Oct 1, 2008 - ABSTRACT. Light-activated pseudospark switches, also called back-lighted thyratrons (BLTs), are low pressure, high voltage (typ. 10-50 kV) ...
IEEE Transactions on Dielectrics and Electrical Insulation
Vol. 16, No. 4; August 2009
993
Quantum Efficiency Measurements of Photocathode Candidates for Back-Lighted Thyratrons Esin B. Sozer, Chunqi Jiang, Martin A. Gundersen University of Southern California Los Angeles, CA 90089, USA and Ryan J. Umstattd Oak Ridge National Laboratory Oak Ridge, TN, USA
ABSTRACT Light-activated pseudospark switches, also called back-lighted thyratrons (BLTs), are low pressure, high voltage (typ. 10-50 kV), high current (typ. 1-100 kA) glow-mode switches. It is of interest to develop BLTs with reliable and practical optical triggering systems for applications of compact pulsed power. This paper reports the results of research into photocathode materials for BLTs to enhance switching performance and provide optimal cathode conditions for optical triggering. Effective photocathode materials have many specific qualities, the most important being low work function, high quantum yield, and long lifetime at typical BLT operation pressures of 1.3-133 Pa (0.01-1 Torr). Photoemission measurements were conducted with 266 nm, 5 ns laser pulses in a pressure range from 4 x 10-5- 13.3 Pa (3×10-7 to 0.1 Torr) using helium as the background gas. Quantum efficiencies up to 1.5×10-5, 1.4×10-5, and 1.2×10-5 were measured for magnesium, copper, and molybdenum samples, respectively. An increase in gas pressure 4 x 10-5- 13.3 Pa (3×10-7 to 0.1 Torr) corresponded to an increase in quantum efficiency (QE) of 13% for magnesium and copper; the same increase in pressure corresponded to a quantum efficiency decrease of 10% for molybdenum. Square root of quantum efficiency shows a linear dependence on the square root of the sample surface’s electric field due to the Schottky effect. 2D electrostatic simulation of the electric field distribution in a typical compact BLT shows that the field strengths are up to hundreds of kV/cm near the surfaces of the electrodes when a voltage potential of 30 kV is applied between the electrodes. This indicates that higher photoelectron yields can be expected when the tested photocathodes are implemented into BLTs. Index Terms — Photocathodes, photoelectricity, back-lighted thyratron, pseudospark switch, magnesium, copper, molybdenum, Schottky effect.
1 INTRODUCTION PSEUDOSPARK switches are high-power, high voltage (typically 10 - 50 kV) gas phase switches known to be superior to high-pressure spark gap switches and thyratrons in several key switch parameters [1], have been developed and employed in a variety of pulsed power applications including high power electron beam sources, high power microwaves, and transient plasma ignition for pulsed detonation engines. As an optically triggered version of pseudospark switches, back-lighted thyratrons (BLTs) offer additional advantages in electrical insulation and flexibility of the triggering system. A hold-off voltage > 37 kV and a peak current > 17 kA were Manuscript received on 1 October 2008, in final form 20 January 2009.
obtained for UV laser and flash-lamp triggered BLT switches [2]. The best jitter and delay of 0.4 ns half width at half maximum (HWHM) and 78 ns, respectively, were achieved when 4.4 mJ of 308 nm light from an optical fiber was incident on the back surface of the hollow cathode of a hydrogen-filled BLT [3]. In addition, electrical insulation between the switch and the trigger has made it possible to design an ultra-compact switch with reasonable lifetime (>108 shots) at medium current operation (several kilo amperes) [4]. Triggering of BLTs is accomplished by using an optical source to generate sufficient number of seed electrons which will initiate the formation of glow-type pseudospark discharge between the anode and cathode. Consequently, improving the sensitivity (quantum efficiency) of the cathode would enhance the photoemission processes and thereby reduce the delay and
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E. B. Sozer et al.: Quantum Efficiency Measurements of Photocathode Candidates for Back-Lighted Thyratrons
994
jitter times of the switch for the same optical triggering condition. This has motivated our work on application of low work function, high quantum yield, rugged photocathode materials for BLTs. Metal photocathodes with their long lifetimes (order of years) and relative insensitivity to contamination are more suitable for BLT application compared to semiconductor photocathodes [5]. However, research reported on metal photocathodes is mainly concentrated on applications for high-energy physics accelerators and free electron lasers [6, 7]. These applications are high vacuum environments (typically 1.3 x 10-7 to 1.3 x 10-6 Pa (10-9 to 10-8 Torr)) with high accelerating fields (up to 100 MV/m). Hence, the data reported on quantum efficiency values of these cathodes are under these high purity environments and are significantly enhanced by high accelerating fields due to Schottky effect [8, 9]. This work reports the quantum efficiencies (QE) of photocathode candidates, in a pressure range of 4 x 10-5- 13.3 Pa (3×10-7 to 0.1 Torr) and up to electric field strength of approximately 550 V/cm.
2 PHOTOCATHODE MATERIAL SELECTION Common high vacuum photocathode materials can generally be divided into two categories: semiconductor and metal. Recent reviews of photocathodes [5, 10] have shown semiconductor cathodes have higher QE, however they suffer from short lifetime (up to hundreds of hours) and high sensitivity to contamination. Metal photocathodes are attractive because of their fast response time, ease of preparation, relative insensitivity to contamination, and the long lifetime (orders of years). However, their high work function usually requires intense UV irradiation of the cathodes to obtain reasonable electron yield. In the case of BLTs, long lifetime is critical and required light energy can be as low as 10 μJ with reasonable delay and jitter times [3]. As a result, a high-yield metal photocathode can improve switches’ triggering parameters while maintaining its performance for years. Table 1 summarizes reported delay and jitter times of BLTs together with the triggering light source, wavelength, the amount of energy delivered to the switch in order to accomplish triggering, and cathode material.
Light Source XeCl laser KrCl UV Flashlamp XeCl laser XeCl laser Nd:YAG laser
Table 1. Reported BLT Triggering parameters Wavelength Cathode Trigger Delay (ns) (nm) Material Energy 308 Mo 4.4 mJ [3] 78 222 Mo 10μJ [3] 78 Mo 3×10 [8] -3
2×10 [6] -5
1.5×10 [7] -3
1.7×10 [19] -3
1.0×10 [9] -5
2.07×10 [20]
1×10-8 1.3 × 10-6
