P.J. Paris

Ellipsoidal diffraction grating as output coupler for quasi-optical gyrotrons

The use of a diffraction grating arranged in the -1 Littrow mount as an output coupler for a quasi-optical Fabry-Pérot resonator at microwave frequencies (100 GHz) was suggested in 1990 [1]. A planar grating with curvilinear grooves (in order to match the Littrow condition everywhere on the surface for a given Gaussian beam) gives a power coupling efficiency of 85%, which is limited by the depolarization induced by the curvature of the grooves. An ellipsoidal grating with linear, equidistant grooves minimized the depolarization but gave approximately the same global efficiency because of distortion. We report low power tests on an improvement of the second scheme, based on curved grooves on an ellipsoidal surface, which gives a global efficiency of 94%.

An Improved Q-Machine Source

The development of a new Q‐plasma source is described. Emphasis is given to the novel parts of the system which have been designed to produce long term, constant density plasmas with low fluctuation levels, as well as long machine operation time. Relevant plasma parameters are obtained by laser induced fluorescence which is used as a diagnostic on the barium plasma. Plasma densities are in the range of 108 cm−3 to 1011 cm−3, with an electron temperature of about 0.2 eV, and a perpendicular ion temperature of the same magnitude. The parallel ion temperature is one half the perpendicular value.

Properties of Diffraction Gratings Used as Output Couplers in a Quasi-Optical Gyrotron

The design of a diffration grating used as an output coupler for a Fabry–Perot resonator is presented. We then consider the problem of determining the distortion and the cross‐polarization of the incident Gaussian beam. Different types of gratings and planes with straight or curvilinear grooves and elliptical, have been designed and built. The output pattern from a resonator using such a grating has been measured experimentally. We found that the elliptical grating generates the least distortion and cross‐polarization. This result is attributed to the geometry of the grooves and is in excellent agreement with numerical calculations.

Test-ion diffusion in a magnetized plasma

Cross-field transport of barium test ions is studied in a Q-machine plasma. Test particles are produced and tracked in their motion by an optical tagging method. The comparison between the measured tag signal and a simple theoretical simulation yields an evaluation of the ion diffusivity. Diffusion is supported by classical mechanisms. The injection of different noble buffer gases modifies the plasma parameters in such a way that the diffusion remains classical but its magnitude is reduced. >

Optical Carriage for Laser-Induced Fluorescence in a Magnetized Plasma

An ‘‘optical carriage’’ has been developed to improve plasma access for LIF diagnostics. Laser light inducing the fluorescence is transported through an optical fiber to the carriage. A telescope fixed on the carriage collects the plasma fluorescence light and sends it through a fiber bundle to an external PMT. The whole carriage is mounted on rails and can be scanned along and across the magnetic field.

Electrodeposition of rhenium on tungsten hot plate used in Q plasma source

The brief note describes diverse aspects of the technique used for rhenium plating the hot plate used for contact ionization of barium in a Q plasma source. The rhenium plating optimizes the probability of barium ionization. Special care has to be taken in order to deposit a uniform layer of rhenium, for this purpose a slow rotation is given to the plate during deposition.

New vaporizing assembly for q-plasma sources

The design and operation of a new vaporizing assembly used for Q-plasma sources are described in this article. The vaporizing system, consisting of two ovens and an effuser, is used to vaporize and direct atoms onto a hot ionizer plate. The hot plate, which may be of tantalum, tungsten, or rhenium, singly ionizes the atoms on contact. The main advantages of this new assembly, compared to previous designs, are the production of higher plasma densities and the control of the radial plasma profile. The heaters of the two atomic beam ovens are independently controlled and monitored so that the gradients of the radial plasma profile can be modified. Plasma production with two ion species can also be performed.

Ion-Cyclotron Wave Excitation by Double-Resonance Coupling

A modulated high frequency wave is used to remotely excite low frequency oscillations in a linear, strongly magnetized plasma column. An electromagnetic wave is launched as an extraordinary mode across the plasma by an external waveguide in the upper‐hybrid frequency regime f≊fUH≊fce≊8 GHz, with P≤2 W. By frequency modulating (at fFM≊1–60 kHz, with fci≂30 kHz) the pump wave, the resonant layer is swept radially across the profile and perpendicularly to the field lines at f=fFM. The resulting radial oscillation of the electron linear and nonlinear pressure can be considered to act as a source term for the ion wave. A localized virtual antenna is thereby created inside the plasma. Measurements of the ion dielectric response (interferograms and perturbed distribution functions) via laser‐induced fluorescence identify the two branches (forward, or ion‐acoustic‐like, and backward, or Bernstein, modes) of the electrostatic dispersion relation in the ion cyclotron frequency range. By changing the modulation band...

The use of high-order TM0m0 modes of a resonant cavity for magnetized plasma density measurements

A study of the application of high‐order TM0m0 modes of a cylindrical cavity to the measurement of the density of a magnetized plasma column is presented. It is shown theoretically that judiciously chosen high‐order modes have the potential advantages of both a wide operational range of densities, and a wide range for which a simple perturbation theory is valid. Furthermore, an experiment is described which shows that the TM060 mode can be excited with a sufficiently high Q value to allow accurate determination of the resonant frequencies, and hence plasma density. Favorable comparison between densities in the range 1010–1012 cm−3 measured by means of the resonant cavity technique and microwave interferometry is presented.

Measurements of turbulent waves by means of microwave scattering and correlation techniques

The turbulence induced by a current drawn in a low‐density argon plasma is investigated by microwave scattering and correlation techniques. The azimuthal 30‐GHz scattering system uses a newly designed variable waveguide in order to detect the scattered signal at any angle between 0° and 90°. Both diagnostic methods show that the turbulence is dominated by low‐frequency waves propagating perpendicular with respect to the current. A comparison of these two important diagnostic methods for turbulent plasma is presented.