Crowbar Replacement Through Solid-state Opening Switches

This paper summarizes DTI’s work in developing and fielding solid-state switches as replacements for crowbars in a wide range of VED systems. The solid-state switch provides a fast opening capability, which does not short the high voltage system, and provides much faster fault handling, and significantly lower deposited energy in the VED than conventional crowbar systems, improving VED reliability and lifetime. These switches also provide a nearly instantaneous return to operation for the VED. This capability improves VED performance in time critical applications, such as radar systems and industrial processes, where loss of RF output can have highly detrimental effects. This paper will describe multiple applications of series solidstate switches to replace crowbars in critical VED applications Crowbars are commonly used to protect VEDs from arc damage. When an arc occurs, the crowbar closes, and rapidly discharges the energy-storage capacitor. The upper part of Figure 1 shows a typical crowbar circuit that shunts energy from the load. An alternative way to protect a klystron is to use a switch that opens during an arc, as shown in the lower diagram of Figure 1. For cathode pulsed systems, this switch can also serve as the modulator. The typical time between fault sensing and switch opening is 700 ns, depending on the specific switch and controls configuration. This is roughly five times faster than a conventional crowbar system, and dissipates several orders of magnitude less fault energy into the load. Figure 2 shows the response to an arc in a system installed at CPI. This can provide substantially extended VED lifetime. After an arc, the switch can resume normal operation in less than 100 ps- the actual time depends on the load recovery. The fast response can also be of high value in a military radar system. VED systems having significantly higher reliability, can be fielded in both new transmitter designs and through retrofits of existing systems. As a result of this crowbar elimination, Figure 3 shows an opening switchlcrowbar system recently installed at Argonne National Laboratory. This 10 MW CW system, built under an SBIR for the Department of Energy, is designed to replace the mercury ignitron crowbars now used to protect each of six large klystrons in the Advanced Light Source. The fast response of the switch, and the retention of the power supplies and capacitors in a fully charged condition during an arc, makes it possible to retain a circulating beam in an accelerator after a klystron fault - which is not true for the current crowbar system. In addition to the solid-state opening switch, this unit contains a solid-state, mod-anode voltage controller, capable of providing high bandwidth voltage adjustment to the mod anode as beam conditions change. At lower power levels, the solid-state switch shown in Figure 4 was recently installed in the Gray Star X-Band radar system to replace a problematic spark-gap crowbar system. This switch is rated at 50 kV, 50 A peak, and handles 150 kW of average power. In this system, the ability to rapidly return to operation after a TWT fault is a significant operational benefit.