Schottky-Enabled Photoemission in RF Guns

1. Schottky-Enabled Photoemission in RF Guns Manoel Conde, Zikri Yusof, and Wei Gai High Energy Physics Division

2. Motivations • To find a clearer signature of the Schottky effect on photocathodes in an RF photoinjector • To find a more direct determination of the field-enhancement factor on the cathode surface • Scheme: Employ Schottky effect in the RF photocathode gun and with low energy photons • This technique may produce an electron beam with ultra-low intrinsic emittance (Yusof et al. PRL 93, 114801 (2004))

3. Photoemission For a typical cathode: However, for a cathode in an electric field E: -Feff where hn : photon energy F : material’s bulk work function a : a constant b : field enhancement factor f : RF phase E : Electric field magnitude Our scheme is to use hn < F, and then employ the Schottky effect to lower the effective work function Feff, where

4. Schottky Effect 0 z Image potential e2/16pe0z DF Electrostatic potential -eEz F Feff Effective potential EF Metal Feff = F - DF

5. Schematics of Beamline Light source: Frequency-doubled Ti:Sapphire laser 372 nm (3.3 eV), 1 – 4 mJ, 8ps. Photocathode: Mg,  = 3.6 eV. Example of Schottky effect on the cathode: at E(q) = 60 MV/m, DF ~ 0.3 eV

6. RF Scans – E-Field on Cathode Surface RF Phase e hn e Laser injection e E(q) = - Emax sin(q) Photocathode RF frequency = 1.3 GHz (Period ~ 770 ps) Laser pulse length = 6 – 8 ps Metallic photocathode response time ~ fs We can safely assume that all the photoelectrons emitted in each pulse see the same E-field strength

7. Charge Obtained from RF Scans Theoretical RF Scan Experimental RF Scan Our scans X.J. Wang et al. Proc. 1998 LINAC Theoretical result from PARMELLA simulation of our RF photoinjector (H. Wang) We see the “expected” flat-top profile Full range of charge detected ~ 130 degrees

8. Experimental Results 1 – RF Phase Scan An RF phase scan allows us to impose different electric field magnitude on the cathode at the instant that a laser pulse impinges on the surface, i.e. E(q) = Emax sin(q). New observation Typical photoinjector conditions hn = 3.3 eV; F = 3.6 eV Laser beam diameter = 2 cm (0.35 mJ/cm2) A noticeable shift of the onset of photoelectron production with decreasing RF power. hn = 5 eV, F = 3.6 eV No change in the phase range over all RF power.

9. Determination of Field Enhancement Factor At threshold, Q = 0. This allows us to make a reasonable estimate of the maximum b. hn = 3.3 eV; F = 3.6 eV This is a new and viable technique to realistically determine the field enhancement factor of the cathode in a photoinjector

10. Experimental Results 2 – Intensity • Parameters: • hn = 3.3 eV • = 3.6 eV E field on cathode: 80 MV/m Laser spot diameter: 2 cm As we increase the laser intensity, we detect more charge. We definitely are detecting photoelectrons and not dark current!

11. Experimental Results 3 – Detection Threshold? Simulated Detection Threshold Using A Sine Function Experimental Observation Emax = 28 MV/m No cutoff With cutoff Two different scans with different amount of charge produced, but with the same RF amplitude, show the same phase angle for the photoemission threshold. The shift in the photoemisson threshold is not due to the detection threshold.

12. Summary • We have shown the ability to use the Schottky effect to tune the RF photoinjector to be right at the threshold of photoemission of our cathode. This is a new technique for RF photoinjectors. • The threshold condition used here is also a new and viable method to accurately determine the maximum field enhancement factor of the photoinjector cathode. • This technique can be used to determine the field emission factor for other materials such as Nb and Cu, which are common materials for RF cavities and accelerating structures.

13. Experimental Results Fitting parameters for 1cm beam: a1 = 0.14; a2 = 0.96 Fitting parematers for 2cm beam: a1 = 0.11; a2 = 0.02 Laser beam diameter = 1 cm (1.3 mJ/cm2). hn = 3.3 eV, F = 3.6 eV Indication that photoemission from 1 cm laser beam is dominated by 2-photon process while that from the 2 cm beam is dominated by the single-photon process.

14. Summary of RF Scans hn = 5 eV, F = 3.6 eV QE ~ 1.1 x 10-4 hn = 3.3 eV, F = 3.6 eV, 1 cm laser diameter QE ~ 2 x 10-6 hn = 3.3 eV, F = 3.6 eV, 2 cm laser diameter QE ~ 0.7 x 10-6