Potential LTD designs based on semiconductor closing switches

Abstract

Summary form only given. Use of linear transformer driver (LTD) technology offers the capability to build fast, high power accelerators without additional pulse shaping. An LTD cavity contains many gas spark-gap switches and capacitor units, called bricks, connected in parallel to achieve fast rise time and high currents from a DC charge. The accelerator is constructed of series and parallel combinations of cavities to achieve desired current and voltage. Performance depends on characteristics of the individual bricks. A desirable improvement to the design of a brick would be to replace the spark-gap switches with semiconductor switches to make the cavities more compact, have a higher repetition-rate capability, and longer lifetime. Current brick design is based on size of available capacitors, but future design could be optimized for available semiconductor switches. This approach likely leads to accelerator designs that have more cavities with more bricks that use smaller capacitors, but that do not have gas switches. The critical question to address is can an assembly of semiconductor switches and smaller capacitors match the cost and performance of gas switches and be configurable in a sufficiently small volume. Using the existing 1 MA LTD cavity design as a standard, a semi-conductor switch would need to have a current density of 50 to 100 A/cm2, a voltage hold-off gradient of 5 to 10 kV/cm, and a rise time of less than 100 ns, all at a cost of less than 50 k dollars/MA. However, almost all of the available technology fails one or more of these criteria. An exception is an inexpensive-to-fabricate silicon avalanche breakdown thyristor developed by the Ioffe Physical Technical Institute in St. Petersburg, Russia. This device, called a deep-level dynistor, has demonstrated 3 kV/cm axial voltage hold-off, 10 kA/cm2 peak current density, and 250 A/ns rise time. In this paper we discuss how this device could be used to design an equivalent 1-MA LTD module without use of gas switches.