T. modules – 5 induction cells simultaneously triggered – 300 k. V @ 30+ ns – SF 6 insulation – Powered by cable Blumleins • Injector structure similar to that in source test stand High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 21

12/31/08 deliverable requires a 300 ke. V proton injector Beam Research Program Lawrence Livermore National Laboratory High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 22

Proton Injector Hardware Vacuum pump 5 -Induction cells Beam Research Program Lawrence Livermore National Laboratory 2 100 -k. V Induction cells Anode stalk Thomson spectrometer Source and Grids camera High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 23

Summary Beam Research Program Lawrence Livermore National Laboratory • DWA promises to dramatically increase the accelerating gradient of high current accelerators • Good progress is being made on the technologies needed for the DWA – New Si. C and Ga. N material has been obtained for switches – Pulse forming line dielectric materials (> 400 MV/M) – High gradient vacuum insulators can be tested at the relevant field stress, sample size and pulsewidth • F. A. S. T. has demonstrated acceleration of electrons and will be repeated with new photoconductive switches F. A. S. T. will accelerate protons by 31 December, 2008 • Spark source is being investigated for the potential compact proton source for the proton DWA for cancer treatment High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 24

Beam Research Program Lawrence Livermore National Laboratory High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 25

High voltage pulser will permit testing of HGI’s at full parameters Beam Research Program Lawrence Livermore National Laboratory • Delivers 3 -5 MV into a 160 matched load, 1~2 ns pulse • Will permit testing of hollow HGI insulators at full radial scale • Now fully operational High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 26

Novel source and electrode system provides great flexibility for intensity modulated beam* Gate electrode Patient Goal: to fit machine in a standard linac radiation vault DWA 100 MV/m DWA Source Grid Spark discharge proton source* Lawrence Livermore National Laboratory Proton beam *patent pending Thin vacuum air window • Only electric focusing fields are used for transporting the Beam Energy (Me. V) Focus electrode Grid Extraction electrode Beam Research Program Energy 100 x Current 80 p. C beam and focusing on the patient • Variable beam parameters from pulse to pulse without using any beam intercepting methods, i. e. , no range shifting wedges or scattering masks — Variable spot size (2 mm - 2 cm diameter) on each pulse — Variable charge (10 - 80 p. C) on each pulse — Variable beam energy (70 - 250 Me. V) on each pulse High Gradient Dielectric Wall Accelerators Yu-Jiuan Chen Muon Collider Design Workshop, Dec. 8 -12, 2008 p. 27