SIXTH - HARMONIC GYROFREQUENCY MULTIPLIER

High power, high frequency rf sources are required for many applications such as imaging radars, rf linear acceleration for the next generation of linear colliders, electron cyclotron heating (ECH) of fusion plasmas and for plasma diagnotics.

The gyrofrequency multiplier is a new class of device capable of efficiently producing high power in the millimeter to submillimeter wave regions for these applications. The device is essentially a high power frequency multiplier. It is well suited for applications requiring a compact source because of its use of a harmonic interaction and low external voltages.

As shown in the diagram below, the gyrofrequency multiplier has three major components: an electron gun, an input low frequency cavity, and an output high frequency cavity. Electrons from the electron gun are accelerated in the input cavity using high power microwaves to form a rotating helical beam. Microwaves at a much higher frequency are generated in the output cavity by matching its resonant frequency to a multiple of the rotation frequency of the beam. Additionally, the spatial structure of the high frequency microwaves is matched to that of the helical beam leading to high efficiency.

Two devices have been developed so far. The first was a third-harmonic device and produced 6.7 kW at 27.7 GHz with 13% efficiency. The second was a sixth-harmonic device and produced 4 kW at 17.4 GHz with 5% efficiency. Currently a 1 THz gyrofrequency multiplier is being developed as a possible source for an alpha particle diagnostic on the next generation fusion energy tokamak, ITER.

Objective:

Develop a highly efficient compact high power source for millimeter/submillimeter waves.

Approach:

Use prebunched nature of electron beam created through rf acceleration to multiply the frequency of existing microwave sources.

Accomplishments:

Third-harmonic device produced 6.7 kW at 27.7 GHz (13% efficient). Sixth-harmonic device produced 4 kW at 17.4 GHz (5% efficient). 1 THz, 80 kW tenth-harmonic device designed for ITER alpha particle diagnostic.

Schematic of Gyrofrequency Multiplier

Disassembled 2.0 microperv Pierce Electron Gun

Axis-Encircling Electron Beam Created Through Gyroresonant RF Acceleration

Assembled Sixth-Harmonic Gyrofrequency Multiplier


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