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Photocathodes for the CTF2

Specifications : For the drive beam (DB), the QE (the ratio of the emitted electrons to the incident photons) must be greater than 1.5% at 262 nm during at least one week with an electric field of 100 MV/m. For the probe beam (PB), the QE must be greater than 0.01% at the same wavelength during at least one month with an electric field of 70 MV/m. The produced charges must be linear with the laser energy up to 1 m C (48 pulses of 21 nC) for the drive and 2 nC for the probe beam. In both cases, the response time must be lower than a few ps.

Table 1 : Electron beams specifications.

		DB	PB
Charge per bunch	nC	13.4	1 - 2
Number of bunches	-	48	1
Pulse width FWHM	ps	10	10
D t between pulses	ps	330	-
Charge stability	% rms	1	5
Jitter laser / RF	ps	ą 1	ą 1
Jitter DB / PB	ps	ą 1	ą 1
Delay PB / DB	ns	-	14
Cathode electric field	MV/m	100	70
Repetition rate	Hz	5	5

Possible photocathodes for the CTF

For the drive beam we used different alkali telluride cathodes, but mainly Cesium Telluride

For the probe beam :

CSI-Ge : From 1992 to 2000 we used these photocathodes because it was possible to transport and install it at ambient atmosphere. The QE was comprise between 0.1 and 0.5 % and the lifetime was about 6 months.

Cs₂Te : In 2001 and 2002 we used these cathodes produced in simplified preparation chamber attached to the RF gun: 15 nm of cesium were deposited at room temperature over 10 nm of Tellurium without QE monitoring. The QE was between 1 and 2 % and the time to decrease the QE down to 0.3 % (minimum QE for the available laser power to produce the nominal charge) was about 1 year (we have produced only 2 operational photocathodes).

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