HIGH POWER RF

A 20 MW X-Band (8.568 GHz) SLAC (Stanford Linear Accelerator Center) SL3 Klystron is currently being setup that will energize the RF photocathode linac used to accelerate photoelectrons in the X-band photoinjector project. The SL3 Klystron is a 5 cavity tube weighing approximately 2000 pounds, including the solonoid. A picture of this klystron is shown below.

The red cylinder is the focusing magnetic solenoid for the klystron tube. The collector and water cooling pipes are seen protruding from the top center of the solonoid in these pictures. So that the gun ceramic may not be punctured by high voltage, it is submerged in oil, contained within a lead tank. A sideview of this tank is shown below.

The Tank is approximately 6 feet long and 3 feet wide containing two compartments; one compartment for the klystron and one for a 15:1 step-up pulse transfomer.

The original operating parameters, designed by SLAC, for the SL3 klystron are shown in Table 1.

SLAC SL3 KLYSTRON DESIGN PARAMETERS
Frequency 8568 MHz
Beam Voltage 330 KV
Beam Current 152 A
Pulse Width 2µs
PRR 120 pps
RF Output Power 30 MW
RF Input Power 100 W
Efficiency 60%
Saturation Gain 54 dB
Max Surface Gradient on Output Gap 1.4 MV/cm
Brillouin Focusing Field 1039 Gauss

The SL3 klystron will be driven by a Varian, 1 KW X-band TWT amplifier, which will yield adequate power to drive the klystron. As shown in Table 1, the klystron requires a beam voltage of 330 KV, a beam current of 152 Amps, with a 2µs pulse width. This will be supplied by a PFN type modulator using an F-187 Hydrogen Thyratron switch. The F-187 Thyratron is capable of handling 750 MW of peak output power. The pulse shaping network is shown below.

This is a voltage fed, line-type pulser, commonly called a pulse forming network (PFN), in that it not only supplies the source of electrical energy during the pulse, but is also the pulse shaping element. The PFN is designed to match the impedance of the klystron at the primary side of the step-up (1:15) pulse transformer, which is approximately 9 ohms. With these impedances matched, the voltage at the primary side of the transformer will be equal to the charging voltage divided by two. The graph below shows a p-spice simulation of the above modulator circuit yielding the 2µs flat top pulse required for the SL3 klystron

The high voltage DC supply was set at 44KV during this simulation, and the graph shows a slight positive mismatch, where the load impedance is higher than the PFN. This will be fine tuned during actual experimentation, as well as the slight ripple observed on the flat top of the pulse by varying slightly the inductance of the PFN.


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