Triggered Vacuum Gaps Research Articles

2138. Breakdown mechanisms and electrical properties of triggered vacuum gaps: G R Govinda Raju et al,J Appl Phys,47 (4), 1976, 1310–1317

2138. Breakdown mechanisms and electrical properties of triggered vacuum gaps: G R Govinda Raju et al,J Appl Phys,47 (4), 1976, 1310–1317

Breakdown mechanisms and electrical properties of triggered vacuum gaps

Some electrical characteristics of a triggered vacuum gap (TVG) having three different dielectric materials have been studied. Silicon carbide which possesses very high resistance (500 MΩ), steatite ceramic of medium resistance (20 MΩ), and boron nitride of very low resistance (50 Ω) have been used. The resistance of the dielectric is found to decrease with increasing trigger current and this is attributed to the deposition of metal vapor on the surface. The minimum trigger voltage and trigger current nessessary for successful operation of the TVG are measured. Deconditioning (reduction) in the minimum trigger voltage, after repeated firings, has been found and is attributed also to the deposition of metal vapor on the dielectric surface. The minimum trigger current for successful firing of the main gap decreases with increasing trigger voltage. The probability of successful firing of the TVG is found to rise rapidly with increasing trigger voltage. The delay time between the application of the trigger pulse to the breakdown of the main gap decreases with increasing trigger current and trigger voltage. A mechanism is suggested for the operation of the TVG.

Multiple-electrode triggered vacuum gaps

Since the design criteria for reliable operation of a single gap at 300 kV are not well known, multiple-electrode triggered vacuum gaps were constructed. A six-gap structure was initially studied. It had excessive time delay in the initiation of the arc. Subsequently, a three-gap structure was built. Time delays in initiation of the arc were satisfactory, but the rate of rise of current was slow. In addition, voltage hold-off capability was marginal. To improve these characteristics, a low pressure of hydrogen, 7 to 25 mtorr, was added. Considerably improved firing characteristics and reliable voltage hold-off at 320 kV were obtained.

Triggered vacuum gaps

Characteristics of a sealed vacuum gap are described and the difficulties encountered in applying this gap as an overvoltage protection device are discussed. It is shown how these difficulties can be ameliorated by the use of gas-free electrode materials and by triggering the gap when breakdown is required. Several methods of triggering are discussed and some practical triggering devices are described that inject minute quantities of ionized hydrogen into the gap. The hydrogen is eventually recovered by the use of a titanium hydride getter. It is shown that breakdown of the gap can be accomplished in less than one-tenth microsecond by first producing a glow discharge that is rapidly transformed into a metal-vapor arc. Properties of the metal-vapor arc are described which have an effect on the characteristics of the vacuum gap. A number of practical sealed-off triggered vacuum gaps are illustrated. These are used to carry microsecond capacitor discharge currents and 60-cycle power line currents for ½ cycle. The operating voltage range is from a few hundred volts to 100 kV. The advantages of vacuum gaps over gas-filled gaps are given and a number of overvoltage protection and switching applications are listed.