Radiation Source

Rossendorf SRF-Gun Cavity CharacteristicsRossendorf SRF-Gun Cavity Characteristics

Member of the Leibniz Association Bautzner Landstr. 128 01328 Dresden/Germany http://www.fzd.de Contact: André Arnold Institute of Radiation Physics, Radiation Source ELBE Email: a.arnold@fzd.de Phone: +49 351 260 - 3382

IntroductionIntroduction

We acknowledge the support of the European Community-Research Infrastructure Activity under the FP6 “Structuring the European Research Area” programme (CARE, contract number RII3-CT-2003-506395) and the support of the German Federal Ministry of Education and Research grant 05 ES4BR1/8.

AcknowledgementAcknowledgement

A. Arnold1, H. Buettig1, D. Janssen1, U. Lehnert1, P. Michel1, K. Moeller1, P. Murcek1, Ch. Schneider1, R. Schurig1, F. Staufenbiel1, J. Teichert1, R. Xiang1, A. Matheisen2, B. v. d. Horst2, J. Stephan3, T. Kamps4, V.

Volkov5

1) FZ Dresden Rossendorf, Dresden, Germany 2) DESY, Hamburg, Germany3) IKS, Dresden, Germany4) BESSY, Berlin, Germany5) Budker Institute of Nuclear Physics, Novosibirsk, Russia

• detuning measured by demodulated LL phase controller signal

• FFT using one minute time signals show discrete PSD (power spectral density)

• strong damping of the membrane pumps and its vacuum tubes was needed to achieve stable cavity operation

Detuning 3-times higher than TESLA 9-cell cavities – but still sufficient for CW

• the electric & magnetic surface field causes a Lorentz Force pressure on the cavity wall

• the induced neg. frequency shift is proportional to the square of gradient 2

peakf k E

0 50 100 150 200 250 300Frequency [Hz]

1.E-07

1.E-06

1E-05

0.0001

0.001

0.01

0.1

1

PS

D [

deg

²/H

z]

0.00025

0.0005

0.00075

0.001

0.00125

0.0015

0.00175

0.002

inte

gra

ted

PS

D [d

eg

²]

tuning lever3-cell cavity tuning lever

1/2-cell cavity

tuning spindle

stepping motor & gearbox

tuning span kHz ± 78 ± 225

mech. resolution

nm/step

1.3 2.1

frequency resolution

Hz/step

0.3 0.9

stored energy U 32.5 J

quality factor Q0 1010

dissipated power Pc 25.8 W

maximum beam power PB 9.4 kW

geometry factor G 241.9 Ω

accel. voltage Vacc

accel. gradient Eacc

9.4 MV18.8 MV/m

shunt impedance Ra=Vacc²/2Pc 1.72x1012 Ω

Ra/Q0 166.6 Ω

Epeak/Eacc 2.66

Bpeak/Eacc 6.1 mT/(MV/m)

½ cell tuner

TESLA cell tuner

main coupler

e--beam

cathode

sc choke filter(to prevent RF leakage)

half Cell

FZD coupler& pickup ant.

2 HOM coupler3 TESLA cell

Under consideration of cooldown shrinking, change of permittivity & final BCP cleaning, the field distribution of the operated cavity inside the module can be calculated using the fundamental mode passband frequencies at 2K and the last known cavity state before final cleaning.

The SRF-Gun development at FZD has made lot of progress. The installation of the injector module next to the linear accelerator ELBE was finished in September 2007. The cavity characteristics measured during the following commissioning are presented here.

On-Axis Field DistributionOn-Axis Field Distribution

• 9th test after high power processing (HPP) up to Epeak=25MV/m results in

stable CW operation up to 17.6MV/m, above this level cavity starts to quench

• maximum achievable field only 1/3 of the designed value of Epeak=50MV/m

• measured Q0 is 10

times lower than in all vertical tests

• no Q degradation found during the 1st year of operation

• first eight Q vs. E measurements limited by FE & He consumption

QQ00 vs. E vs. Epeakpeak

Tuning SystemTuning System

MicrophonicsMicrophonics

Microphonics (closed loop σ = 0.055° rms) comparable to ELBE & sufficient for CW

CW Lorentz Force DetuningCW Lorentz Force Detuning

The resolution and the tuning span of both tuners are sufficient but the hysteresis has to be improved within the next maintenance!

• pressure sensibility (230Hz/mBar) is seven times higher than known from ELBE modules which is probably caused by different cavity design (weak half cell)

• no problem seen for the LL controller and max. available RF Power due to helium pressure stability of 0.1mbar

Pressure SensibilityPressure Sensibility

dampedmembrane

pumps Heliumrefrigerator

63.1%

100%

96.6%

97.0%

electrical field distribution along cavity axis for Ez,max=50MV/m

desired field flatness: [0.6 - 1 – 1 – 1]