Non-conservation of the charge lifetime at high average current

Non-conservation of the Non-conservation of the charge lifetime at high charge lifetime at high average currentaverage current

R.Barday

University Mainz, Germany

INFN Milano-LASA

4-6 October 2006

05.10.06 2

Many accelerator facilities use GaAs photocathodes to produce

a) polarized electron beam:

SLAC, CEBAF, MIT-Bates, ELSA, MAMI

Iaver~100 A, high polarization

b) unpolarized electron beam for Light Sources:

JLab FEL, Cornell ERL

Iaver~10 mA, no polarization

With new projects such as EIC or antiprotonpolarization, increase the demant of much higher average current of polarized electron beam.

Iaver~(10-100) mA

05.10.06 3

Photoemission from GaAsPhotoemission from GaAs

EF

EG

ECB

EVB

EVAC

EF

EG

ECB

EVB

E

z

EVAC

EA

VBB

d Cs+

O2(

NF

3)

bare GaAs: emission is not possible p-dopped GaAs+(Cs+O2): emission is possible

GaAs photocathode is activated by exposure of the monolayer of caesium and an oxidant to the clean semiconductor surface.

1) Exciting of electrons from the VB to the CB with circularly polarized light

2) Lowering of the work function with Caesium and an oxidant (O2, NF3)

05.10.06 4

During operation at low average current we found out two lifetime limitations:

1) Chemical poisoning of the photocathode surface by residual gas. Oxidizing gas species as H2O, O2 and CO2 decrease QE drastically. H2, CH4 or CO do not affect the Cs/O activation layer.

Chemical poisoning degrades the cathode QE uniformly

2) Ion back-bombardement. Residual gas molecules are ionized by electrons and are accelerated towards the photocathode, causing photocathode damage. This effect is proportional to pressure and to the average current.

Ion back-bombardment degrades the cathode QE between the illuminated laserspot and the electrostatical centre.

The lifetime of the GaAs photocathode is a major issue, because of high sensivity to the vacuum environment.

05.10.06 5

We assume that all processes destroying Cs/O layer act parallel and independently.

?)...1

(111

IFENeut

The goal of our experiments is to explore, how relevant our lifetime measurement is at low average current for operation at mA average current, i.e. whether the charge lifetime I is inversely proportional to the beam current (i*I=const?).

Lifetime operation under different Lifetime operation under different operation conditionsoperation conditions

Neut no operation Residual gas

FE high voltage is on

1+field emission

I beam current (1+2)+additional back streaming+ additional ion back bombardement

At high average current 1/I>>1/Neut+1/FE and ~I

05.10.06 6

Polarized electron sourcePolarized electron source

VERDI V5

NEG500l/s

NEG200l/s

Beam Dump

NE

G20

0l/s

IGP

150 cm90 c

m

JBLNEG200l/s

NE

G20

0l/s

80 cm

apertured=10 mm

100kV DC gunE=0.9MV/m

Insu

lato

r

05.10.06 7

Water-cooled Faraday cup with NEG

Polarized electron sourcePolarized electron source

Beam

Water

Water Copper

05.10.06 8

GaAs PhotocathodeGaAs PhotocathodeDopant Zink

Orientation 100

Thickness, Thickness, mm 350

Carrier Carrier concentration, cmconcentration, cm-3-3

2*1019

Polarization, %

(at =808 nm)

25

QE, %

(at =808 nm)

3

WAFER TECHNOLOGY LTD.

1) a lot of cheaper

2) similar operating conditions to highly polarized photocathode

05.10.06 9

FFiber iber AArray rray PPackage (FAP) Laserackage (FAP) Laser

Power, W 15

Wavelength, nm 808 (fixed)

Pulse length, ms 0.1-10

Frequency, Hz 100

Beam divergence, N.A.

0.16

VERDI: Power 5 W, =532 nm

05.10.06 10

Optical excitation process

For low energy EG<h<E+ all excited electrons are thermalized into the -minima.

For higher photon energy the electrons are thermalized into the L-minima.

For photon energy above 1.9 eV the electrons are scattered into the vicinity of the X-CB minima and thermalized there.

-valley

L-valley

X-valley

k

Ex

E

EL

E=1.42 eVEL=1.71 eVEx=1.90 eV=0.34 eV

Lifetime of polarized and unpolarized Lifetime of polarized and unpolarized Electron Beam.Electron Beam.

The hotter photoelectrons will be less sensitive to change in the work function.

05.10.06 11

Ered~1.53 eV (polarization)

Egreen~2.33 eV (no polarization)

EF

ECB

EVB

E

z

eff

dgreen~0,15m

dred~0,9m

A low energy cutoff for cold electrons due to the rise of the vacuum level.

Lifetime of polarized and unpolarized Lifetime of polarized and unpolarized Electron Beam.Electron Beam.

The lifetime at 532 nm is at least factor 4 better than at 808 nm.

05.10.06 12

Cathode heating

In order to achieve mA beam current, the laser power should be increased up to several tens (hundreds) mW. Most of the applied energy (~70%) will be absorbed in the GaAs photocathode, causing heating of the photocathode.

What happens at high temperature?

1) Decompasition of the Caesium-Oxide activation layer at high temperature

2) Thermally induced chemical reaction

3) The energy gap decreases with increasing temperature, which increases the escape probability of the electrons in the vacuum:

T

TETE gg

2

0

T(T(00K)K) EEgg(eV)(eV)

00 1,521,52

300300 1,421,42

350350 1,41,4

Non-linear effectsNon-linear effects

05.10.06 13

Non-linear effectsNon-linear effectsCathode heating

0

0,2

0,4

0,6

0,8

1

1,2

0 50 100 150 200 250

Laser power, mW

Vac

uum

life

time

Photocathode vacuum lifetime normalized to the vacuum lifetime at the laser power 23 mW.

We are here at

I=1mA (QE=20mA/W)

05.10.06 14

Cathode heating

GaAs

Laserthermal contact between cathodeand holder

Spring

GaAs photocathode in holder

The temperature was measured by a photoluminescence technique.

The thermal coefficient is about 0.4 K/mW.


To ensure a operation of the electron gun with high average current (strong laser illumination), cooling of the GaAs photocathode is required!!!

For example TSR 10K/W

0.4K/mW*100mW=40K

05.10.06 15

Ion trapping


The electron beam collides with residual gas molecules are producing positive ions. A negatively charged electron beam can capture positive ions, if the potential well is higher as the energy of ions.

The electrostatic potential of the electron beam with radius a and a uniform charge density which propagate in a vacuum tube of radius r0:

00

02

2

0 ,ln

0,ln2

1

22 rra

r

r

ara

r

a

r

c

IV

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

0 5 10 15 20

Distance from the center of the beam, mm

Pote

ntia

l, m

V

I=1mA

I=5mA

Potential well for different beam current. Ee=60 kV, beam diameter 4 mm.

05.10.06 16

Ion trapping leads to an increased flow of ions towards the photocathode. But it is possible to remove these ions from the gun by suppressing the ion flow with a repeller behind the anode of the gun at a positive potential.

Non-linear effectsNon-linear effectsIon trapping

0

0,2

0,4

0,6

0,8

1

1,2

0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5

Time, hr

Bea

m c

urre

nt, m

A

U=+65V

U=-65V

VERDI V5

NEG500l/s

NEG200l/s

Beam Dump

NE

G20

0l/s

IGP

150 cm90 c

m

JBLNEG200l/s

NE

G20

0l/s

80 cm

apertured=10 mm

100kVDC gun

Insu

lato

r

U

05.10.06 17

An increase of relative transmission loss caused by space charge forces at high average current


2,5

3

3,5

4

4,5

5

5,5

0,0 0,5 1,0 1,5 2,0

Time, hr

Bea

m c

urre

nt,

A

beam was dumped in the vacuumchamber of the alpha-magnet

beam was dumped in the Faraday Cup

ESD can lead to vacuum degradation during operation with beam.

Important issue for obtaining sufficient long lifetime is the precise control of electrons which leave cathode.

A beam loss of 4 A located 1 m from the source limits the lifetime to 3 hours.

At 10 mA average current:

4A/10mA=4*10-4!!!

05.10.06 18

An increase of relative transmission loss caused by space charge forces at high average current


Beam loss:

1) between gun and -magnet: <10-7 (was measured at 2,5 mA)

2 -magnet: ~10-6 at 1 mA

dlaser=2.1mm

dbeamline=(28-38)mm

0,0E+00

5,0E-07

1,0E-06

1,5E-06

2,0E-06

2,5E-06

3,0E-06

3,5E-06

4,0E-06

4,5E-06

5,0E-06

0 0,5 1 1,5 2 2,5

Beam current, mA

Rel

ativ

bea

m lo

ss

NEG200l/s

NEG200l/s

Isolated

to the Faraday Cup

05.10.06 19

Linear effectLinear effectBackstream gases from the beam dump

The basic pressure inside the Faraday cup itself is 6*10-10 mbar (which was measured with vacuum gage). The Faraday cup is located 2.5 m from the gun.

The gun vacuum is isolated from the vacuum in the beam dump by differential pumping, BUT gases can reach the photocathode from higher pressure regions (Beam Dump).

An increase of the pressure at Faraday cup of the order of 10-8 mbar/mA is observed.

Our photocathode lifetime is almost completely dominated by neutral molecules (from the beam dump) and not by ion back bombardement.

720~2

2..

r

Rl

S

S

beam

CF

Cu+

NE

GH2

eeCO2

eeCH4

CO2

H2OCO

H2

05.10.06 20


We made three observations which can tend to a not proportional to the electron current decrease of lifetime.

1) Thermal heating of the photocathode. (not yet)

2) Ion trapping in the beam line. (yes)

3) Increase of relative transmission loses caused by the space charge forces at high current. (is under control)

4) Backstream gases from the Faraday cup (soon)Cleaning of the Faraday cup is „slow“.

Solution: cleaning with thermal cathode.

Our experiments reveal that nonlinear effects exist.

05.10.06 21

New projects require electron beam with average current of (10-100) mA and peak current of order (1-10) A. The feasible way to attain so high current is through beam recirculation.

Advantages:1) Dissipated Energy is lower2) ESD is lower3) High Voltage Supply

Beam CollectorBeam Collector

VERDI V5

NEG500l/s

NEG200l/s

Beam Dump

NE

G20

0l/s

IGP

150 cm90 c

m

JBLNEG200l/s

NE

G20

0l/s

80 cm

apertured=10 mm

100kV DC gunE=0.9MV/m

Insu

lato

r

HV: 100kV

3kV

05.10.06 22

Beam CollectorBeam CollectorCharge saturation

Charge accumulation at the surface of the photocathode.

EVAC

ECB

EVB EVB

ECB EVAC

PVlow intensity light

high intensity light

+P

V

Jgen

Jemit

Jsurf

Jrec

VBB

05.10.06 23


In order to overcome charge saturation problem, two conditions should be satisfied:

1) high escape probability of electrons through the potential barrier (high NEA (QE))

2) high probability of holes to overcome the surface band bending region.

The thickness of the BBR: pe

Vd BBr

202

(heavily p-doped)

Charge saturation

05.10.06 24


Cathode peak photocurrent vs. laser power with laser spot size 2.1 mm. The cathode was biased at -60 kV. Dopant concentration

2*1019cm-3.

Current density is presently limited to 1.6 A/cm2.

57 mA in 100 s long pulses at 100 Hz repetition rate.

Q=5.7 C per Impulse

emitted area *(1.05mm)2~3.5 mm2

hole concentration 2*1019 cm-3

Charge saturation

05.10.06 25

SummarySummaryWe made four observations which can tend to decrease of lifetime.

1) Ion trapping in the beam line.

2) Increase of relative transmission loses caused by the space charge forces at high current.

3) Backstreamgases from the Faraday cup.

4) Thermal heating of the photocathode.

Our experiments reveal that nonlinear effects exist, but it is possible to keep them under control.

We have already demonstrated 11.4 mA average current of polarized electron beam and 57 mA in 100 s peak current (charge 5.7 C, current density 1.6 A/cm2).

Non-conservation of the charge lifetime at high average current

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