There are several modulator techniques being investigated as candidates for driving NLC klystrons in groups of between two and eight per modulator. The baseline design is a conventional line type modulator that would use a thyratron as the switch to drive two klystrons. This paper describes a thyratron designed to meet these operational requirements. The target lifetime is 50,000 hours. Cost and reliability are of critical importance due to the large number of modulators required. Several design decisions were taken at the outset including cathode type, basic tube diameter and number of high voltage gaps required. The development process included measurement of cathode temperature as a function of average current and electrostatic field analysis of the high voltage geometry. The rationale underpinning the design that resulted is explained. Since the specification was first defined, the pulse width has been doubled with a corresponding doubling of the average current. The impact of this updated requirement on the existing design is also discussed.Presenting Author : Sheldrake R.