beam research program lawrence livermore national laboratory high gradient dielectric wall...

BeamResearch Program

Lawrence Livermore National Laboratory

High Gradient Dielectric Wall Accelerators*High Gradient Dielectric Wall Accelerators*,#,#

Yu-Jiuan Chen


Muon Collider Design WorkshopDecember 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-AC52-07NA27344.

# Patents Pending



High Gradient Dielectric Wall AcceleratorsHigh Gradient Dielectric Wall AcceleratorsYu-Jiuan ChenMuon Collider Design Workshop, Dec. 8-12, 2008 p. 2

Compact Proton DWA Accelerator TeamCompact Proton DWA Accelerator Team

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. Weir1

TomoTherapy Team: D. W. Pearson

1. Also Compact Particle Acceleration Corporation, USA




• 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

A new type of induction accelerator promises to provide a compact proton source for cancer therapy




DWA technology originated with a desire for more compact flash x-ray sources

DARHT-I18 MeV, 2 kA

FXR18 MeV, 2 kA

8 MeV, 2 kA DWA

Existing LIA sources have gradients < 0.5 MV/m

Existing LIA sources have gradients < 0.5 MV/m




Dielectric Wall Accelerator (DWA) Dielectric Wall Accelerator (DWA) incorporates pulse forming lines into a high incorporates pulse forming lines into a high gradient cell with an insulating wallgradient cell with an insulating wall

State of the Art Electron Induction Accelerator

≈ 0.3 - 0.5 MV/meter Gradient

E-field in gaps only

≈ 1 meter

High gradient insulator

Z0

Z0

Z0 /2

Z0 /2

Switch

Switched load (beam)

* Patent Pending

Novel Zero Integral Pulse (ZIP) Forming Line with potential for > 10 MV/m*

Important elements for the DWA

• High gradient insulators

• PFL architecture

• Switches

• Large size dielectrics with high dielectric constant and high bulk breakdown strength

Important elements for the DWA

• High gradient insulators

• PFL architecture

• Switches

• Large size dielectrics with high dielectric constant and high bulk breakdown strength




High gradient insulators (HGIs) High gradient insulators (HGIs) perform 2 - 5 x better than perform 2 - 5 x better than conventional insulatorsconventional insulators

* U. S. Patent No. 6,331,194

Kapton

Leopold, et. al., IEEE Trans. Diel. and

Elec. Ins. 12, (3) pg. 530 (2005)

HGI structure forms a periodic electrostatic focusing system for low energy electrons

Closely spaced conductors inhibit the breakdown process

- -

++

High Gradient InsulatorConventional Insulator

Emitted electrons repeatedly bombard surface

Emitted electrons repelled from surface

floating conductors

field emitted electrons

3 mm HGI

sample

100 MV/M, 3 nsHigh Gradient Insulators

Conventional Insulators




A Blumlein pulse generator is formed from two transmission lines

-

-

+

Closing switch Matched load

-

+

Output voltage

Output voltage

V

time0

Beam

conductors

E

dielectric

wd

Z120rdw

Impedance:

One way transit :

Lc r

In each transmission line:

IVZ

EdZ

rwE120

Current in the line with a gradient E:

Example:

E = 50 MV/m, w = 1 cm, r = 3 => 2.3 kA

→ inherently has low impedance

→ Has large current flowing in transmission lines for high gradients (kA’s)




The Blumlein systems are more efficient with kA’s beams for high gradient DWAs

• System efficiency increases with beam current up to a theoretical maximum of 50% for our “Blumlein” design

• Proton therapy system requires ~ 1 A maximum … virtually no beam loading

– System efficiency on the order of 0.01 %

• Radial “ZIP line” architecture, with varying dielectric constant in layers to make Z constant, can theoretically deliver 100% of stored energy into the beam

• Switch resistive losses and use of less than full accelerating pulsewidth will reduce this figure

Z0

Z0

Z0 /2

Z0 /2

High gradient insulator

Switch

radial ZIP line - vary dielectric

“constant” in layers to

make Z constant

Switched load (beam)




A new castable dielectric is one of the A new castable dielectric is one of the possible materials for a DWA*possible materials for a DWA* acceleratoraccelerator

Cast dielectric has high bulk breakdown strength > 400 MV/m (small samples) and can have epsilons from ≈ 3 up to ≈ 50 for transmission lines

Nominal Gap = 11 mils

Polished Steel Ball

Polished Steel Ball

Cast CompositeDielectric

* Patent pending

r= 45, 1 meter length

0

1000

2000

3000

4000

5000

6000

7000

8000

0 5 10 15 20

Number

kV

/cm

100 MV/m

Epoxy Buried Ball Electrodes Breakdown Strength = V/gap

average gap = 0.287 mm




Cast dielectric has been successfully used for a long pulse DWA

• 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 kV) across stack and HGI’s with 1 kA electron beam load

r = 10, 1.2 m long, cast ZIP line




SiC photoconductive switch has SiC photoconductive switch has demonstrated fast operation at 27.5 demonstrated fast operation at 27.5 MV/m average gradient*MV/m average gradient*

* Patent pending

SiC offers the possibility of high voltage, high current operation at elevated temperature with long lifetime and low jitter

SiC offers the possibility of high voltage, high current operation at elevated temperature with long lifetime and low jitter

SiC

Electrical Contacts

Wasted Optical EnergyOptical Energy

Injection

13 mJ, 1064 nm Nd:YAG laser




Control of the temporal laser profile potentially can compensate for beam loading in high-current accelerators

• 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

… How well this can work is TBD

High Gain Amplifier Region

(Idark~nA)

Modulator/ Switching

Region




b

DWA can be used in the single pulse DWA can be used in the single pulse “traveling wave" mode for any charge “traveling wave" mode for any charge particleparticle

= full width at half maximumu = speed of wall excitation = Lorentz factor

HGI characteristics imply that the highest gradients will be attained for the shortest pulses

*patent pending

Ez(

ax

is)/

Ez(

wa

ll)

Gaussian

bu

Sech

Super Gaussian

A high on-axis gradient is maintained as long as 0.3This implies pulses in the range of a fraction to several ns

Longitudinal Electric Field Plot

Uncharged transmission lines in this region are not shown

Charged Blumleins

u




Stacks of Blumleins with independent Stacks of Blumleins with independent switch triggers implement the virtual switch triggers implement the virtual traveling wave*traveling wave* accelerator accelerator

* Patents pending

“Blumlein”HGI

Beam

Optical fiber distribution

system

Proton source HGI

Stack of “Blumleins”

SiC photoconductive

switches

Monitor

Focusing

Laser




Spark source, capable of providing Spark source, capable of providing large proton current, is a potential large proton current, is a potential compact proton sourcecompact proton source

Near Faraday cup

Far Faraday cup

Near Faraday cupFar Faraday cup

Spark sources

Time of flight between 2

Faraday cups indicates

proton extraction

Grid assembly




We are working with Tomotherapy, We are working with Tomotherapy, Incorporated and CPAC to develop a Incorporated and CPAC to develop a compact proton DWAcompact proton DWA

• 200 MeV 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

• TomoTherapy has licensed the DWA technology from the Lawrence Livermore National Laboratory and CPAC has a Cooperative Research and Development Agreement (CRADA) with LLNL

“artist's rendition of a possible

proton therapy system”




We have designed a small integrated test of the accelerator

• 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.




F.A.S.T. is designed for rapid F.A.S.T. is designed for rapid iterationiteration

7 Blumleins containing photoconductive switches

HGI Sensitive beam current monitor

F.A.S.T. mounted in an oil tank

Laser line of sight




F.A.S.T. was initially operated F.A.S.T. was initially operated with sub-standard switcheswith sub-standard switches

• Breakdown of the low quality dielectric in the Blumleins was expected, instead there were unexpected switch failures at low charge voltage

• Original SiC switches were defect free

• Replacement material used for FAST had structural defects and failed at stresses 20 - 30 x lower than original material

• We now have a new (proprietary) source of SiC

1 mm X 1 mm corner of original substrate (5X)

1 mm X 1 mm corner of FAST substrate (5X)




Near term plans: Near term plans: working towards an integrated test of proton source, HGI’s and Photoconductive switched Blumleins

• A small length of accelerator sufficient to 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-of-principle device

• Proton acceleration by 31 December, 2008

Proton injector

F.A.S.T.




12/31/08 deliverable requires 12/31/08 deliverable requires a 300-keV proton injector a 300-keV proton injector

Proton injector

F.A.S.T.

• F.A.S.T. pulsewidth of 3 ns = crossing time of injected protons

=> 300 keV initial energy

• Acceleration provided by induction modules– 5 induction cells

simultaneously triggered

– 300 kV @ 30+ ns

– SF6 insulation

– Powered by cable Blumleins

• Injector structure similar to that in source test stand




12/31/08 deliverable requires 12/31/08 deliverable requires a 300 keV proton injector a 300 keV proton injector




Proton Injector HardwareProton Injector Hardware

5-Induction cells

Vacuum pump 2 100-kV

Induction cells

Anode stalk

Source and Grids

Thomson spectrometer

camera




• 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 SiC and GaN 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

SummarySummary




High voltage pulser will permit testing of HGI’s at full parameters

• 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




Novel source and electrode system provides great flexibility for intensity modulated beam*

Source

Grid

DWA 100 MV/m

Proton beam

Extraction electrode

Focus electrode

Grid Thin vacuum air window

Patient

Gate electrodeDWA 100 MV/m

Grid electrode

Spark discharge proton source* *patent pending

• Only electric focusing fields are used for transporting the 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 pC) on each pulse

—Variable beam energy (70 - 250 MeV) on each pulse

Be

am

En

erg

y (

Me

V) Energy

100 x Current

80 pC

Goal: to fit machine in a standard

linac radiation vault

beam research program lawrence livermore national laboratory high gradient dielectric wall...

Documents

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high dielectric constant

high gradient insulators

dwa technology

mvmeter gradient efield

load beam

mvm slide

insulating wall state