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Design Study of a Plasma-Loaded CRM Using TPD-II Machine
Date of publication:
Cavity, Cherenkov instability in azimuthal direction, CRM instability,
cyclotron resonance maser, fast wave, gyrotron, negative absorption,
microwave, slow wave, TPD-II, wiggler.
Design study of an experiment for plasma-loaded cyclotron resonance maser
(CRM) utilizing TPD-II Machine at NIFS, Japan is described in some detail.
The principle of gyrotrons has been believed the CRM instability. However,
all the existing linear theories of CRM instability include unphysical modes
unstable at infinite values of axial wavenumber that can never be observed
experimentally. To overcome the difficulty, we derive and analyze
numerically an exact linear dispersion relation of a large orbit electron
beam for CRM, and removed the unphysical modes. However, the relation is
found to include two principles of cyclotron emission with oscillation
frequencies above and below the branch of fast electron cyclotron wave. The
former is CRM instability, and the latter is named Cherenkov instability in
the azimuthal direction (CIAD). It is noted that the CIAD we found remains
only a proposal of a new mechanism for cyclotron emission until the physical
existence is verified experimentally. To verify the CIAD, the design study
of a plasma-loaded CRM has been carried out. The apparatus consists of two
portions installed in the TPD-II: A pair of helical wiggler windings to
create a mono-energetic beam with 15 keV and pitch factor V_theta/V_z ≥1,
and microwave circuits including a cylindrical TE011 mode cavity with
resonant frequency 3.45 GHz. For high plasma density n≥1.5¡ß10^11 cm^-3 from
TPD-II, the CRM instability may be suppressed and the CIAD will take turn.
The present experimental study contributes to a deeper understanding and a
widened future prospect in gyrotron physics.
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