Lawrence Livermore National Laboratory Livermore, CA USA 2. Tomotherapy, Inc. Madison, WI USA 3

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Louisa Holmes
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1 Status of the Dielectric Wall Accelerator* G. J. Caporaso 1, Y.-J. Chen 1, S. Sampayan 1, G. Akana 1, R. Anaya 1, D. Blackfield 1, J. Carroll 1, E. Cook 1, S. Falabella 1, G. Guethlein 1, J. Harris 1, S. Hawkins 1, B. Hickman 1, C. Holmes 1, A. Horner 1, S. Nelson 1, A. Paul 1, B. Poole 1, R. Richardson 1, D. Sanders 1, J. Sullivan 1, L. Wang 1, J. Watson 1, J. Weir 1,3, D. Pearson 2 and K. Selenes 4 1 Livermore, CA USA 2 Tomotherapy, Inc. Madison, WI USA 3 Compact Particle Acceleration Corporation Madison, WI USA 4 TPL Corporation Albuquerque, NM USA 2009 Particle Accelerator Conference Vancouver, Canada May 4 8, 2009 *Patents Pending. This work was performed under the auspices of the U.S. Department of Energy by under Contract DE-AC52-07NA

2 High gradient induction linacs are possible for short pulses Dielectric wall accelerator (DWA) Early concepts High gradient possibilities Component status Issues with the current approach New architecture concept Summary 2

3 An early dielectric wall accelerator concept* Coreless induction accelerator 40 MeV, 100 ka, 25 nsec* *A. I. Pavlovski, et. al. Sov. At. En. 28, 549 (1970) LIU 30 Sarov, Russia Gradient 1 MV/m *Courtesy of Anatoly Krasnykh, SLAC 3

HGI structure forms a periodic electrostatic focusing system for

breakdown process Conventional Insulator High Gradient Insulator

bombard surface floating conductors Leopold, et. al., IEEE Trans.

530 (2005) Kapton + Emitted electrons repelled from surface High

4 HGI structure forms a periodic electrostatic focusing system for low energy electrons Closely spaced conductors inhibit the breakdown process Conventional Insulator High Gradient Insulator - - field emitted electrons + Emitted electrons repeatedly bombard surface 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 4

5 All DWA configurations employ parallel plate transmission lines dielectric L conductors d w E One way transit time Impedance of each transmission line Typical current flow in the line with a gradient E 5

6 SiC offers the potential of high voltage, high current operation at elevated temperature with long lifetime and low jitter Optical Energy Injection Electrical Contacts SiC Modulator/ Switching Region High Gain Amplifier Region * Patent pending 6

$pulses HGI E z (axis)/e z (wall) A high on-axis gradient is maintained as long as This implies pulses in the range of a fraction to several ns Sech Super Gaussian Gaussian E z (wall) *patent$

7 DWA can be used in the single pulse traveling wave" mode to accelerate any charged particle* Along the wall HGI characteristics imply that the highest gradients will be attained for the shortest pulses HGI E z (axis)/e z (wall) A high on-axis gradient is maintained as long as This implies pulses in the range of a fraction to several ns Sech Super Gaussian Gaussian E z (wall) *patent pending 7

Proton source Focusing HGI SiC photoconductive

8 artist's rendition of a possible proton therapy system Laser Optical fiber distribution system Proton source Focusing HGI SiC photoconductive switches * Patents pending Monitor Stack of Blumleins 8

Blumleins HGI Spark sources Initial tests used as an

9 F.A.S.T. was built to test components in an integrated system 7 Blumleins HGI Spark sources Initial tests used F.A.S.T. as an electron diode Signal proportional Electron to A-K Current (A) Voltage Flashboard cathode Time [ns] HGI 9

10 Proton injector and F.A.S.T. accelerator section Vacuum pump 5-Induction cells Laser line of sight Thomson spectrometer Spark sources Spectrometer & camera 10

11 F.A.S.T. acceleration of protons is measured with spectrometer Accel. phase H + C ++ H 2 + or D + OFF Injector voltage Decel. phase accel decel Energy Signal proportional to F.A.S.T. voltage 11

12 Cast dielectric sustains interesting field levels in relevant configurations 4 cm x 56 cm x 0.8mm gap between electrodes Typical Trace Failure point ns At Failure

New SiC material fails at enhanced stress > 200 MV/m Failure of 3 Blumlein switches at > 30 kv Failures occur at electrode edges where computed field is > 200 MV/m Intrinsic breakdown field

13 New SiC material fails at enhanced stress > 200 MV/m Failure of 3 Blumlein switches at > 30 kv Failures occur at electrode edges where computed field is > 200 MV/m Intrinsic breakdown field for pure material 10 mm x 10 mm x 1mm Average stress > 30 MV/m electrode Present work is focused on developing an integrated switch package that eliminates these enhancements solder SiC oil 13

14 Stacked Blumleins are subject to parasitic coupling Stripline impedances can be tens of Ohms, ~ ka currents Switch tenths of Ohms, ~ 100 ka currents Magnetic field lines close through adjacent layers, inducing currents in neighboring lines V Ideal stack output voltage t Output including parasitic coupling Z 0 /2 Z 0 Z 0 Z 0 /2 Radial lines completely isolate adjacent layers but have a very low impedance (sub- Ohm), requiring massive currents to support high gradients C L 14

15 Classical bipotential lens can be used to accelerate particles V conductor conductor 15

16 If we could move the lens at the correct speed, a particle could be continuously accelerated V conductor u conductor Move lens with speed u 16

17 Move the lens without moving the assembly - a moving virtual gap* V Move this region at speed u High conductivity material u High conductivity material V Low conductivity material Voltage is concentrated into a small gap 0 Need a material whose conductivity can be rapidly changed from a high conductivity state to a low conductivity state and back again * Patent pending 17

18 One way to do this is with photoconductivity* V Move this region at speed u Illuminated SiC u Illuminated SiC V 0 SiC not illuminated Need a material whose conductivity can be rapidly changed from a high conductivity state to a low conductivity state and back again * Patent pending Laser light Voltage drop occurs across high resistivity region 18

19 Induction machines have been used for decades to concentrate voltage Typical electron injector Induction cells gap Inductive voltage adders are induction concentrators 19

20 To what extent can this be achieved dynamically? Induction cells solenoid w i gap region l a Photoconductive switch C L Virtual gap* is created by shutting off photoconductive switches * Patent pending 20

21 Continuous model for moving virtual gap* Transmission line equations Acceleration field Define dimensionless variables u (w) is speed (width) of the virtual gap * Patent pending 21

22 Seek a traveling wave (similarity) solution for a long system There are two distinct regimes*: LCu 2 < 1 subluminal LCu 2 > 1 superluminal * Patent pending 22

23 Idealized solutions for constant accelerating field (subluminal) 23

24 Idealized solutions for constant accelerating field (superluminal) 24

10 0 5 10 15 20 25 30 35 40 45 50 time (ns) On-axis accelerating field in the middle of each

25 3D EM simulations (XFDTD)verify the effect* grid coaxial feeds core switch tube Electric field (V/m) grid conductivity (S/m) time (ns) On-axis accelerating field in the middle of each switch tube segment (1 Volt drive per cell) 20 * Patent pending time (ns) 25

26 Magnetic cores can enable the superluminal regime* Induction cells w Magnetic cores i gap region l a Photoconductive switch C L * Patent pending 26

27 A circuit dual exists using a helical inner conductor* Virtual gap* is created by shutting off photoconductive switches Induction cells w helix gap region l a C L * Patent pending 27

28 Traveling wave and finite length system solutions are similar Charging phase Finite length solution Traveling wave solution 28

29 Summary Key material strengths look to be consistent with 100 MV/m gradients for short pulses Near term goals Improve switch material Develop integrated switch package Add focusing to injector and characterize Source lifetime and repeatability quality New, moving virtual gap architecture idea to overcome parasitic effects 29

High Gradient Induction Linacs for Hadron Therapy*

High Gradient Induction Linacs for Hadron Therapy* G. J. Caporaso Livermore, CA 94551 USA 1 st International Workshop on Hadron Therapy for Cancer Erice, Sicily April 24 May 1, 2009 *Patents Pending. This