First Tests of the 250 keV Electron Source and Beamline for the TESLA Test Facility Injector

The TESLA Test Facility (TTF) uses an electron source consisting of a 30 kV thermionic triode gun followed by an electrostatic accelerating column to increase the beam energy to 250 keV. The gun cathode is modulated at 217 MHz to provide sub-nanosecond micropulses within the 800 μs macropulse. The beam is then transported to superconducting 'capture' cavity through a 3 m long beamline equipped with solenoidal focusing lenses, a 217 MHz pre-buncher cavity and several beam monitors. The gun, its associated electronics and the transfer beamline have been assembled and tested. We present the first measurements of the beam emittance under the nominal operating condition of 8 mA average current in the macropulse. These measurements are found to be consistent with measurements made on the 30 kV gun alone and with simulations of the transport through the electrostatic column. Beside the anode-cathode voltage, three additional voltages determine the gun current. The gun is normally held in "cut-off" by a negative d.c. bias applied to the grid. A combination of two pulses are used to drive the gun into conduction. The first is a positive square wave pulse of variable width (10 μs - 800 μs) applied to the grid at a repetition of 10 Hz or less. The second is a train of negative pulses at 216.7 MHz applied to the cathode. The negative pulses are furnished from a commercial broad band RF amplifier capable of providing outputs of up to -100 V and of duration <1.5 ns. The "portion" of this pulse which switches on the gun can be varied by altering the amplitudes of the grid bias and square wave pulse. Thus the width and amplitude of the micropulse can be independently controlled. The settings of these values were determined following a measurement of the gun characteristic(3).