fCsCs22Te and NEA GaAs Te and NEA GaAs

photocathode activities at photocathode activities at FermilabFermilab

Raymond FlillerFNAL

Workshop on High Quantum Efficiency Photocathodes for RF guns

INFN Milano - LASA4-6 October 2006

f Past and Current ActivitiesPast and Current Activities A0 Photoinjector

Time Dependant QE with Cs2Te Secondary Emission studies

NML New photoinjector built to be an ILC cryomodule test

stand and AARD machine Polarized RF Electron Gun

New Preparation chamber PWT type gun Participation with BNL/AES/MIT on SRF gun

f A0 PhotoinjectorA0 Photoinjector Cs2Te photocathode 1.3GHz, 1.5cell normal

conducting RF gun 35MV/m cathode field 9 cell Tesla type

superconducting cavity, 12 MV/m accelerating gradient

15MeV beam energy 2.4ps laser pulse width 16 J/laser pulse 25-35 s RF pulse width 1 Hz rep rate

<5nC bunch charge (recently)

>1mA dark current

f A0 Preparation ChamberA0 Preparation ChamberCathode plane

f A0 Cathode Preparation ChamberA0 Cathode Preparation Chamber

The A0PI cathode preparation chamber is located inside of the cave.

The future upgrade/move to NML will have the preparation chamber will be of the Milano design.

Cesiation Chamber

Transfer chamber

Gun

f CathodeCathode Current cathode has been in operation since prior to October

2004 (nobody remembers when it was last inserted) Cathode plane shows groves that have been made since gun

was installed.

Images taken May 17,2006

HeNe spots

16 mm

f Time Dependant QE and Dark CurrentTime Dependant QE and Dark Current A0PI has a QE that

asymptotically increases or decreases with time depending on solenoid settings.

With all solenoids approximately the same current, Bz=0, the quantum efficiency rises about a factor of 2.

Other solenoid configurations show an asymptotic decrease in QE.

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

0.00 1.00 2.00 3.00time (hours)

Qua

ntum

Effi

cien

cy (%

)

0.00

0.20

0.40

0.60

0.80

1.00

Dar

k C

urre

nt (m

A)

QE - RoundBeamQE - Flat Beam

Idark - RoundBeamIdark - FlatBeam

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 5 10 15 20 25 30 35E (MV/m)

Dar

k C

urre

nt (m

A)

off - 0 h

round - 0 h

flat - 0 h

off - 12.5 h

round - 12.5 h

flat - 12.5 h

f Multipactoring in the A0 gunMultipactoring in the A0 gun A “multipactoring spike”

has been observed when the QE is increasing.

The theory is that multipactoring cleans the cathode surface, increasing the QE and dark current.

With multipactoring off, the contaminants settle back to cathode lowering them.

Photoelectrons

Dark Current“Multipactoring spike”

f Secondary EmissionSecondary Emission

0

5

10

15

20

25

0 60 120 180 240 300 360

Phase (deg)

Cha

rge

(pC

)

Simulation Pink (no secondary emission) Green (secondary emission)Data Blue

Studies at DESY show that secondary emission from the cathode can cause multipactoring in the RF gun (Han, Ph.D. thesis.

We have decided to try to understand the secondary emission from our cathode.

Below data taken at Ecath=15MV/m. Secondary emission is obvious.

Data/simulations by summer student Rob Inzinga

f Secondary EmissionSecondary Emission By comparing simulations of phase scans with

data for a variety of Ecath and solenoid configurations we plan to characterize the secondary emission characteristics of our cathode.

0

5

10

15

20

25

0 60 120 180 240 300 360

Phase (deg)

Cha

rge

(pC

)

0

5

10

15

20

25

0 60 120 180 240 300 360

Phase (deg)

Char

ge (p

C)

35.4 MV/m Round Beam 35.4 MV/m Flat Beam

f NMLNML A 750 MeV test accelerator will be built in a “building formerly known as”

the New Muon Lab (now just NML). 50 MeV injector Eventually 1 ILC RF Unit (1st cryomodule to arrive at NML in Summer 07)

1 RF unit = 3 cryomodules w/ 8 cavities each driven from 1 klystron ILC Bunch charge/length/train Gun Testing planned in the future. Also AARD machine in future.

f NML InjectorNML Injector Milano type cathode prep chamber (MOU is with the lawyers….) Modified FLASH type RF gun CC1 (presently in A0) and CC2 (presently at Meson lab) (Tesla type 9 cells) 3.9 GHz accelerating mode cavity for longitudinal Phase space linearization Bunch compressor 50 MeV dogleg for beam experiments (such as 3.9 GHz deflecting mode

cavity). To fit third cryomodule either the building needs to be extended or the

injector shortened.

f NML Cathode Prep chamber – Open questionsNML Cathode Prep chamber – Open questions

Base design is for Milano system.

Is there anything to be gained by moving the transverse stalk to the other side? Stalk limits minimum

distance from beam centerline to wall.

Need aisle on east side for servicing anyway, stalk is no problem there.

Does it really gain you anything? A small aisle is needed on

west side to insert cathodes and service croymodules on “backside”.

Is this really any smaller than what is shown?

f Polarized RF gunPolarized RF gun By using a strained GaAs semiconductor and using the

correct laser, polarized electrons can be produced from the cathode.

The QE is improve by coating the surface with a monolayer Cesium Fluoride

Figure taken from R.L. Bell, Negative Electron Affinity Devices

f Polarized RF gunPolarized RF gun Strained NEA GaAs photocathodes have been

used at SLAC and JLab for a number of years in DC guns.

Experiments at Novosibirsk (Alexandrov, EPAC98) showed that an NEA GaAs photocathode lasted in an RF gun only a few tens of RF pulses.

At low gradient the cathode could be reactivated.

At high gradient (>30MV/m) the cathode was damaged.

We would like to make an RF gun that will support a GaAs photocathode

f Cryogenically Cooled Normal conducting RF Cryogenically Cooled Normal conducting RF gungun

Using a spare 1.3 GHz gun, we cooled it using liquid N2 to attempt to improve the vacuum.

1.0E-15

1.0E-14

1.0E-13

1.0E-12

1.0E-11

0 10 20 30 40 50

Mass (AMU)

Part

ial P

ress

ure

(torr

)

WarmCold

H2

CO/N2

H2OCH4

O2 Ar

CO2

Results show that this does not work because the

gun is not cold enough. The gas migrates from the warm section to the cold producing a lower

pressure, lower temperature, higher

density gas than existed prior to cooling. Liquid

He is necessary to reduce the vapor

pressure of the residual gas to effectively

remove the gas from the volume.

RGA comparision of gun at 300K and 90 K. Little difference in spectrum.

f Major ChallengesMajor Challenges Vacuum – The NEA GaAs photocathode is

susceptible to surface damage from carbon compounds. SLAC DC gun operates at a H2 pressure <1e-11torr. The A0 RF gun operates at 1e-9 torr. How do we achieve lower pressure??

Ion backbombardment Simulations done at AES in collaboration w/FNAL

show that ions may not be such a large issue because the ion energy is limited to 14keV or so. This is much lower than in DC guns.

Electron backbombardment This issue is nonexistent in DC guns, and needs

study in RF guns.

f PWT GunPWT Gun

Schematic of an L-band, 1+2/2 cell, PWT polarized electron injectorBeing designed by DULY Research Inc.

Open RF structure allows use of NEG strips or SNEG coating outer vessel.

f PWT gunPWT gun

Operating the L-band PWT at a low peak field helps prevent backstreaming electrons emitted from the first PWT iris from

reaching the photocathode.

-

-

0

20

40

60

80

100

120

140

160

180

200

220

0.11 0.22 0.33 0.44 0.55 0.66 0.77 0.88 0.99 1.1 1.21 1.32Axial Distance from Iris Center (cm)

Thre

shol

d P

eak

Fiel

d on

Axi

s (M

V/m

)

i

0° rf phase at emission 90° rf phase at emission

FNAL/TESLA

1.6 cell gun

DULY L -band PWT

1.6 cell L-

band gun scaled from S

-band

f BNL/AES SRF GunBNL/AES SRF Gun FNAL is collaborating with BNL/AES/MIT Bates

on a half cell SRF gun. The vacuum environment should be good

enough to support at GaAs photocathode if sputtered Cs does not hurt the superconductor.

I’ll let the folks from BNL say more on this.

f GaAs Preparation ChamberGaAs Preparation Chamber We are in the process of

building a cathode preparation chamber for GaAs.

The main vessel was designed and built by AES.

It will initially be a stand alone chamber to gain experience with making bulk GaAs cathodes and initial survivability tests.

With a stalk replacement it can be fit to an RF gun.

Currently on the cusp of being assembled, and awaiting safety review for H2 and NF3.

H2 thermal gas cracker

Cs getter and collector bellows

Spare port for future load lock

500 L/s ion pump

f GaAs stalk designGaAs stalk design Mo cup with Mo cap GaAs soldered to cup

with indium Ceramic break to

measure charge emitted from cathode

Stalk heater (not shown) Stalk is only for initial

tests to gain experience. Adding a bellows type

actuator and additional stainless spool will allow for insertion into a gun

f ConclusionsConclusions Cs2Te research at FNAL focuses on

Time dependant QE Secondary Emission Multipactoring

Starting a program to produce an RF gun to support an NEA GaAs photocathode Prep chamber under construction Collaborating with BNL and Duly Research Inc, to

produce candidate guns with focus on• Producing a high vacuum environment during RF

operation• Attention to electron backbombardment issues