110 GHz, 1 MW power level gyrotron oscillator

Characteristics and Applications of Gyrodevices

Gyrodevices are currently being developed for a variety of applications where high power levels are required at millimeter wave frequencies. Gyrodevices, also known as cyclotron resonance masers or gyrotrons, take advantage of the cyclotron resonance maser instability to transfer energy from an electron beam to an electromagnetic wave. In the interaction, an annular electron beam, composed of mildly relativistic electrons traveling in helical paths, interacts with the electromagnetic fields of a circuit or cavity in the presence of an applied axial magnetic field.
One important way in which gyrodevices differ from conventional microwave devices, such as klystrons or helix traveling-wave-tubes, is that the phase velocity of the electromagnetic wave in a gyrodevice is equal to or greater than the speed of light. Devices such as gyrotrons, exhibiting this characteristic, are known as "fast-wave" devices. In cyclotron resonance masers, the synchronism between the electron beam electromagnetic fields of the circuit is achieved by chosing the applied magnetic field such that the cyclotron frequency of the electrons is nearly equal to the desired microwave frequency of the device. The benefits of gyrodevices are derived from the combination of the cyclotron resonance interaction and the fast-wave interaction circuit. In fast-wave circuits, the electromagnetic field strength can be quite high, independent of the proximity of the metallic circuit structure. This enables the electron beam to be situated in regions of high field, for optimum coupling, without necessarily placing the beam too close to the circuit. The interaction mode is selected by the magnitude of the applied magnetic field, which determines the electron cyclotron frequency, and the placement of the electron beam. If one makes use of a high-order waveguide or cavity mode, the transverse dimensions of the interaction structure can be several times the free-space wavelength.

On the contrary, in traditional slow-wave devices, the interaction circuits are designed to reduce the phase velocity of the electromagnetic wave to values below the speed of light so that synchronism between the beam and wave can be maintained. The electric field strength falls off rapidly with distance from the circuit structure. Typical transverse circuit dimensions required to effectively slow the phase velocity of the wave are on the order of 10% of the free space wavelength. The small circuit sizes, necessary to achieve the interaction in a slow-wave device, severely limit the peak and average power capabilities, particularly at millimeter wave frequencies.

The cyclotron maser interaction has been used to produce high-power millimeter waves in both oscillator and amplifier configurations. Gyrotron oscillators, developed primarily for fusion applications, have achieved power levels near 1 MW for pulse durations in excess of 1 second at frequencies above 100 GHz. Gyrotron amplifiers are being developed for applications requiring phase coherence and instantaneous bandwidth, such as linear accelerators and millimeter wave radar. Recent successful demonstrations of gyroklystron amplifiers from X-band to W-band suggest the promise of these devices.

A tour of the experiment follows the talk.

Monica Blank received the B.S. degree (Electrical Engineering) from the Catholic University of America, Washington, D.C. in 1988, and the M.S. and Ph.D. degrees (Electrical Engineering) in 1991 and 1994, respectively, from the Massachusetts Institute of Technology, Cambridge, where her dissertation work involved theoretical and experimental studies of quasi-optical mode converters for high power gyrotron oscillators. In 1994 she joined the Vacuum Electronics Branch of the Naval Research Laboratory, where she was responsible for the design and demonstration of high-power millimeter wave vacuum electronic devices for radar applications. In 1999 she joined the gyrotron team at Communications and Power Industries (formerly Varian) where she continues her work on high-power millimeter wave gyrotron amplifiers and oscillators.
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