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Thermal emittance and response time measurements of negative electron affinity photocathodes
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Image of FIG. 1.
FIG. 1.

Beam line used in photocathode studies. The beam moves from right to left.

Image of FIG. 2.
FIG. 2.

Typical transverse profile of a laser spot incident on the photocathode (a) after a circular aperture and (b) for a Gaussian imaged beam.

Image of FIG. 3.
FIG. 3.

(a) Typical beam profile as measured by the wire scanner and the fit. (b) Typical solenoid scan data and emittance fit (GaAs, 860 nm).

Image of FIG. 4.
FIG. 4.

Transverse phase space of the beam with thermal emittance. Current signal in nA. (a) GaAs and (b) GaAsP. Calculated emittances as a function of encompassing ellipse area after the noise subtraction for (c) GaAs and (d) GaAsP data, respectively (refer to the text).

Image of FIG. 5.
FIG. 5.

Comparison between different emittance measurement techniques for GaAs at 532 nm. Filled squares correspond to a Gaussian laser profile.

Image of FIG. 6.
FIG. 6.

Thermal emittance of GaAs as a function of rms laser spot size for various illumination wavelengths.

Image of FIG. 7.
FIG. 7.

Thermal emittance of GaAsP as a function of rms laser spot size. (a) and (b) .

Image of FIG. 8.
FIG. 8.

Thermal energy of GaAs as a function laser wavelength.

Image of FIG. 9.
FIG. 9.

Thermal energy of GaAsP as a function laser wavelength.

Image of FIG. 10.
FIG. 10.

Fraction of the electron pulse emitted up to time upon arrival of a delta-function laser pulse for the photoemission response function given by Eq. (11).

Image of FIG. 11.
FIG. 11.

Calculated dependence of the transverse beam size of the beam at the location of the wire scanner for different values of parameter . Charge per bunch 100 fC, gun voltage 250 kV, , initial rms laser spot size . (a) Solenoid scan and (b) change of the spot size relative to the case of negligible space charge for unpowered solenoid as a function of parameter .

Image of FIG. 12.
FIG. 12.

Fit of the measured solenoid scan when excited by 100 fs laser pulse at 710 nm. (a) 200 kV and (b) 250 kV.

Image of FIG. 13.
FIG. 13.

Example of fit to solenoid scan data for GaAs when excited with 300 fs laser pulses at 460 nm. The bunch charge is 97 fC, the initial electron distribution is assumed to be identical to the laser (prompt response). All parameters except for the gun voltage are identical for the two fitted curves.


Generic image for table
Table I.

Results of data fitting for GaAs response time.

Generic image for table
Table II.

Results of data fitting for GaAsP response time.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Thermal emittance and response time measurements of negative electron affinity photocathodes