These photocathodes are, in first analysis, very attractive for our applications because they accept very high electric fields, higher than 100 MV/m, their relaxation time is very short, of the order of some femtosecondes [ 1 ] and their lifetime is long. However quantum efficiency is low, in the best of cases, with special treatments, QE is at most of the order of some 10-4 electrons by incident photon at 266 nm. The table below reports the main performances of metallic photocathodes measured in different laboratories. Except other specification QE's measurements were realized between 5 and 10 MV/m.
The best performances were obtained with a special surface treatment (see [2] for example). Unfortunately, these performances degrade as the surface contaminates in the RF gun. The lifetime does not exceed few days; however, it is generally possible to restore the initial properties. Furthermore, the low QE of these cathodes implies a strong laser power density at the surface. From a certain threshold there is an "explosive" emission which produces a plasma, then a huge electron production after the end of the laser pulse. For the copper and at 266 nm, this threshold is about 1GW/cm2 (12 mJ/cm2 in 10 ps) [3], for the magnesium it is about half, 400 MW/cm2 [ 4 ] .
QE of some metals
|
l [nm] |
193 |
213/209 |
248 |
266/262 |
308 |
355 |
fs |
Ref. |
|
E [eV] |
6.4 |
5.8 / 5.9 |
5 |
4.7 |
4 |
3.5 |
eV |
|
|
Al |
|
8.4*10-4 |
|
3.2*10-5 |
|
3.4*10-7 |
4.3 |
[5] |
|
Ag (a) |
|
|
|
2*10-5 |
|
|
4.3 |
[2] |
|
Au (a) |
|
|
|
4.7*10-5 |
|
|
5.1 |
[2] |
|
Au |
|
4*10-4 |
|
1.310-5 |
|
|
5.1 |
[5] |
|
Ca |
|
|
4*10-5 |
|
|
|
2.9 |
[4] |
|
Cu |
2.0*10-4 |
1.5*10-4 |
|
2.2*10-6 |
1.6*10-7 |
8*10-9 |
4.6 |
[5] |
|
Cu (b) |
1.5*10-3 |
4.2*10-4 |
|
|
|
|
4.6 |
[5] |
|
Cu (a) |
|
|
|
1.4*10-4 |
|
|
4.6 |
[2] |
|
Mg |
|
|
|
5.1*10-5 |
|
|
3.7 |
[5] |
|
Mg (c) |
|
|
|
2.7*10-4 |
|
|
3.7 |
[5] |
|
Mg (d) |
|
|
|
5*10-4 |
|
|
3.7 |
[6] |
|
Mo |
|
|
|
<7*10-7 |
|
|
4.6 |
[5] |
|
Nb |
4.5*10-4 |
|
3.2*10-6 |
|
|
|
4.3 |
[7] |
|
Ni (a) |
|
|
|
2.5 10-5 |
|
|
5.2 |
[2] |
|
Pd (a) |
|
|
|
1.2*10-5 |
|
|
5.1 |
[2] |
|
Ac 316 LN |
|
9*10-5 |
|
1.6*10-6 |
|
|
? |
[5] |
|
Sm |
|
|
|
|
1.6*10-6 |
|
2.7 |
[5] |
|
Sm (a) |
|
|
|
7.3*10-4 |
|
|
2.7 |
[2] |
|
Ta (a) |
|
|
|
10-5 |
|
|
4.3 |
[2] |
|
Tb (a) |
|
|
|
2.3*10-4 |
|
|
3 |
[2] |
|
W (111)(e) |
|
|
|
|
|
2*10-5 |
4.5 |
[8] |
|
WK+ (b), (f) |
|
|
|
|
1.2*10-5 |
|
2.8 |
[5] |
|
Y |
|
|
|
5*10-4 |
|
|
3.1 |
[2] |
|
Y |
|
|
|
2.7*10-6 |
1.1*10-6 |
|
3.1 |
[5] |
|
Y (b) |
|
|
|
1.8*10-4 |
|
|
3.1 |
[5] |
|
Y (a) |
|
|
|
5*10-4 |
|
|
3.1 |
[2] |
|
Zn (a) |
|
|
|
1.4*10-5 |
|
|
4.3 |
[2] |
|
Zr (a) |
|
|
|
10-5 |
|
|
4.1 |
[2] |
|
fs = work function from [9].
(a) = Surface preparation and activation under vacuum, from [2].
(b) = Cleaning by Argon ion bombardment [10]
(c) = Used in the CTF RF gun at 100 MV/m
(d) = ATF (BNL) measurements at 70 MV/m without surface treatment
(e) = Photoemission assisted by high electric field, E = 3 GV/m
(f) = Potassium ions implanted in tungsten substratum (150 keV implantation energy, ion density 1.3x1017 ions / cm2 at the surface [11]) |
||||||||
|
[ 1] |
W.E. Spicer and A. Herrera-Gomez, Modern Theory and Applications of
Photocathodes, Proceedings of International Symposium on Optics, Imaging and Instrumentation, San Diego, CA, July 1993 |
|
[ 2] |
T. Srinivasan-Rao, J. Fischer, and T. Tsang, Photoemission studies on metals using picosecond ultraviolet laser pulses,
Jour. of App. Physics, vol. 69 No 5, 1 March 1991 |
|
[3] |
X.J. Wang, T. Tsang, H. Kirk, T. Srinivasav-Rao, J. Fischer, K. Batchelor, R.C. Fernow, P. Russel, Intense Electron Emission Due to Picoseconde Laser-Produced Plasma in High Gradient Electric Fields, Submitted to Journal of Applied Physics, BNL 45031, January, 1992. |
|
[4] |
P. Schoessow, E. Chojnacki, G. Cox, W. Gai, C. Ho, R. Konecny, J. Power, M. Rosing, J. Simpson, N. Barrow, M. Conde, The
Argonne Wakefield Accelerator High Current Photocathode Gun and Drive Linac, |
|
[5] |
E. Chevallay, J. Durand, S. Hutchins, G. Suberlucq, M. Wurgel, Photocathodes tested in the dc gun of the CERN photoemission
laboratory, NIM A 340 (1994) |
|
[6] |
X.J. Wang, T. Srinivasan-Rao, K. Batchelor, I. Ben-Zvi, and J. Fischer, Photoelectrons
Beam Measurement from a Magnesium Cathode in a RF Electron Gun, proceedings of LINAC 1994. |
|
[7] |
L.N. Hand, U. Happeck, Photoelectric Quantum Efficiency of Niobium for l = 193 nm and l = 248 nm, SRF 95030103 |
|
[8] |
Y. Gao and R. Reifenberger, Yield of photofield emitted electrons from tungsten, Physical Review B, Third series, vol. 35 No 16, 1 June 1987 |
|
[9] |
H.B. Michaelson, The work function of the elements and its periodicity, Journal of Applied Physics, vol. 48 No11, November 1977 |
|
[10] |
M. Wurgel, Nettoyage ionique sous vide des Photocathodes, PS/LP Note Technique No 91-03, janvier 1991 |
|
[11] |
J.P. Girardeau-Montaut et A. Pérez, private communication, Claude Bernard university, Lyon (F) |