# Presentation: 932353

## Further Measurements of Photocathode

- Enhance our understanding of photocathode decay mechanism.  Will undoubtedly allow us to improve existing polarized guns operating at lower average current and unpolarized guns at milliAmp beam currents (e.g., Lightsources).

- Operational Lifetime at Beam Intensity > 1mA
- with the NEW 100 kV DC GaAs Photogun

- _J. Grames_, _M. Poelker_, P. Adderley, J. Brittian, J. Clark,
- J. Hansknecht, E. Pozdeyev, M. Stutzman, K. Surles-Law

## CEBAF Polarized Source

- CEBAF is doing very well.  Busy, productive Nuclear Physics program.
-  35 weeks of beam delivery per year
-  100 A at 85% polarization is fairly routine
-  One photocathode operates for year(s), and three or four activations
-  2-3 Users simultaneously; one is always Parity Violation experiment

- Before heat/activation

- After heat/activation

## CEBAF Gun Charge Lifetime (2001-2004)

- Charge Lifetime Steadily Decreasing

- NEG replacement Summer 2003 improves lifetime

**Data compiled by M. Baylac**

## Can increasing the laser spot size improve charge lifetime?

- Bigger laser spot, same # electrons,  same # ions

## Experiment Requires Managing Electron Beam

- Cathode

- Anode

- Hits
- beampipe

- Hits gun
- chamber

- Hits Wien
- faceplate

-  Limit photocathode active area
-  Eliminate stray light
-  Large diameter beampipes
-  NEG coated chambers to limit ESD
-  Proper electrode geometry
-  Proper lens configuration

- Hits
- anode

## Experimental Setup

## High Voltage

**Top View**

- Chamber

- Suitcase & Load Lock Chamber
-  Mount wafer on puck in lab
-  Holds 4 pucks (e.g., bulk, SL, SSL)
-  Load Lock: 8 hour bake @ 250 C
-  No H-Cleaning

- Beam

-  Activation Chamber
-  Manipulators 150 C bake
-  New & Used puck storage

## High Voltage Chamber 

- High Voltage Chamber
-  “Side ceramic” design
-  load chamber at ground potential
-  No moving parts at HV

**Side View**

- Activation Chamber
-  Mini-stalk heater
-  Mask selects active area
-  UHV IP supplies gauge activation
-  Keyed & eared pucks

## High Voltage Chamber 

- High Voltage Chamber
-  “Side ceramic” design
-  load chamber at ground potential
-  No moving parts at HV

**Side View**

- Activation Chamber
-  Mini-stalk heater
-  Mask selects active area
-  UHV IP supplies gauge activation
-  Keyed & eared pucks

## Improvements to the High Voltage Chamber

- NEG coating
- (Ti/Zr/V)
- 100 hrs @ 70 C
- 200 L/sec

- 304 SS: Electropolished & Vacuum Fired
- (AVS: 3 hrs @ 900 C @ 3x10-6 T)

-  Careful electrode alignment
-  Lipped to flatten field profile
-  Bias anode or support
-  Rear windows view “tee”

## Experimental Setup

- Load lock
- (GaAs on puck)

- NEG pipe

- Laser
- (1 W @ 532 nm)

- Faraday Cup

- High Voltage
- (100 kV)

- Activation
- (Cs/NF3, 5 mm)

- 350 m

- 1500 m

- Spot Size
- Adjustment

## Example Run (5 mA)

-  Run laser power (<1 Watt) PID to fix beam current
-  Record ion pump current at 7 beam line locations
-  Record laser power/setpoint via “pickoff” detector

## NEW vs. OLD Load Lock Design (small laser spot)

- OLD

- NEW

- Damage ~ (a∙I + b∙I2)

## Dependence of Gun Pressure on Beam Intensity

- Sept

- July

## SMALL vs. LARGE Laser Spot (both guns)

- Tough to measure >1000 C lifetimes with 100-200 C runs!

- 5

- 15

## Is Ionized Gas from the Beamline Limiting Charge Lifetime?

## Repelling Beamline Ions with Biased Anode

- Beamline
- Ions

- Anode

- Contributed by E. Pozdeyev

## Biased Anode: Null Result

## Preparation Chamber: Hydrogen Degradation of QE

- 30 min
- ~2 L

## Tantalyzing Discovery: Hydrogen Barrier Enhances Lifetime

- Wafer QE improves as the hydrogen barrier is removed.
- All three spots ~25 C before QE starts to fall.
- Once the barrier is removed QE falls as usual.

## QE Largest at Beam Spot Location

- Trenching?

- ~103 L

## Experience with Superlattice @ 1 mA

- ~8% at bias (300V)
- ~4% at HV (100 kV)

- Lifetime ~200 C at 1 mA (532 nm)

## What’s Next ?

- Laser Position Stability

## Conclusions

- Large spot provides better Charge Lifetime, consistent with previous experiments.
- Exceptionally good Charge Lifetime >1000 C at high currents >1mA
- In fact, difficult to measure when using large laser spot.
- Anode biasing to +/- 2.5kV yields no measurable improvement; ions created downstream of anode not a problem, at least not in test stand with good vacuum.
- First demonstration of surface barrier that enhances operating lifetime, albeit at expense of initial QE.  Look for other coating material that preserves QE, but does not reduce QE.
- Polarized beam studies at >1 mA and 80% polarization during Fall 2006.
- Install load lock in tunnel in Spring 2007.

## New Load Lock Gun Assembled & Running Spring ‘06

## Reality is more complicated...