commissioning of the high current erl injector at cornellcommissioning of the high current erl...

FLS2010 Workshop, Stanford, March 1-5, 2010 Florian Loehl (Cornell University)

Commissioning of the High Current ERL Injector at Cornell

Florian Loehlfor the Cornell ERL team


photocathodeDC gun

buncher

cryomodule

beam dump

experimental beam lines

deflector

Design parameter:Nominal bunch charge 77 pCBunch repetition rate 1.3 GHzBeam power up to 550 kWNominal gun voltage 500 kVSC linac beam energy gain 5 to 15 MeVBeam current 100 mA at 5 MeV

33 mA at 15 MeVBunch length 0.6 mm (rms)Transverse emittance < 1 mm-mrad


Initial thermal emittance measurements

Measured normalized beam emittance at ~fC bunch charge (5 MeV):

0.2 to 0.4 mm mrad (in both planes)

Good agreement with predictions from thermal limit at cathode and utilized laser spot size.

Next step: optimizing injector for 77 pC operation• Implemented fast (~5 s), “single button” measurement• Will be used for parametric optimization of the injector


Comparison of beam measurements with simulations

Fixed slits phase space measurements• Corrector coils for beam scanning• 10 to 20 micron precision slits• 1 kW beam power handling

Good agreement with theory givesconfidence that the very small simulated emittances can be achieved.

From

Phy

s. R

ev. S

T-A

B 1

1, 1

0070

3 (2

008)

Beam properties at the cathode

Phase space measured after the beam was transported through theaccelerator.


Beam Orbit Control

Implemented global beam position feedback which uses all BPMs, corrector, and dipole magnets.


Transverse Deflecting Cavity

beam energy

time

streakedbeam

unstreakedbeam

Number of cavities 1 Max transverse kick voltage 200 kV Max RF power 3.8 kW Average power 200 W Pulse duration 60 µs Max rep. rate 1 kHz

Pumping port

Protrusion

Tuner and tuner mechanism

Water cooling channel

Input coupler

Beam pipe

Field probe


Initial high current run

0

2

4

6

8

10

12

10:00:00 PM 10:30:00 PM 11:00:00 PM

beam

cur

rent

(mA)

• Achieved maximum current is about 9 mA• Main limitations: - Gun high voltage instabilities

- Laser amplitude / position instabilitiesWork on solving the stability issues is in progress


Strange beam response of 250 kV beam

Steering the beam differently through the cryomodules changed the beam shape (no RF field in cavities)


Strange beam response

horizontal corrector (A)

verti

cal c

orre

ctor

(A)

verti

cal b

eam

pos

ition

(mm

)

horizontal beam position (mm)

cryomodule beam position measurement

correctorpair


Localizing stray fields in the cryomodule with DC coupler-kicks

horizontal position (mm)

verti

cal p

ositi

on (m

m)

electrical field vectors

+-

horizontal position

+ +

dipole-like field quadrupole-like fieldGeneration of electrical DC fields in the coupler regions of the SRF cavities



coupler 5coupler 4coupler 3coupler 2coupler 1

hor. corrector strength





vert.

cor

r. st

reng

th

deflection of the beam after the cryomodule due to the coupler field


correctorpair

dipole-like coupler kick



coupler 5coupler 4coupler 3coupler 2coupler 1


correctorpair

quadrupole-like coupler kick

deflection of the beam after the cryomodule due to the coupler field






vert.

cor

r. st

reng

th



3000 3500 4000 4500 50000.00

0.05

0.10

0.15

0.20

Center of 3rd HOM absorber

Coupler 2

Coupler 3

Coupler 4

Determined the origin of the stray fields:


In-situ demagnetization

Warm up and in-situ demagnetization removed stray fields


Stray fields

horizontal corrector (A)

verti

cal c

orre

ctor

(A)

verti

cal b

eam

pos

ition

(mm

)

horizontal beam position (mm)

• Stray fields reappeared after a beam loss in the cryomodule• Coupler conditioning changed the stray fields Charging up of HOM absorbers!


HOM absorbers

17

Cooling Channel (GHe)

Flange to Cavity

Flange to Cavity

RF Absorbing Tiles Shielded

Bellow

Total # loads 3 @ 78mm + 3 @ 106mmPower per load 26 W (200 W max)HOM frequency range 1.4 – 100 GHzOperating temperature 80 KCoolant He GasRF absorbing tiles TT2, Co2Z, Ceralloy

Antennas to study HOM spectrum


Charging up of HOM absorbers

Low conductivity of HOM absorber tiles: Can hold charge for many days / weeks! Worst offender: Ceramic 137Zr10, followed by ferrite Co2Z and TT2

Small beam loss charged up absorber tiles -> kV electric fields at beam position!

Opposite side Cu coated

Uncoated between electrodes

Opposite side Cu coated

Uncoated between electrodes

1.E-111.E-101.E-091.E-081.E-071.E-061.E-051.E-041.E-031.E-021.E-011.E+001.E+011.E+02

70 170 270 370 470

σ[1

/Ω/m

]

T [deg K]

TT2 #2TT2 #1TT2 #3Co2Z #2Co2Z #3Co2Z #1137Zr101 sec time const

>1 second time const


Removing inner tiles

Stress relief slots

Beamside tiles removed

Consequence of low resistivities of absorber materials:• Completely removed ceramic 137Zr10• Tried gold coating of TTE absorbers but coating may fall off Removed all tiles from the inside of the HOM absorber

Found one loose tile during cryomodule disassembly• Thermal stress tests confirmed

this problem Solved by cutting stress relief

slots in the tiles


Power limitations with modified HOM absorbers

2000 2500 3000 3500 4000 4500 5000100

102

104

106 X: 2630Y: 2.75e+005

R/Q

*Q [Ω

]

f [MHz]

X: 3751Y: 364.8

X: 3904Y: 692.3

100 mA, 1 kW limit

Blue: inside and outside ferritesRed: outside ferrites only


Power limitations with modified HOM absorbers

2000 2500 3000 3500 4000 4500 500010-4

10-3

10-2

10-1

100

X: 2630Y: 0.06555

f [MHz]

X: 3751Y: 0.6855

X: 3904Y: 0.06082

Pw

all /

Pto

tal

Blue: inside and outside ferritesRed: outside ferrites only

Power loss in the metal walls


Cavity Quality Factors

109

1010

0 2 4 6 8 10 12Eacc [MV/m]

Q

August 2008April 2009

Averaged quality factor Q0 vs. Eacc for all cavities together Had difficulties with low cavity

quality factors

• Q factors degraded over time Q disease?

During the rebuild, all cavities were high pressure rinsed Q restored to 1.6 x 1010 at 1.8 K no Q disease cavities were possibly contaminated with particles?


Current status of the injector cryomodule

• Cryomodule is rebuilt and back in the injector

• Cooled down to 4 K • 2 K cool-down planned for next Monday

Ready to see beam in the next weeks!


Conclusion and Outlook

Charging up of the HOM absorbers caused difficulties during the first commissioning Rebuild cryomodule during the last 6 months and

removed problematic absorbers

Still, many critical systems could be successfully commissioned and prepared.

• Measured thermal beam emittance as expected• Could increase beam current to 9 mA for short times

(limitations understood and being worked on)

First beam operation after cryomodule rebuilt expected end of March

• Emittance optimization at 77 pC• Work on high current beam operation

commissioning of the high current erl injector at cornellcommissioning of the high current erl...

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