Rf Magnetic Probe Research Articles

Measurement of the Lower Hybrid Wave Using RF Magnetic Probes on the TST-2 Spherical Tokamak

Measurement of the Lower Hybrid Wave Using RF Magnetic Probes on the TST-2 Spherical Tokamak

Identification of waves in the lower-hybrid frequency range in the scrape-off layer plasma of Alcator C-Mod

Polarization resolved measurements of the parallel refractive index of the driven RF waves in the lower hybrid (LH) range of frequencies are performed using arrays of RF magnetic probes in the scrape-off layer plasma of Alcator C-Mod. The measured of the RF magnetic field component parallel to the background magnetic field is about −1.6, which corresponds to the peak of the launched LH spectrum. Based on the wave dispersion relationship, this wave is identified as the LH slow wave. On the other hand, the RF magnetic field component perpendicular to the magnetic field is found to have a lower of −1.2, and is detected only near the last closed flux surface. This wave is identified as the LH fast wave generated by slow-fast wave mode conversion.

Identification of Waves by RF Magnetic Probes during Lower Hybrid Wave Injection Experiments on the TST-2 Spherical Tokamak

Identification of Waves by RF Magnetic Probes during Lower Hybrid Wave Injection Experiments on the TST-2 Spherical Tokamak

The resonant radio-frequency magnetic probe tuned by coaxial cable

In this paper, the resonant rf magnetic probe is upgraded by replacing the rotary capacitor in the old version with the series-connected coaxial cable. The numerical calculation and the measurement with the prototype probe show that the rf magnetic probe can achieve resonance at a middle length of the series-connected coaxial cable. The good electrical symmetry of the new rf magnetic probe is ensured by both the identity of series-connected coaxial cables and the new structure of the primary winding. Practical measurements conduced on an rf inductively coupled plasma source demonstrate that performances of the new rf magnetic probe are good.

A tunable radio-frequency magnetic probe

A tunable center-tapped transformer is proposed to increase the output of a rf magnetic probe and improve the signal-to-noise ratio. The tuning is implemented by a variable capacitor connected parallel with the primary winding of the tunable center-tapped transformer. Undesirable common-to-differential conversion is reduced by installing a compensating capacitor. In addition, a planar Faraday shield is installed between the windings of the transformer to further suppress the electrostatic coupling. It is found that tuning the variable capacitor can result in a resonance in the output voltage of the rf magnetic probe. The largest output voltage, achieved with the tunable magnetic probe under the optimal condition, is higher than that with a conventional one by an order of magnitude. Effects of the compensating capacitance on the common-mode output voltage are studied and discussed. Influences of parameters such as cable length, the coupling coefficient, and the step-up ratio of the transformer on the output voltage are also presented. Analytical derivations and numerical calculations based on the equivalent circuit are performed to elucidate the characteristics of the differential mode.

Parametric decay instability during high harmonic fast wave heating experiments on the TST-2 spherical tokamak

A degradation of heating efficiency was observed during high harmonic fast wave (HHFW) heating of spherical tokamak plasmas when parametric decay instability (PDI) occurred. Suppression of PDI is necessary to make HHFW a reliable heating and current drive tool in high β plasmas. In order to understand PDI, measurements were made using a radially movable electrostatic probe (ion saturation current and floating potential), arrays of RF magnetic probes distributed both toroidally and poloidally, microwave reflectometry and fast optical diagnostics in TST-2. The frequency spectrum usually exhibits ion-cyclotron harmonic sidebands f0 ± nfci and low-frequency ion-cyclotron quasi-modes (ICQMs) nfci. PDI becomes stronger at lower densities, and much weaker when the plasma is far away from the antenna. The lower sideband power was found to increase quadratically with the local pump wave power. The lower sideband power relative to the local pump wave power was larger for reflectometer compared with either electrostatic or magnetic probes. The radial decay of the pump wave amplitude in the SOL was much faster for the ion saturation current than for the floating potential. These results are consistent with the HHFW pump wave decaying into the HHFW or ion Bernstein wave (IBW) sideband and the low-frequency (ICQM). Two additional peaks were discovered between the fundamental lower sideband and the pump wave in hydrogen plasmas. The frequency differences of these peaks from the pump wave increase with the magnetic field. These decay modes may involve molecular ions or partially ionized impurity ions.

A calibration method of a radio frequency magnetic probe

We demonstrate a simple calibration method of a rf magnetic probe by utilizing a two port network analysis method. The sensitivity (V/G) of the probe is determined by measuring the scattering parameters of S11 and S21. The magnetic field generated by the Helmholtz coil is calculated from S11 and the probe sensitivity is obtained from S21. The measured probe sensitivity is 0.1–0.6 V/G in the frequency range of 1–15 MHz.

Measurements of rf emission and transmission in the 5–500 MHz range in TFTR (abstract)

Active and passive measurements using an array of broadband rf magnetic probes in TFTR have suggested several diagnostic applications. Swept, fast wave propagation experiments using one probe as a launching antenna have revealed absorption peaks from the dominant minority species (hydrogen and helium-3) in ohmic plasmas. A new synchronous modulation/detection system has been installed to allow operation during beam heated plasmas, where spontaneous emission is significant. Proposed experiments will investigate the localization of the absorption and evaluate the utility of this measurement as a diagnostic for tritium. The signal processing system for the probe array has also been recently modified to permit fast gating during ICRF heating pulses. This change allows measurement of the rf emission spectrum immediately following a heating pulse and will be used to look for features arising from slowing down of the high-temperature minority tail. The experiment, by analogy, may give an indication of the spectral signature that will result from alpha particles during future DT operation. This work was supported by U.S. Department of Energy Contract No. DE-AC02-76-CHO-3073.

Novel RF magnetic probe using LED linked with optical fibre

A novel radio-frequency magnetic probe composed of a small loop antenna and a light emitting diode (LED) linked with an optical fibre is presented, and its characteristics are experimentally examined. In order to confirm its validity, some of the RF field measurements made by the probe are also presented.

Experiments on Ion Cyclotron Waves

Experiments have been performed on the generation of ion cyclotron waves and their propagation into a magnetic beach. The experiments were carried out on the B-66 machine, which is currently a magnetic mirror device. Studies of the production of neutrons have provided evidence for the absorption of the energy of these waves via ion cyclotron damping. Microwave phase-shift measurements have now been made, and the addition of electron density completes the list of parameters required for direct comparison of experimental and theoretical dispersion relations. The experimental data yield a smooth monotonic relation between density and frequency which is qualitatively similar to that predicted by theory. There are, however, unexplained quantitative differences. Wave propagation into the magnetic beach region was observed with a single turn rf magnetic probe. The variation of the amplitude of these waves in the magnetic beach is in qualitative agreement with the theory of ion cyclotron wave propagation and cyclotron damping.

Observation of Ion Cyclotron Waves

The existence of ion cyclotron waves had been inferred from theoretical calculations and indirectly from experimental observations. In this paper direct experimental observations are reported of phenomena appropriate to these waves in a hot plasma. An induction coil energized at 11.5 Mc with approximately 200 000 w of rf power surrounds a deuterium plasma confined in a 16-kilogauss magnetic field. A single-turn rf magnetic probe located in the plasma about 50 cm down the axis from the center of the induction coil is used to detect the propagated wave. The probe is movable. Radial motion of the probe shows the expected penetration of the transverse rf field into the center of the plasma. Axial motion of the probe shows a 300-cm damping length, and an oscillation wavelength 64% longer than the calculated value. The probe signals disappear when the ion cyclotron frequency is less than the induction coil frequency, which is a result predicted from the ion cyclotron wave dispersion relation.