Status and Recent Developments at the CTF3 Photoinjectors and Laser System

Christoph Hessler, Eric Chevallay, Steffen Doebert, Valentin Fedosseev, Irene Martini, Mikhail Martyanov

(CERN)

04 February 2014CLIC Workshop 2014, CERN

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 2

Photoinjectors at CTF3

04.02.2014

DRIVE beam MAIN beam

Electrons

PHIN CALIFEScharge/bunch (nC) 2.3 0.6

Number of subtrains 8 NANumber of pulses in subtrain 212 NA

gate (ns) 1272 20-150bunch spacing(ns) 0.666 0.666bunch length (ps) 10 10Rf reprate (GHz) 1.5 1.5

number of bunches 1802 32machine reprate (Hz) 5 5margine for the laser 1.5 1.5

charge stability <0.25% <3%QE(%) of Cs2Te cathode 3 0.3

Photoinjector laser lab (1st floor) and optical transfer

lines to PHIN and CALIFES

Dedicated photoemission laboratory in Bldg 101 for photocathode production,

testing and R&D

CALIFESMain beam photoinjector in

CLEX

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 3

News from CALIFES

High availability for daily operation in 1st half of 2013. Charge requirements fulfilled despite of some degradation of laser beam

transmission caused by dust on optical surfaces. CALIFES not operational since CTF3 restart in October

Failure of the photocathode manipulator Manipulator exchange required bake-out of the ultra-high vacuum system, a

re-conditioning of the gun and a preparation of a new photocathode. Work is in progress.

1st major downtime since its commissioning in 2008→ CALIFES still very reliable

Upgrade of laser control and diagnostics computers Single-pulse picker installed and ready to be tested with beam.

04.02.2014

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 4

PHIN Layout

FCT: Fast current transformerVM: Vacuum mirror SM: Steering magnet BPM: Beam position monitorMSM: Multi-slit Mask OTR: Optical transition radiation screenMTV: Gated cameras SD: Segmented dump FC: Faraday cup

04.02.2014

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 5

Motivation for a Drive-Beam Photoinjector

A conventional system (thermionic gun, sub-harmonic buncher, RF power sources) is not necessarily more reliable than a photoinjector. At CTF3 e.g. the availability of the CALIFES photoinjector is high.

Present system (thermionic gun, sub-harmonic buncher) generates parasitic satellite pulses, which produce beam losses. Reduced system power efficiency Radiation issues due to the beam losses of the satellite pulses

These problems can be avoided using a photoinjector, where only the needed electron bunches are produced with the needed time structure.→ Has been demonstrated for the phase-coding in 2011.

04.02.2014

M.Csatari Divall et al., “Fast phase switching within the bunch train of the PHIN photo-injector at CERN using fiber-optic modulators on the drive laser”, Nucl. Instr. And Meth. A 659 (2011) p. 1.

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 6

Main Challenges for Drive Beam Photoinjector

04.02.2014

High macro pulserep. rate (50 Hz)*

High bunch charge (8.4 nC)*

High average current (30 mA)*

Long train lengths (142 µs)*

High bunch rep. rate (500 MHz)*

High charge stability (<0.1%)*

Long cathode lifetimes (>150 h)*

Must be achieved at the same time!

Cs2Te cathodes UV laser beam Crystaldamage

Cs3Sb cathodes Green laser beamOK

Can in principal be achieved individually with a photoinjector

DurabilityLaser rods?

* CLIC parameters

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 7

Strategy to Overcome the Challenges

04.02.2014

High macro pulserep. rate (50 Hz)*

High bunch charge (8.4 nC)*

High average current (30 mA)*

Long train lengths (142 µs)*

High bunch rep. rate (500 MHz)*

High charge stability (<0.1%)*

Long cathode lifetimes (>150 h)*

Must be achieved at the same time!

Cs3Sb cathodes Green laser beamOK

DurabilityLaser rods?

Vacuum improvement

Feedback stabilisation

New CLIC type front end

* CLIC parameters

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 8

Improvement of Vacuum in PHIN

04.02.2014

1/e lifetime 185 h

1 nC, 800 ns, l=524 nm, Cs3Sb

1/e lifetime 26 h

1 nC, 800 ns, l=262 nm, Cs3Sb

7e-10 mbarDynamic

vacuum level:4e-9 mbar

March 2012March 2011

Activation of NEG chamber

around gun

Installation of additional

NEG pump<2e-10 mbar

July 2013

1/e lifetime n/adue to differentproblems duringPHIN run

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 9

Problems with Lifetime Measurements

Despite good QE in DC gun, photocathodes had low QE on arrival in PHIN gun

Despite good dynamic base vacuum level there were a lot of breakdowns causing desorption spikes:

24h oscillations on beam current Rapidly decreasing QE, (partially) recovering during “QE jumps”

04.02.2014

Cathode #189 (PHIN run 2012) Cathode #191 (PHIN run 2013)

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 10

QE Jumps

04.02.2014

Cathode #191, Cs3Sb, train length 800 ns, bunch charge 1 nC

Sudden improvement of quantum efficiency(“QE jumps”)

→ Not yet understood!

Laser energy was regulated to keep the charge constant at 1 nC

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 11

Some Explanations

Low initial QE and the breakdowns can be partially explained by the fact that Cu plugs of a different batch were used (Photos taken after usage in PHIN):

24h oscillations probably due to non-working air conditioning in klystron gallery → Phase shift RF reference signal for laser

04.02.2014

Cathode #193 (PHIN run 2013)Surface treatment: Diamond turning

Cathode #189 (PHIN run 2012) Surface treatment: Diamond powder polishing

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 12

Operation with Long Trains

In 2012 measurements with 350 ns and 1200 ns train length were taken, yielding similar lifetimes:

However, due to low QE of photocathodes and other problems these measurements were not possible during last PHIN run.

Beam intensity of 5 µs trains is too high for present Faraday cup.

04.02.2014

2.3 nC, 350 ns,l=524 nm

2.3 nC, 1200 ns, l=524 nm

1/e lifetime 168 h 1/e lifetime 135 h

2.3 nC, 5000 ns

lifetime ?

Cs3Sb Cs3Sb

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 13

Faraday Cup for Long Trains

In 2012 strong pressure increase due to Faraday cup heating was observed for relatively high beam intensity.

Beam power for 5 µs trains with 5 Hz rep rate and 2.3 nC/bunch: 430 W → too much for uncooled FC!

Solution: FC in air + vacuum window Only possible material for full intensity:

Beryllium However, Be must be supported by

carbon-composite plate, which might be harmful for photocathodes (outgassing due to beam heating?)

Solution for intermediate intensities: Stainless steel window + IR camera

04.02.2014

Cathode #189 (PHIN run 2012)

Bunch charge 2.3 nCTrain length 1000 ns

Courtesy:M. Delonca

Pressure increase to 2e-7 mbar

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 14

Measurement in DC gun with 1 kHz laser beam and Cs3Sb cathodes :

Total integrated charge produced: 321 mC (cathode #188), 33 C (cathode #192) For low charge lifetime is 3 times as long as in PHIN with same average current. For high charge vacuum is still good (<8e-11 mbar) but the lifetime is as short as

in PHIN under 4e-9 mbar. → Possible reason: Ion back-bombardment in DC gun Maybe short lifetime of cathode #192 is due to the very thin photo-emissive layer

(13 nm), compared with 170 nm (#188) and 79 nm (#189, PHIN) More measurements needed.

High Charge Studies in Photoemission Lab

04.02.2014

1 µA average current, 1 nC/bunch 120 µA average current, 120 nC/bunch

p < 8e-11 mbar, p < 1e-11 mbar,

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 15

New Cathode Transfer Arms

Two transfer arms built in collaboration with LAL, Orsay. Transfer arm for XPS analysis

LAL transfer arm

04.02.2014

Cathode transfer under UHV from the photoemission lab to the XPS setup.

Surface analysis to get a better understanding of production and degradation process.

New off axis cathode holder will be machined for complete compatibility with XPS setup.

Up to 4 cathodes can be transferred to LAL after being produced in the CERN photoemission lab.

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 16

Feedback Stabilisation Scheme

04.02.2014

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 17

Feedback Stabilisation Tests

04.02.2014

-20 -15 -10 -5 0 5 10 15 200

500

1000

1500

2000

2500

%

Laser Energy distribution, FWHM = 4.92 %, rms = 2.09 %

fitted curve

-20 -15 -10 -5 0 5 10 15 200

200

400

600

800

1000

1200

1400

1600

1800

%

Bunch charge distribution, FWHM = 7.14 %, rms = 3.03 %

fitted curve

-20 -15 -10 -5 0 5 10 15 200

200

400

600

800

1000

1200

1400

%

QE distribution, FWHM = 9.19 %, rms = 3.90 %

fitted curve

Cathode #193Feedback OFF

-5 -4 -3 -2 -1 0 1 2 3 4 50

50

100

150

200

250

300

350

400

450

%

Laser Energy distribution, FWHM = 2.07 %, rms = 0.88 %

fitted curve

-5 -4 -3 -2 -1 0 1 2 3 4 50

50

100

150

200

250

300

350

400

450

%

Bunch charge distribution, FWHM = 2.27 %, rms = 0.97 %

fitted curve

-5 -4 -3 -2 -1 0 1 2 3 4 50

50

100

150

200

250

300

350

%

QE distribution, FWHM = 3.03 %, rms = 1.29 %

fitted curve

Cathode #188Feedback ON

-5 -4 -3 -2 -1 0 1 2 3 4 50

500

1000

1500

2000

2500

x

y

FB OFF, Output signal distribution, FWHM = 3.17 %, rms = 1.34 %

fitted curve

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.50

0.5

1

1.5

2

2.5

3x 10

4

x

y

FB OFF, Signal channel noise distribution, FWHM = 0.31 %, rms = 0.13 %

fitted curve

-5 -4 -3 -2 -1 0 1 2 3 4 50

2000

4000

6000

8000

10000

12000

14000

x

y

FB ON, Output signal distribution, FWHM = 1.00 %, rms = 0.43 %

fitted curve

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.50

0.5

1

1.5

2

2.5

3

3.5x 10

4

x

y

FB ON, Signal channel noise distribution, FWHM = 0.28 %, rms = 0.12 %

fitted curve

Feedback OFF1.34 % rms

-5 -4 -3 -2 -1 0 1 2 3 4 50

500

1000

1500

2000

2500

x

y

FB OFF, Output signal distribution, FWHM = 3.17 %, rms = 1.34 %

fitted curve

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.50

0.5

1

1.5

2

2.5

3x 10

4

x

y

FB OFF, Signal channel noise distribution, FWHM = 0.31 %, rms = 0.13 %

fitted curve

-5 -4 -3 -2 -1 0 1 2 3 4 50

2000

4000

6000

8000

10000

12000

14000

x

y

FB ON, Output signal distribution, FWHM = 1.00 %, rms = 0.43 %

fitted curve

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.50

0.5

1

1.5

2

2.5

3

3.5x 10

4

x

y

FB ON, Signal channel noise distribution, FWHM = 0.28 %, rms = 0.12 %

fitted curve

fact

or 3

im

prov

emen

t !3.03% rms 0.97% rms

Feedback ON0.43 % rms

Lase

r bea

mE

lect

ron

beam

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 18

Upgrade of the PHIN Laser System

Current laser setup: Common 1.5 GHz, 10 W HighQ front-end Simultaneous laser operation of CALIFES and PHIN photoinjectors is possible

(with limitations)

04.02.2014

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 19

Upgrade of the PHIN Laser System

04.02.2014

Future laser setup: CALIFES and PHIN lasers are completely decoupled Simultaneous laser operation of CALIFES and PHIN photoinjectors PHIN laser timing parameters pushed towards CLIC requirements due to new

front-end ordered from OneFive company.ORDERED

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 20

Stability Tests of OneFive Test System

400 MHz demo oscillator provided by OneFive for testing. 400 000 pulses in the train captured and processed on the pulse-to-pulse basis

04.02.2014

0 1 2 3 4 5 6 7-0.5

0

0.5

1

1.5

2

2.5

3

3.5x 10

4 EOT ET-3010, single trace 20us, 60Gsa/s

t, ns

PhD

Sig

nal,

a.u.

noisesignalsignal

0 50 100 150 200 250 300 350 400-120

-100

-80

-60

-40

-20

0

20EOT ET-3010, single trace 20us, 60Gsa/s

q, MHz

Spe

ctru

m a

mpl

itude

, a.u

.

noisesignal

Real time signal digitized with 60 GSa/s

FFT spectrum in 0 to 400 MHz interval

Detected noise 60dB below carrier !

-2 -1.5 -1 -0.5 0 0.5 1 1.5 20

2000

4000

6000

8000

10000

12000

14000

16000

18000

dCCFmax, %

Num

ber o

f pul

ses

EOT ET-3010 CCFmax distribution

Gaussian fit FWHM = 0.95 %, RMS = 0.40 %

fitted curve

Pulse-to-pulse energy distribution

0.4% rms !

-20 -15 -10 -5 0 5 10 15 200

50

100

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300

350

400

450

500

dE, %

Num

ber o

f pul

ses

EOT ET-3010 Energy distribution, 100ns long train averagingGaussian fit FWHM = 0.209 %, RMS = 0.089 %

fitted curve

Same conditions as for HighQ measurement:→ OneFive: 0.09% rms

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 21

Plans for PHIN, Photoemission Lab and Laser System

PHIN: Excellent vacuum conditions have been achieved

→ Lifetime studies under these conditions with Cs3Sb and Cs2Te cathodes Study the effect of surface roughness on cathode lifetime

→ Electro-polished cathode plugs Continue studies of Cs3Sb cathodes using a green laser beam Studies with 5 µs long pulse trains and 5 Hz repetition rate

→ Be window needed → separate PHIN run required Photoemission lab: Continue high-charge studies (with different cathode layer thicknesses) XPS studies Upgrade of cathode preparation system with load-lock systemLaser: Study of performance of laser system with CLIC specs (new 500 MHz front end,

50 Hz tests) Continue stability studies with new front end

04.02.2014

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 22

Future Photoinjector Activities for CLIC and at CERN

CTF3 operation will stop in 2016 → PHIN program will also stop. Logical continuation of PHIN program would be an R&D program for a new 1 GHz

RF photoinjector with full CLIC specs. Due to the budget situation, this is currently not foreseen. But the photoinjector activies might continue in the following projects:

CALIFES might be used at a new facility in CLEX PHIN or a new photoinjector is planned to be used at the AWAKE facility

However, these two facilities do not require photocathode R&D.→ High risk to lose the expertise in photocathode R&D at CERN!

Possibility to send cathodes to LAL and perform experiments there.→ However, only single bunch operation possible, not suitable for solving the problems for the CLIC drive beam photoinjector.

Possibility to focus on photocathodes for polarized electrons for CLIC / ILC→ However, production technique totally different from Cs2Te/Cs3Sb photocathodes and risk to lose expertise still present

Without an RF photoinjector available for photocathode R&D, the risk to lose the expertise in photocathode production and R&D at CERN is very high!

04.02.2014

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 23

Acknowledgement

Controls: Sergio Batuca, Mark Butcher, Mathieu Donze, Alessandro Masi, Christophe Mitifiot

Load-lock system: Szymon Sroka FLUKA simulations: Markus Brugger, Melanie Delonca Beam instrumentation: Benoit Bolzon, Thibaut Lefevre Vacuum: Paolo Chiggiato, Berthold Jenninger, Esa Paju RF: Stephane Curt, Luca Timeo Wilfrid Farabolini XPS studies: Holger Neupert, Elise Usureau Collaborators at LAL and IAP-RAS … and many others

… and thank you for your attention!04.02.2014

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 2404.02.2014

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 25

PHIN and CLIC Parameters

Parameter PHIN CLICCharge / bunch (nC) 2.3 8.4Macro pulse length (μs) 1.2 140Bunch spacing (ns) 0.66 2.0Bunch rep. rate (GHz) 1.5 0.5Number of bunches / macro pulse 1800 70000Macro pulse rep. rate (Hz) 5 50Charge / macro pulse (μC) 4.1 590Beam current / macro pulse (A) 3.4 4.2Bunch length (ps) 10 10Charge stability <0.25% <0.1%Cathode lifetime (h) at QE > 3% (Cs2Te) >50 >150

Norm. emittance (μm) <25 <100

04.02.2014

26

24h Oscillations on Beam Current

04.02.2014 C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

Klystron phase was kept constant

Beam current measured with the Faraday cup:

24h oscillations probably caused by non-working air-conditioning in klystron gallery:→ Temperature variation of RF reference signal cable for laser→ Phase shift of laser with respect to klystron