ASTACapabilities and Projects

Accelerator Structure Test AreaHigh power X-band accelerator structure test facility

Deliver high power pulsed RF to high gradient accelerator structures

Based on lessons learned at the NLCTA

Stephen Weathersby

CapabilitiesHigh gradient X-band testing of accelerator structures regularly exceeding 100 MV/m gradient

Up to ~300 MW peak power delivered to the structure by combining the power of two XL4 klystrons into a delay line pulse compression system.

Consists of two modulators, two klystrons, delay lines for pulse compression, a radiation enclosure, PPS system, interlocked vacuum and cooling water systems, llrf system, diagnostics and data acquisition…

Past RF gun testing incorporated a UV cathode laser system and small beam line with a beam spectrometer.

S-Band capability

High power RF capabilities

From Two 50 MW Klystrons

Variable iris Variable Delay line length through variable mode converter

Gate Valves

Two experimental stations inside the enclosure, one with compressed pulse and the other without the benefit of the pulse compressor.

With two 50MW XL4 klystrons ASTA can produce: 100MW @ 1.5 μs --> 550MW @ 63 ns at X-band and feed twoexperimental outputs in the enclosure.

Courtesy of Valery Dolgashev

components to support the experimental facilities

Tee for variable iris

Bends for low loss transmission and reliable RF systems

Dual moded delay lines with variable delay for a flexible pulse width

Courtesy of Sami Tantawi

Gate valve

SRS60 Hz

AFG

AFGTWT K

TWT K

llrf configuration

I&QMIXER

I&QMIXER

SRSDG645

4 port combiner

no SLED

SLED

Power meters Dark current signals

DUT FE/REKlystron RE

Vacuum

Pulse compression and pulse shaping

Each bin of has independent I&Q modulation via two channel AFGs

Forward power RF signals are I&Q demodulated and can be used in pulse shape feedback

Delay line tuning is handled by feedback

Pulse compressor forward power

Breakdown rates vs gradient

forward powerreflected powerfaraday cup 1faraday cup 2

Faraday cup signals register breakdowns and inhibit further pulsesGradient is calculated. Several weeks for typical structure characterizationGS/s acquisition ratesBreakdown traces are savedAutomated processing

50 100 150 20010 -7

10 -5

0.001

0.1

Gradient [M V /m ]

Brea

kdw

nPr

obab

ility

[1/p

ulse

/m]

Breakdown rate vs. pulse length for C10-VG07

260 ns

130 ns

Accelerating structures

RF power

PETS

Main beamDrive beam Courtesy of Alessandro Cappelletti

Peak powerAvg powerEnergyBD

CERN CLIC PETS3 Testing133 ns 266 ns

Revised: April 7, 2010

Jake Haimson

Recirculation Implementation

Some ongoing and planned HG studies

Test of a Vacuum Brazed CuZr and CuCr Structures

High Gradient Structures--AAC 2010Page 11

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G rad ien t M V m Bre

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a 0.215, t 4.6mm, C uC r SLA C 1a 0.215, t 4.6mm, C uZ r SLA C 1a 0.215, t 4.6mm, F rascat i 2

Normal copper

Diffusion bonding and brazing of copper zirconium are being researched at SLAC.

Clamping Structure for testing copper alloys accelerator structure

•The clamped structure will provide a method for testing materials without the need to develop all the necessary technologies for bonding and brazing them.•Once a material is identified, we can spend the effort in processing it.•Furthermore, it will provide us the opportunity to test hard materials without annealing which typically accompany the brazing process

Clamped Structure

Test of Hard Copper

Hard Copper showed an observable improvements of annealed brazed structures

Clamped Structure with Hard Copper cells

High Gradient Structures--AAC 2010Page 13

8 0 9 0 10 0 11 0 1 20 1 30 1 40 1 5 01 0 7

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a 0.215, t 4.6mm, C lam p ed SLA C 1

a 0.215, t 4.6mm, F rascat i 2

Cryogenic RF material testing at SLAC

• Test bed for novel SRF materials– Finding materials with higher quenching RF magnetic field

• Leading to higher gradient in SRF accelerator structures• Samples in different forms, thin film or bulk, multilayer, etc

– Unique X-band system with compact size and short pulses, resulting lower pulsed heating

– Quick testing cycles with small samples – Surface resistance characterization

Cavity design

High-Q cavity under TE013 like mode

Q0,4K=~224,000Q0,290K=~50,000(measured from bulk Cu samples)

Fres, design=~11.399GHzFres, 290K=~11.424GHzFres, 4K=~11.46GHz

Q0,4K=~350,000(Estimated for zero resistivity samples, using measured Cu sample results)

Sample R=0.95”

Tc~3.6µs(using Q value for copper at 4K) Qe~310,000

• High-Q hemispheric cavity under a TE013 like mode

– Zero E-field on sample– Maximize H-field on the sample, peak

on bottom is 2.5 times of peak on dome– Maximize loss on the sample, 36% of

cavity total– No radial current on bottom

• Copper cavity body– Stable, no transition or quenching– Higher surface impedance– Coupling sensitive to iris radius

• Nb cavity body being designed– Lower loss for more accurate surface

impedance characterization– Qext is much higher with smaller iris

H E

Selected test results: MgB2 on Sapphire

0

5 104

1 105

1.5 105

2 105

2.5 105

3 105

3.5 105

4 105

0 5 10 15 20 25 30 35 40

300nm MgB2 thinfilm on SapphireQ vs T

H=10mT vs low power04082010

Q0 from scope w/ Qe_NWQ0(MgB2_LP_04062010_corr)

Q0

from

sco

pe w

/ Qe_

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TSample(K)

5 104

1 105

1.5 105

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2.5 105

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MgB2 thinfilm on SapphireQvsH

T=3K, 04082010

Q0 from scope w/ Qe_NW

Q0

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/ Qe_

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Hpeak from Pf/Ue(mT)

Experimental Evaluation of Magnetic Field role in Breakdown Rate

Experiments with short standing wave structures and specifically with structures where magnetic field is increased due input slots or field-confining rods (PBG) showed that magnetic field plays an important role in determining the gradient limit.

Before we studied effect of rf magnetic fields on rf breakdown high-magnetic-field and low-magnetic-field waveguide tests (V.A. Dolgashev, S.G. Tantawi, RF Breakdown in X-band Waveguides, EPAC02)

Here we suggest a test that separately controls electric and magnetic fields using the TE01 and the TM02 modes

A standing wave accelerator cell with iris dimensions similar to standing wave accelerator structure

Electric Field along the surface

TM02 Mode with resonance frequency 11.443GHz

Feed with TM01 mode converter

S. Tantawi

A standing wave accelerator cell with iris dimensions similar to standing wave accelerator structure

Feed with TE01 mode converter

Magnetic Field along the surface

TE01 Mode with resonance frequency 11.4244GHz

S. Tantawi

Rf Breakdown at Cryogenic Temperatures at ASTA

We plant to test hypotheses that connect statistical properties of rf breakdowns to dislocation dynamics in metals: this dynamics dramatically changes at cryogenic temperatures

Single-Cell-SW structure

TM01 input waveguide

S. Tantawi et al.

Cryostat

“Cold head” of refrigerator

• Crystal migration due to pulse heating― Interferometer― High resolution microscopy

• Pulse temperature measurement by High-Speed Radiation Thermometer

• Particles observation by Laser scattering

In-Situ Observation of Metal Surface (KEK, SLAC)

SW structure New pulse heating cavity

Future plans for ASTA• EPICS for remote monitoring and control• Spectrometer to measure gradient• Phase measurements and breakdown

localization• 24 hour unattended operation• Move cryostat to ASTA

Thanks for your attention