Development and Testing of Advanced Accelerator Structures and Technologies

at 11.424 GHz

Interagency Agreement #DE–AI02–01ER41170

Steven H. GoldNaval Research Laboratory

US High Gradient Research Collaboration WorkshopStanford Linear Accelerator Center

23–25 May 2007

Program Overview

Dielectric-Loaded Accelerator (DLA) Program (NRL, ANL, SLAC, Euclid)Program: DLA Structure Development and High Power TestingGoal: A compact 20-MeV Dielectric-Loaded Test Accelerator

High Power RF Component Development (NRL, Omega-P, IAP, …)Program: Active pulse compressors using plasma discharge switch tubes or ferroelectrics; high-power component testing; etc.

NRL Magnicon Facility (developed jointly with Omega-P, Inc.)Goal: A dedicated X-band facility for collaborative accelerator research

11.424-GHz Magnicon Amplifier

Magnicon Facility Summary

• Magnicon Facility offers a flexible venue for hands-on experiments requiring high power 11.4 GHz radiation, with reasonable work and safety rules and minimal red tape

• Magnicon properties: high power (10–20 MW), adjustable pulse length (200-ns FWHM – 1-µs flattop), moderate repetition rate (up to 10 Hz), stable gain, precise frequency control, very narrow bandwidth, a modest (~0.1%) tuning range, phase stability, and stable operation into resonant or mismatched loads; power limited by pulse-shortening effects

• Magnicon facility has been used for two series of collaborative experiments since 2001:

• Twelve high-power tests of DLA Structures (3 since last Workshop)• Seven high-power tests of Active Pulse Compressors

fdrive = 5716 MHz

Magnicon Response vs Drive Frequency

fdrive = 5714 MHz

fdrive = 5716 MHz

Magnicon Response vs Drive Frequency

fdrive = 5714 MHz

11.432 GHz

11.428 GHz

fdrive = 5716 MHz RADLAB vol. 18

Magnicon Response vs Drive Frequency

fdrive = 5714 MHz

y = 0.1242x + 1001111428

11428.5

11429

11429.5

11430

11430.5

11431

11431.5

11432

11432.5

11415 11420 11425 11430 11435 11440 11445

Drive Frequency x 2 (MHz)

Gun Voltage = 472 kVPulse Length = 200 ns

Frequency vs Drive Frequency

y = -0.1667x + 11508

11425

11426

11427

11428

11429

11430

11431

11432

11433

11434

11435

440 450 460 470 480 490 500

Gun Voltage (kV)

fdrive = 5712 GHz

Frequency vs Voltage

-47 dB

-51 dB

2.382”0.900”

-177.6°

-87.6°

matchphase

2.143”

0.500”

9.765”

sliding piece

Circular TE01 In-Line Phase Shifter

New Magnicon Power Combiner(To combine magnicon output arms, and split power with

continuous control of power ratio and relative phase)

Rectangular TE10 Hybrid

Basic Circuitwaveguide hybrid

mode converters

phaseshifter

Mode Converter

height taper (.400” .800”) planar

TE10 TE20converter

rect.-to-circ.TE20 TE01mode converter

C. Nantista & S.Tantawi

C. Nantista

V. Dolgashev

WR90

WC150

WC150

10.82 degrees/turn

Tuning

TE01 Phase Shifter Cold Test Results at SLACS21

S11

y = -10.566x - 31.696

-70

-60

-50

-40

-30

-20

-10

0

10

-4 -3 -2 -1 0 1 2 3 4

Position (cm)

Cold Test of Power Combiner at NRL

Phase Angle vs Positio

y = -10.778x - 52.65-90.00

-80.00

-70.00

-60.00

-50.00

-40.00

-30.00

-20.00

-10.00

0.00-4 -3 -2 -1 0 1 2 3 4

Position (cm)

Phase Angle vs Positio

y = -10.798x + 128.8

80.00

90.00

100.00

110.00

120.00

130.00

140.00

150.00

160.00

170.00

-4 -3 -2 -1 0 1 2 3 4

Position (cm)

Power Split vs Positio

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-4 -3 -2 -1 0 1 2 3 4

Position (cm

Power Split vs Positio

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-4 -3 -2 -1 0 1 2 3 4

Position (cm

Test of Power Combiner Using Magnicon Output

Normal Magnetic Field Reversed Magnetic Field

Magnicon Output Power Combiner Output

LeftRight

LeftRight

20 MW, 200 ns

Power Combiner Traces

Magnicon Output Power Combiner Output

LeftRight

LeftRight

20 MW, 200 ns

Power Combiner Traces

LeftRight

LeftRight

10 MW, 1 µs

Power Combiner Traces

Magnicon Output Power Combiner Output

16 MW, 350 ns, 11.424 GHz

Power Combiner Traces

Power Combiner Summary

• Power combiner is conditioned and has produced up to 21 MW, 200-ns and 10 MW, 1-μs pulses into a single load

• Phase adjust of ~65º permits adjustment of power split ratio from approx. 99:1 to 65:35

• Reversing magnicon magnetic fields permits interchange of power between two output waveguides

• Waveguides have been fabricated to bring power to experimental stations, and soon to bunker

• Power combiner has now been operated into first external experiment, a TiN-coated silica DLA structure

Dielectric-Loaded Accelerator Program

Goals: Study RF acceleration in dielectric structures; Develop a compact 20 MeV test accelerator

Collaboration: Argonne National LaboratoryStanford Linear Accelerator CenterEuclid Techlabs LLC

Experimental Setup for High Power DLA Structure Tests

Modular DLA Structure

Faint Light (can be conditioned away)

Bright Light

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Recent Resultsfrom TiN-Coated Quartz Structure

First Day Second Day

DLA Progress to Date

Three different dielectric materials (Alumina, Silica, MCT) tested at gradients up to 8 MV/m, the first two with and without TiN-coatings.Strong multipactor loading observed in most cases, but

substantially less for TiN-coated silicaNo dielectric breakdown observed, but breakdown at dielectric joints frequently a gradient limitA new paper (L. Wu and L.K. Ang, Phys. Plasmas 14, 013105, 2007) suggests that multipactor may vanish at high accelerating gradients, when electron impact energy is above second crossoverof secondary emission curve

Dielectric-LoadedTest Accelerator

Magnicon

Power Combiner/Phase Shifter Load

5 MeV Injector 20 MeV Dielectric-Loaded Accelerator Spectrometer

Planned Accelerator Layout

(1) (2) (3) (4) (5) (6)(8)

(7)

ParametersEnergy 5.2 MeVCharge/Bunch >5 pC RF Frequency 11.434 GHzRF Power in ~2 MWNorm.Emittance 3.1 π mm mradEnergy Spread ~6%

MatchingQuads

Injector

NRL X-Band Injector

LaB6 Cathode

Accelerator Lab of Tsinghua University胡源 (Y. Hu, Student), 杜晓福 (X. Du),

唐传祥 (Prof. C. Tang), and 林郁正 (Prof. Y. Lin)

Measured Axial Field Profile and PARMELA Results

Temperature Tuning the Injector

Linear Fit

Experimental Data

Frequency –Temperature Curves1×10−9~3×10−9Torr

DLA Program Plans

• Continue study of ceramic structures, with goal of reducing multipactor and eliminating joint breakdown

– jointless DLA structures (to avoid joint breakdown)– smaller i.d. structures (to reduce multipactor loading

and increase accelerating gradient)– dual layer structures (to reduce rf attenuation); – TiN-coated and other structures to further explore

multipactor issues

• Complete installation and commissioning of 5 MeV electron injector

High Power RF Component Development and Testing

Recent Program: Active pulse compressors using plasma discharge switch tubes

Collaboration: Omega-P, Inc. Institute of Applied Physics

Active Pulse Compressor Summary

• Series of seven 6–8 week experiments on different pulse compressor configurations, addressing problems of switching stability, multipactor, self-breakdown

• The best result was ~50 MW, 43-ns output pulse at ~10x compression from a two-channel, single-mode compressor in the reflection configuration

• An improved dual mode configuration, designed for higher power, was limited by self-breakdown of the gas switch tube

• Next experiment in series, an improved dual-mode configuration to produce >100 MW compressed pulse using 10 MW, 1 μs drive pulse,scheduled for August–October 2007

• Program transitions to SLAC for tests of an active SLED II configuration

Dual-Mode Reflection Configuration

Energy Storage

Traces from Two-ChannelDual-Mode Reflection Experiment

Pinc = 1.8 MWPcom = 16.5 MWGain = 9.2Efficiency = 55%

• Power limited by self-breakdown (believed due to rf field nonuniformities near electrodes)

• No breakdown during switching

11/16/04–12/23/04

Improved Switch for Pulse Compressor

Calculations suggested that self-breakdown in previous switch resulted from excitation of non-axially symmetric modes in the cavity due to the presence of holes in the switch tube wall needed to feed the electrodes of the gas discharge tubes. In the redesigned cavity, the holes are set in a groove in the cylindrical wall of the switch to eliminate this problem.

RF Test of Axisymmetric X-Band Gun(October 12–20, 2006)

RF Conditioning Normal Pulse Breakdown

240 MV/m

H. Bluem

I. High-power pulse compressor using plasma switch tubes—100 MW test (Omega-P—DoE Phase II SBIRs)

II. High-power pulse compressor using ferroelectric switch—first test (Omega-P—DoE Phase II SBIRs)

III. RF tests of advanced DLA structures—tests of gapless structure

IV. Commissioning of 5-MeV X-band injector for DLA experiments

Planned Near-Term Projects

Longer-Term Goals

• Operate compact X-band accelerator as test bed for study of DLA and other advanced structures (ANL, Euclid, SLAC)

• Continue collaboration with Omega-P on development and testing of high-power RF components, advanced pulse compression schemes, etc.

• Make NRL Magnicon Facility and test accelerator available to other users for high-power X-band experiments

• Experiments can transition to SLAC for higher-power tests, once initial success has been demonstrated