Seminar - A.Terekhov, 18th April 2007

4GLS photoinjector problems:
10ps response decreases QE
Low (30 meV) energy spread decreases QE
100 mA on cathode needs 50 W laser, needs cooling
100 mA gives short lifetime due to ion back-bombardment

Solve the first 3 using new cathode heterostructure design - the response time depends on the number of random walks through the thickness of the cathode. A cathode with a thin active later is therefore better. But QE drops because of incomplete light absorption, and mean transverse energy can increase.
In reflection mode (RM), can use Distributed Bragg Reflector
In transmission mode (TM), can use Graded Band Gap Layer - can increase QE by maybe 10x.

Band bending near surface can increase transverse energy spread via momentum scattering (and 'acceleration'), up to value of negative electron affinity. This is not the longitudinal energy spread, however, the longitudinal and transverse energy spreads are similar (difference between conduction band minimum and vacuum energy level, around 150 meV at 300 K and 250 meV at 100 K.

If active layer thickness is smaller than the characteristic thermalisation length (of the order of 100 nm) then you get bigger energy spread.

Notes from ERLP Planning Meeting 5th April 2007

Late Saturday Shift 2/3 go to 350 kV. If there is something wrong,
Heat cleaning on Monday.
Activation on Tuesday.
No problems at 250 kV

Halo - overheating of the cathode? Diffusion of arsenic to the surface of the cathode causing scattering of the incoming laser light rather than reflection through the anode.

Tasks:

Highest Q available
Emittance measurement? No sensible results possible with the halo. For example, a pepperpot measurement revealed a lot of strange images.

Notes from ERLP Planning Meeting 4th April 2007

The halo is around 50% of the beam. This number is not confirmed, so we should try to repeat the measurement using a scanning slit.

Make sure that no cryo equipment is left on the floor at the end of cryo commissioning.
Yuri/Philippe on Shift 1

Notes from ERLP Planning Meeting 3rd April 2007

Next 2 shifts should look at fringing field in the first correct after SOL-01 (HCOR/VCOR-06). There appears to be a lot of (cruciform) distortion at large apertures (i.e. large corrector angles).
The present halo could be because of laser scattering inside the gun chamber (has the foil been removed? yes it has). The halo is more-or-less round (as seen on screen A) - it could be an image of the whole cathode (with the laser spot in the middle). The edge of the halo is the edge of the wafer?

Anode has a 20mm radius. Since the laser beam comes in at ~6 deg, the image at the cathode should be a clipped 'ellipse' shape. Maybe about half the charge is in the halo? Needs to be measured (by Steve Jamison).

Shield wall has not been replaced since removal for installation of another transfer line girder module.

Any time we get a good average current (e.g. 20 pC), then call Martin Holbourn to do some radiation measurements.

2 options - work at 250 kV to look at source of halo. Or go to 350 kV hoping that the halo will diminish, but could kill cathode. For the next 2 shifts we will therefore work at 250 kV.