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Thermionic Electron Gun

The Electron Gun operating in the NSLS linac, is a pulsed-grid design, it is also possible though to have a pulsed-cathode if desired. However, in the pulsed-grid configuration, electrons emitted from the cathode are held on the cathode by applying a negative bias of sufficient voltage on the grid (Electrons flow from negative to positive). The grid is then pulsed from negative to positive with a series of five nano-second pulses, or better known as the macro-pulse, we normally operate with an eight-pulse bunch, macro pulse length. When the pulse amplitude reaches a certain level (-40 volts for a deck voltage of 100kv), electrons start to flow; the higher the pulse voltage is, the more electrons and the higher the current.

Thermionic Gun

Thermionic Gun

The gun and the control electronics sit at negative 97Kv potential. The electrons that flow from the cathode to the grid are now accelerated toward the anode [LEBT], which is ground potential, at an energy of 97 Kev.

The gun is housed in the ceramic column, or reentrant as it is sometimes called, heater leads and a coax cable run from the gun to the High Voltage deck. The deck is mounted on a 120VAC to120VAC isolation transformer, which supplies the AC to operate the 12VDC power supply that runs the electronics for the gun. The grid bias level control and gun pulser are connected to the ground level controls via fiber optics.

The Gun Electronics

  1. Filament [heater] and pulser power supplies
  2. Grid bias supply
  3. Grid driver
  4. Pulser optical receiver

The filament supply is a voltage regulator, incorporating a LM723 as the main device. It also has the feature of current limitation, which is set at two amps. Test points are provided to measure both voltage and current, the current monitor is I=2*V. The voltage is set by a trim pot on the card, normal operation is 6.2V / 1.5A. Featured also is a low bias fault, which will run the filament to zero volts. This feature is required, because the loss of bias would result in the gun running in a CW state, resulting in possible damage to the gun. The grid drive power supply is also mounted on this card. It is an EMCO E05 or E10, 500v or 1000v respectively. These are small switching, non regulated supplies, the 500v model is run directly off the 12v supply the 1000v is run off a 5v regulator to give an output on the order of 450v, the pulser only requires > 350v, and has it's own regulator

Electron Gun High Voltage Deck

Electron Gun High Voltage Deck

The grid bias supply was designed as a multi purpose card. It was used as the grid bias, amplitude control and long pulse pulser. The amplitude is no longer used and the long pulse, micro-second, was never used. The upper section of the card is the only part in use; it is a negative switching supply with a capability of 250 volts. The output is controlled by a fiber optic link or may be set by a pot on the card, the minimum level is set in this manner, test point are provided. {min is set at neg. 80v]

The grid driver supplies the positive pulses to drive the gun. The gun command pulse, a macro pulse with a frequency of 10.57mhz with 8 pulses, is sent to the card via a fiber optic link. The pulses widths are from 20 to 30 nano seconds, this is adjustable at the ground level transmitter. This must be the case as the optical link is not capable of sending the 5 nano second pulse required by the gun. Pulse shaping is done on the card to get a 1 nano second rise time. The optical receiver only has an output of millivolts, this is amplified by a MAR 3 rf amplifier, the level is increased to 5v and drives a series of 74F04's and a 2N6661 to improve the pulse shape and produce a 10v negative pulse. The coax cable and the pulse transformer combination, provide a 30v pulse on turn off with a rise time of a few hundred pico-seconds. This circuit is critical and changing cable lengths or terminating resistor will affect the pulse. The circuit is transformer coupled using an IRF710 as a series element to drive the gun grid. The rise time is about 1 nano second but the width at this point is about 10 nano seconds, a second IRF710 is connecter as a shunt or tail bitter element. A current transformer monitors the grid current, this signal drives the shunt FET through a length of coax, and the length of this cable determines the pulse width. Although the signal does not go to zero volts, it goes low enough that the bias cuts the gun off. The match to the gun is not perfect causing the first pulse in the bunch to be lower in amplitude, this is not a problem for operations and no effort has been made to improve the match.