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Klystrons capable of 75 MW output power at 11.4 GHz have been under development at SLAC for the last decade. The work has been part of the program to realize all the components necessary for the construction of the Next Linear Collider (NLC). The effort has produced a family of solenoid-focused 50 MW klystrons, which are currently powering a 0.5 GeV test accelerator at SLAC and several test stands, where high power components are evaluated and fundamental research is performed studying rf breakdown and dark current production. Continuing development has resulted in a Periodic Permanent Magnet (PPM) focused 50 MW klystron, tested at SLAC and subsequently contracted for manufacture by industry in England and Japan. A 75 MW version of that PPM klystron was built at SLAC and reached 75 MW, with 2.8 microsecond pulses. Based on this design, a prototype 75 MW klystron, designed for low-cost manufacture, is currently under development at SLAC, and will eventually be procured from industry in modest quantities for advanced NLC tests. Beyond these developments, the design of Multiple Beam Klystrons (MBKs) is under study at SLAC. MBKs offer the possibility of considerably lower modulator costs by producing comparable power to the klystrons now available, at much lower voltages. SLAC's X-band klystron development began with the XC series of tubes designed to produce 100 MW output power with a relatively high perveance (1.8 µK) electron beam. These klystrons allowed study of several design approaches, including multiple gap output structures and multiple rf output waveguides with separate windows. Unfortunately, the peak power goal proved too high for the existing technology, with rf breakdown in the output cavities and windows, and high voltage breakdown in the electron gun, limiting design operation of the tubes. After a series of six prototypes it was decided to reduce the design goal from 100 MW to 50 MW peak output power for the next series of experimental tubes. This next series of klystrons, designated the XL series, utilized a lower perveance (1.2 µK) than the XC series. Drawing on the results of the XC series, methods to reduce rf fields in critical components were explored. A traveling wave TE 01 output window was utilized for the first time to reduce rf gradients on the window and eliminate electric fields at the vulnerable window edge. Various multiple gap standing wave and travelling wave output structures were tested, with a four-cell travelling …