FMT Develops Robust, High-Current, Self-Bunched Electron Gun
FM Technologies has developed an innovative electron gun. The Micro-Pulse Electron
gun (MPG) utilizes a secondary emission cathode in an rf cavity driven to space-charge
saturation by a relatively low-power rf source. The MPG has many superior properties,
- Robust - no heater or laser required; can be exposed to air without harm
- High performance - capable of high current and high duty factor
- Self-bunched - generates short, prebunched pulses of electrons
- Low cost - does not require sophisticated control/timing system
The MPG is well suited to many applications including medical and research linacs and
high-frequency, high-power rf-sources. Devices explored to date include:
The micro-bunches have been measured directly and found to be 50-70 ps long
(~8% of the rf period) with a peak current of 20 A and charge of 1 nC per bunch (the
design value). Lifetime testing has been conducted for 18 months at nearly 24 hours
per day at 300 Hz with 5 msec macropulses. A gated L-band MPG is also being developed.
A 2.85-GHz MPG was operated at 30-250 Hz with a 2 ms macro-pulse,
producing micro-bunches at ~350 A peak from a 1.5 cm diameter cathode. A Gatling MPG
(GMPG) at S-Band has been developed which provides electron bunches at multiples of the
rf drive frequency. In this case a 2.85 GHz, TM110 rotating mode sweeps past four
emitters to produce 11.4 GHz bunches. The photograph shows a 3-GHz MPG brazed to a 2-1/2 cavity linac,
yielding a final beam energy >1.25 MeV with 200 kW applied to the linac. The MPG cavity
requires 60 kW.
A 9-GHz MPG has been operated routinely at 10 Hz with a 2-msec
macropulse. The beam current was ~ 7.5 A in the gun with a 0.5A beam extracted through
a 1.4 mm aperture.
The availability of very short beam bunches leads to designs
that make use of an output cavity tuned to a multiple of the bunch frequency. For
example, two-cavity high-power klystron designs are currently being evaluated for
frequency tripling, from 11.4- to 34 GHz.
Efficiency depends on the application, especially if post acceleration is employed.
The MPG typically uses ~50% of its cavity input power to generate the bunched beam.
This is generally a small fraction of the final output power of most devices.