Non conservation of the charge lifetime at high average current
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Non-conservation of the charge lifetime at high average current. R.Barday University Mainz, Germany. INFN Milano-LASA 4-6 October 2006. Many accelerator facilities use GaAs photocathodes to produce a) polarized electron beam: SLAC, CEBAF, MIT-Bates, ELSA, MAMI

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Non-conservation of the charge lifetime at high average current

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Non conservation of the charge lifetime at high average current

Non-conservation of the charge lifetime at high average current

R.Barday

University Mainz, Germany

INFN Milano-LASA

4-6 October 2006


Non conservation of the charge lifetime at high average current

Many accelerator facilities use GaAs photocathodes to produce

a) polarized electron beam:

SLAC, CEBAF, MIT-Bates, ELSA, MAMI

Iaver~100 mA, 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


Photoemission from gaas

Photoemission from GaAs

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)

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.


Non conservation of the charge lifetime at high average current

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

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

  • 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

  • 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.


Non conservation of the charge lifetime at high average current

Lifetime operation under different operation conditions

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

At high average current 1/tI>>1/tNeut+1/tFE and t~tI

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 tI is inversely proportional to the beam current (ti*I=const?).


Non conservation of the charge lifetime at high average current

Polarized electron source


Non conservation of the charge lifetime at high average current

Polarized electron source

Water-cooled Faraday cup with NEG


Gaas photocathode

GaAs Photocathode

1) a lot of cheaper

2) similar operating conditions to highly polarized photocathode

WAFER TECHNOLOGY LTD.


F iber a rray p ackage fap laser

Fiber Array Package (FAP) Laser

VERDI: Power 5 W, l=532 nm


Non conservation of the charge lifetime at high average current

Lifetime of polarized and unpolarized Electron Beam.

For low energy EG<hn<EG+D all excited electrons are thermalized into the G-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.

Optical excitation process

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


Non conservation of the charge lifetime at high average current

Lifetime of polarized and unpolarized Electron Beam.

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

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

Ered~1.53 eV (polarization)

Egreen~2.33 eV (no polarization)


Non linear effects

Non-linear effects

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?

  • Decompasition of the Caesium-Oxide activation layer at high temperature

  • Thermally induced chemical reaction

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


Non linear effects1

Non-linear effects

Cathode heating

We are here at

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

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


Non conservation of the charge lifetime at high average current

Non-linear effects

Cathode heating

The temperature was measured by a photoluminescence technique.

The thermal coefficient is about 0.4 K/mW.

0.4K/mW*100mW=40K

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

GaAs photocathode in holder


Non conservation of the charge lifetime at high average current

Non-linear effects

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:

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


Non conservation of the charge lifetime at high average current

Non-linear effects

Ion trapping

U=+65V

U=-65V

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 conservation of the charge lifetime at high average current

Non-linear effects

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

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

ESD can lead to vacuum degradation during operation with beam.

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

At 10 mA average current:

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


Non conservation of the charge lifetime at high average current

Non-linear effects

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

dlaser=2.1mm

dbeamline=(28-38)mm

  • Beam loss:

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

  • a-magnet: ~10-6 at 1 mA


Non conservation of the charge lifetime at high average current

Linear effect

Backstream 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.


Non conservation of the charge lifetime at high average current

Non-linear effects

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

  • Thermal heating of the photocathode. (not yet)

  • Ion trapping in the beam line. (yes)

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

  • 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.


Beam collector

Beam Collector

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 lower

2) ESD is lower

3) High Voltage Supply


Beam collector1

Beam Collector

Charge saturation

Charge accumulation at the surface of the photocathode.


Beam collector2

Beam Collector

Charge saturation

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

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

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

  • The thickness of the BBR:

(heavily p-doped)


Beam collector3

Beam Collector

Charge saturation

emitted area p*(1.05mm)2~3.5 mm2

hole concentration 2*1019 cm-3

Current density is presently limited to 1.6 A/cm2.

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

Q=5.7 mC per Impulse

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.


Summary

Summary

  • We made four observations which can tend to decrease of lifetime.

  • Ion trapping in the beam line.

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

  • Backstreamgases from the Faraday cup.

  • 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 ms peak current (charge 5.7 mC, current density 1.6 A/cm2).


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