REPETITIVE NANOSECOND HIGH-VOLTAGE GENERATOR BASED ON SPIRAL FORMING LINE S. D. Korovin,ξ V. P. Gubanov, A. V. Gunin, I. V. Pegel, and A. S. Stepchenko Institute of High Current Electronics SD RAS 4 Akademichesky Ave., Tomsk 634055, Russia here L = 2πr1 N is the length of the spiral conductor, N is
Abstract This paper presents a nanosecond periodically-pulsed generator based on a spiral line charged by means of a high-coupling Tesla transformer. At repetition rate of 100 p.p.s., 130-ns, 500 – 700-kV pulses were produced in a 100 – 150-Ω load.
I. INTRODUCTION
the number of its turns, and r1 and r2 are radii of outer and inner conductors of the line. In the particular case of r2 = r1 + ∆r with ∆r ρ 0 / 2 . In particular, for
RL ≈ 100 Ω at optimum ratio of conductors radii and transformer oil insulation we have η (α ) ≈ 1 , if k sl ≈ 4 − 6 . Under the same conditions, the maximum efficiency of load energy transfer for a conventional coaxial line is only 70%.
tel. 7-3822-258706, fax -259134, e-mail
[email protected]
0-7803-7120-8/02/$17.00 © 2002 IEEE
efficiency
4α 2
III. DESIGN OF THE GENERATOR AND EXPERIMENTAL RESULTS Charging a spiral line from a transformer is possible in several different ways. The most simple way uses sequential connection of the spiral line with a conventional coaxial line, in which a Tesla transformer is built. Figure 1 shows the interior schematic of the spiral line-based pulse generator developed. The FL of the generator consisted from two sections of long lines. The first one was the spiral line (1) with 100 Ω wave
2.1 m
impedance and slowing coefficient k sl ≈ 5.1 . The other one was a 30-Ω conventional coaxial line (2) with built-in Tesla transformer. The outer conductor of the line was 700 mm in diameter. The line was insulated with transformer oil. The Tesla transformer charged the FL up to maximum voltage of ~1.1 MV with pulse repetition rate up to 100 p.p.s. The energy stored in the FL was 600 J. The HV two-electrode gas gap switch filled with technical nitrogen at up to 15 Atm operated in selfbreakdown regime. To stabilize the breakdown voltage, forced gas circulation between the electrodes was used.
2.1 m
U Diode
I Diode
0.7 m
U Line
1
2
3
4
5
6
Fig. 1. Diagram of the pulse source: 1 – spiral FL, 2 – Tesla transformer, 3 – spark gas gap switch, 4 – vacuum diode, 5 – magnetic coil, 6 – REB In experiment, two different spiral line construction were studied. In the first case, the spiral conductor was made from metal strip in a form of single-start helix. In the second case, a multiple-start helix was made from many wires insulated from each other. The wires were laid onto a dielectric framework. The electric field strength at the inner conductor of the line reached 190 kV/cm. At pulse rates of 50 – 100 p.p.s., the single-start spiral line fell out after 105 pulses because of breakdown between adjacent turns along the framework dielectric. The multi-start spiral stood long periodically-pulsed operation under the same electric field strength conditions. Using multiple conductors allowed to substantially reduce the strength of electric field occurring between adjacent wires during the risetime and fall of the propagating electric pulse. At low FL charge voltages (
