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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, including:

  • 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.

S-Band Linac
S-Band Linac

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.


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