A Two-Stage Spark Gap for Blumlein-Driven Transversely Excited Atmospheric Nitrogen Laser WeeFong KAU, TeckYong TOU, WeeOng SIEW, SeongShan YAP, RenBing YANG and OiHoong CHIN 1 Faculty of Engineering, Multimedia University, Cyberjaya, Selangor 63100, Malaysia 1 Physics Department, University of Malaya, Kuala Lunpur 50603, Malaysia (Received April 24, 2003; accepted September 22, 2003; published January 13, 2004) A two-stage spark gap is used to implement a two-stage Blumlein circuit for the efficient operation of a transversely excited atmospheric nitrogen laser. This enables a more compact circuit arrangement which employs an array of doorknob capacitors as the dummy capacitors instead of large parallel-plate capacitors but maintains a comparable laser output perfor- mance. [DOI: 10.1143/JJAP.43.144] KEYWORDS: two-stage spark gap, TEA N 2 laser, spark-gap inductance 1. Introduction An efficient operation of a transversely excited atmos- pheric (TEA) nitrogen (N 2 ) laser usually requires ultraviolet (UV) preionization and a fast discharge in the laser channel. 1–6) A fast discharge is often created using a parallel-plate Blumlein circuit while UV preionization of the laser channel is activated by promoting a corona discharge at the sharp edge of a high-voltage plate (aluminum foil) which is purposely protruded underneath the laser electrodes into the laser channel. Satisfying these two criteria usually ensures a homogeneous discharge and a high peak current (> 20 kA) for a short brief moment of laser excitation since the upper laser level, C 3 II ( ¼ 0), has a relatively short lifetime of about 40 ns for a TEA N 2 laser. A high peak current in a high-speed Blumlein circuit requires a large overvoltage compared with static breakdown to be established across the laser channel prior to its electrical breakdown. It was found that the maximum overvoltage was 1:5V O for a parallel-plate Blumlein circuit, although the expected value should be 2V O , where V O is the charging voltage. 7) More critical circuit parameters such as the inductance, capacitor ratio, plasma resistance and spark- gap inductance have been investigated to improve the laser performance. 8,9) Despite these studies, the output of a TEA N 2 laser is generally less than 500 mJ at capacitor-charging voltages of up to 20 kV; the exception was a six-channel oscillator-amplifier system which was operated with a modified Marx circuit and produced up to 6 mJ per pulse. 10) In a previous article, 11) we presented a two-stage Blumlein circuit for doubling the output energy of a TEA N 2 laser [see Fig. 1(a)] which doubled the laser output energy. The laser electrodes were 25 cm of length and 5 mm thick, and these were set 3 mm apart to form the laser-discharge channel. The capacitance of C 1 ¼ C 2 was approximately 3.6 nF, while that of C D was 14 nF for maximum laser output perform- ance. The area of C D was measured as 50 cm  50 cm, which was four fold those of C 1 and C 2 . The total length of (C 1 þ C 2 þ C D ) was about 120 cm. The total inductance of the three discharge capacitors, (C 1 þ C 2 þ C D ), was less than 1.5 nH owing to the parallel-plate arrangement. There are three time-delayed discharge loops in the two-stage Blumlein circuit loops which can be described as follow: (a) C D to the ground via the first spark gap, SG1; (b) C 1 into C D via the second spark gap, SG2; and (c) C 2 into C 1 when the laser channel fires. The second spark gap, SG2, while allowing the dummy capacitor C D to switch to a negative voltage, delayed the discharge of C 1 into C D . Upon C 1 -to-C D discharge, a fast voltage swing ramped up across the laser channel. The first discharge loop of C D to ground remained in the reverse cycle which effectively enabled an over- voltage to be established across the laser channel. The optimum delay of the breakdown in the laser channel was set to about 70 ns by varying the breakdown gap separation in SG2. We showed from computer simulations of a discharge- coupled circuit that the overvoltage across the laser channel reached a value of 1:92V o and the peak discharge current by a factor of 2.3. [Fig. 1(b)]. Figure 1(b) shows the time sequence of the breakdowns of the spark gap, SG2, and the laser channel with respect to the discharge current of the dummy capacitor via SG1. The laser channel current was measured to increase by a factor of 2.4x, which agreed with the predicted value of 2.3. The overvoltage across the laser channel, however, could not be measured using a high- HV SG2 R Laser channel C 1 C D Mylar C 2 Trig. SG1 R SG1 Fires d/mm= 1 2 3 4 5 SG2 Fires Laser Output C D -Current (a) (b) 50ns Fig. 1. (a) Schematic of the two-stage Blumlein circuit for TEA N 2 laser operation. SG1: a trigatron spark gap; SG2: a self-breakdown, two- electrode spark gap; C D : dummy capacitor. (b) Discharge current for the dummy capacitor via the trigatron, SG1, and delayed firing of SG2. Japanese Journal of Applied Physics Vol. 43, No. 1, 2004, pp. 144–146 #2004 The Japan Society of Applied Physics 144