The ALICE DC Photoelectron Gun is a nearly exact replica of the JLab design with the exception of a single piece HV ceramic insulator employed. Despite the HV PS and the ceramic insulator can both operate at the voltages of up to 500kV, the gun operating voltage is normally 350kV to reduce the field emission current and outgassing within the vacuum vessel.

The negative electron affinity (NEA) GaAs photocathode is located on the face of a spherical cathode ball. During the wafer surface preparation (by heat cleaning) and a subsequent cathode activation, however, the wafer is retracted inside the ball to avoid contamination of surfaces exposed to high electric fields. The aperture in the cathode ball that determines the effective area of the photocathode is 25.4mm in diameter.

Activation of the photocathode to NEA is performed by applying subsequent layers of Cs and O₂ (or NF3 recently) in a carefully controlled procedure. At present, we can routinely activate the cathode to the quantum efficiency (QE) of above ~3%. In ideal conditions however, QE levels of 20% and higher could be also achieved. The QE map across the whole area of the photocathode is monitored with the QE scanner.

The activated cathode is highly susceptible to the vacuum environment and can be easily poisoned by molecules, like O₂, normally found in the residual gases and by ion back-bombardment due to ionisation of atoms remaining in the vacuum chamber. These factors, if not carefully controlled, can reduce the cathode lifetime to just a few tens of hours. That is why the DC photoguns similar to that of ALICE require an extra-high vacuum of, ideally, ~10^-12 mbar.

To achieve these vacuum levels, the DC gun must have the highest possible pumping speed capacity (ion pumps and NEGs), must be thoroughly baked out, and must be patiently conditioned by high voltage.

Field emission is a serious issue in HV DC guns because the electric fields reach the levels of ~8MV/m on the cathode ball surface and ~12MV/m on the cathode stem (both are given for 500kV gun voltage). During the HV gun conditioning process, the voltage is raised slowly in steps. Each step that could be as small as 1-2kV above ~300kV is normally followed by a sharp increase in electron field emission and electron stimulated gas desorption. After a few tens of minutes, the gun current and vacuum activities subside and a new voltage increment could be applied. To avoid a catastrophic damage to the gun electrodes or the ceramic insulator, a conditioning resistor of ~200 MΩ must be inserted between the HV PS and the gun. This resistor itself is a crucial part of the gun system and two new types have been developed at Daresbury Laboratory.

To make sure that the gun operates reliably and with a high cathode lifetime at nominal voltage, the HV conditioning should continue until a much higher voltage is reached, i.e. ~450kV in our gun. With a new “freshly made” gun, the HV conditioning may take as long as 100-150 hours. After the gun was once conditioned but opened to atmospheric pressure, this time could reduce to less than 50 hours.

Electron pulses are generated from the NEA GaAs photocathode by green light from a Nd:YVO₄ mode-locked laser frequency-doubled to produce a 532nm laser beam with a transverse size on the cathode of ~4mm FWHM. The intrinsic laser pulses are Gaussian in profile with a 7ps FWHM length. ALICE is normally operated with longer laser pulses of 28ps generated with the use of a vanadate pulse stacker.