Modulation Reflectometry Research Articles

Design of an O-mode frequency modulated reflectometry system for the measurement of Alborz Tokamak plasma density profile

Reflectometry is a common method for plasma diagnostic, in which microwaves are launched into the plasma and reflected at the critical surfaces. Comparing the reflected microwave signals with the launched waves would give rise to the plasma density profiles. In the present study, an ordinary mode (O-mode) frequency modulation (FM) reflectometry system has been designed for the electron density profile measurement of the Alborz Tokamak plasma. This system has been considered to operate at K-band (18–26.5GHz) frequency range and scan the frequency band between 18 to 26GHz in 40μS. The density profile from major radius r=47.9–51.55cm can be measured in Alborz Tokamak plasma. Based on the Alborz Tokamak operational conditions, the characteristic frequencies, and some dimensional limitations, all parts of reflectometer have been designed so that an appropriate efficiency with minimum attenuation, especially in transmitting/receiving system would be achieved. A dual antenna and an oversized waveguide of X-band (8–12GHz) for transmitting and receiving purposes and a balanced detector for absolute phase determination have been utilized. The details of the Alborz Tokamak FM reflectometry components focusing on the antenna and waveguide design and mounting are described in this paper. Additionally, the procedure of plasma profile reconstruction using the system output signal is discussed. This system uses signal phase shift to determine the position of the cutoff layer.

Design of an X-mode fast-scanning reflectometry for edge density profile measurement on HT-7 tokamak

An amplitude modulation reflectometry system with fast-scanning microwave source is designed for edge density profile measurement on the HT-7 superconducting tokamak (R0=1.22 m, a=0.28 m, Bt: 1.8–2.3 T, plasma pulses of 1–20 s). The reflectometer launches microwave power from a very fast-scanning backward wave oscillator in 53–78 GHz to probe the plasma and receive the reflection by the density cutoff layer. It is an extraordinary mode reflectometer, which can measure the edge density profiles in normalized radius r/a from scrape-off layer up to 0.5 when Bt=2.2 T, Ne0=3×1019 m−3 in typical HT-7 operation parameters. The microwave amplitude modulation frequency is 200 MHz. A 2 MHz reference signal and the probing signal are obtained by mixing a 198 MHz local oscillator power with the demodulated microwave signals. Phase comparison is then accomplished at 2 MHz by I/Q phase detector. The scanning time over the full band of 53–78 GHz is about 0.5 ms. The reflectometry will fill in the space left by the five-chord interferometer, which measures in the core plasma region and the Langmuir probes, which work in the scrape-off layer; especially the reflectometry is used to study the lower hybrid wave coupling into plasmas for the current drive and the radio frequency wave coupling problems for heating on HT-7. The reflectometry is also considered to be useful on the future HT-7U superconducting tokamak.

Density profile measurements by AM reflectometry in TJ-II

An amplitude modulation reflectometry system is in operation at thestellarator TJ-II. Recently, the first electron density profiles wereobtained, showing good agreement with profiles measured by Thomsonscattering and lithium beam diagnostics. In order to measure densityprofiles from the plasma edge, the extraordinary mode of polarization isused. A hyper-abrupt varactor-tuned oscillator used in combination withactive multipliers generate two frequency segments: 25-36 and 36-50etc GHz sharing a unique common wave-guide system (Ka band). The signalis amplitude modulated at 200 MHz and the phase demodulation is done atlower intermediate frequency. The time evolution of the electron densityprofile was measured under different experimental conditions. In thispaper, we present the temporal evolution of the profiles obtained duringa transition to an enhanced confinement mode and during cold pulsepropagation experiments. We also report the modification in the shape ofthe density profile measured during a magnetic configuration scan inwhich a low-order rational surface is moved from the scrape-off layerinto the plasma confinement region.

Sensitivity of fast swept frequency modulation reflectometry to changes in magnetic configuration in ASDEX upgrade

Microwave reflectometry is a diagnostic well suited to measure the plasma density profile in large fusion devices. The possibility to make reflectometry measurements with unfavourable propagation of the incident microwaves is especially important in devices where significant modifications of the flux surfaces are expected, as will be the case of ITER. The sensitivity of reflectometry to D-shaped and circular plasma configurations was studied in ASDEX Upgrade. Ray tracing was used to model the experiments and to determine the limits for reflectometry measurements.

Experimental results of amplitude modulation reflectometry on the FTU tokamak

The amplitude modulation (AM) reflectometer realized for the FTU tokamak has been successfully operated in a wide range of plasma conditions. The reflectometer has been calibrated using the signal reflected by the inner part of the vacuum chamber (back wall) in absence of the plasma. Density profiles have been obtained for steady-state discharges by inverting the AM phase measured as a function of the carrier frequency. The results are in good agreement with density profiles obtained from multichannel far infrared interferometry.

First results with amplitude modulation reflectometry on the PBX-M tokamak

Electron density profile measurements have been obtained on PBX-M by amplitude modulation reflectometry. With this technique, the measurement of the time delay is obtained from the phase delay of the modulating envelope (200 MHz in PBX-M). The system operates with the extraordinary mode, in the range 32–50 GHz. Under these conditions the density profile of PBX-M can be probed from the scrape-off layer up to typically r/a=0.7. With a final bandwidth of 40 kHz, the reflectometer is able to obtain the edge profile in 1 ms. The profiles obtained are relatively noise free and in good agreement with Thomson scattering measurements. Perturbations due to the strong-edge turbulence are kept to a minimum, and no software filtering or other signal processing was necessary to extract the time delay information from the raw data. Profiles have been measured for ohmic and Neutron Beam Injection heated discharges.

Amplitude modulation reflectometry for large fusion devices

One of the main problems which arises in density profile measurements by reflectometry is the need for continuous tracking of the phase delay; fast density fluctuations and strong modulation on the amplitude of the reflected signal can cause ‘‘fringe jumps’’ in the phase signal. Amplitude modulation reflectometry performs a time delay measurement by the determination of the phase delay of the modulating envelope of a millimeter wave reflected by the plasma. The phase delays involved are small, the measurement is not affected by the fluctuations and can be directly performed without complicated fringe counters; the method provides a promising possibility for real time determination of the plasma position and density profile. In the present paper the principles of the method are presented as well as the analysis on the effects of phase and amplitude fluctuations, dispersive effects, and accuracy requirements.