Basic principles of heterodyne techniques are introduced and the various components of a heterodyne system are summarized. Special applications in ECE, interferometry and reflectometry are discussed after introducing the diagnostic principles. Realized systems as described in the literature are briefly outlined. Ordering principles are radiometer types in the case of ECE, mixing scheme and generation and stabilization of local oscillator and intermediate frequency signals in the case of interferometry and reflectometry. Special techniques and their impact on the performance of the diagnostic instruments are discussed.

https://iopscience.iop.org/article/10.1088/0741-3335/39/11/001/pdf

Heterodyne methods in millimetre wave plasma diagnostics with applications to ECE, interferometry and reflectometry

Results are presented of experiments carried out in JET with the Mark II divertor to study ELMy H modes at high density. In these experiments the effective ion mass of the plasma (pure hydrogen, deuterium and tritium), input power, plasma current and edge triangularity were varied. The variation of density was achieved by gas fuelling. The focus is on two main issues: the variation of global energy confinement with density and the scaling of the H mode pedestal. It is found that the density dependence in the ITER97-P(y) scaling is not confirmed by experiment when the density limit is approached. For the high density regime in ELMy H modes a density independent scaling is proposed that includes the plasma triangularity in the regression variables. The core energy content of the plasma is correlated to the edge pedestal parameters, consistently with a mixed Bohm-gyroBohm transport model for H mode plasmas. It is found that with type I ELMs, the edge pedestal pressure increases with the ion mass of the plasma, the input power and the edge magnetic shear. A scaling is derived for the pressure at the top of the edge pedestal which is consistent with a model where the pedestal width is assumed to be proportional to the ion gyro-radius and the pedestal gradient is limited by ideal MHD.

The influence of isotope mass, edge magnetic shear and input power on high density ELMy H modes in JET

In this review of plasma polarimetry, first the theory is discussed in general, then exact analytic solutions of the evolution equation for polarization are presented, as well as approximate analytic solutions. Numerical integration of the evolution equation is also discussed. The design of experiments is then considered taking into account the maximum amount of information which can be obtained from polarimetric measurements. Special attention is devoted to the techniques of polarization modulation (including both progressive and alternating modulation). Different alternative configurations are described which are of special interest because they can be realized in the far infrared and because they allow a measurement of phases rather than amplitudes. The effects of refraction are then briefly considered. Finally, the combination of polarimetry and interferometry on the same instrument is discussed, including the effects of polarization modulation.

http://iopscience.iop.org/article/10.1088/0741-3335/41/2/001/pdf

A review of plasma polarimetry—theory and methods

Impurity injection in the JET ELMy H-mode regime has produced high-confinement, quasi-steady-state plasmas with densities close to the Greenwald density. However, at large Ar densities, a sudden loss of confinement is observed. A possible correlation between loss of confinement and the observed MHD phenomena, both in the core and in the edge of the plasma, was considered. The degradation in confinement coincided with impurity profile peaking following the disappearance of sawtooth activity. In addition, impurity density profile analysis confirmed that central MHD modes prevented impurity peaking. Experiments were designed to understand the role of sawtooth crashes in re-distributing impurities. Ion-cyclotron radio frequency heating was used to control the central q-profile and maintain sawtooth activity. This resulted in quasi-steady-state, high-performance plasmas with high Ar densities. At and high Ar injection rates, quasi-steady-states, which previously only lasted <1τE, were now maintained for the duration of the heating (Δ t ~ 9τ E). The increased central heating may have an additional beneficial effect in opposing impurity accumulation by changing the core power balance and modifying the impurity transport as predicted by neo-classical theory.

/pdf/role-of-sawtooth-in-avoiding-impurity-accumulation-and-1ra9u8saiq.pdf

Role of sawtooth in avoiding impurity accumulation and maintaining good confinement in JET radiative mantle discharges

A double-pass, radially viewing, far-infrared laser-based POlarimeter-INTerferometer (POINT) system utilizing the three-wave technique has been implemented for diagnosing the plasma current and electron density profiles in the Experimental Advanced Superconducting Tokamak (EAST). POINT has been operated routinely during the most recent experimental campaign and provides continuous 11 chord line-integrated Faraday effect and density measurement throughout the entire plasma discharge for all heating schemes and all plasma conditions (including ITER relevant scenario development). Reliability of both the polarimetric and interferometric measurements is demonstrated in 25 s plasmas with H-mode and 102 s long-pulse discharges. Current density, safety factor (q), and electron density profiles are reconstructed using equilibrium fitting code (EFIT) with POINT constraints for the plasma core.

Initial measurements of plasma current and electron density profiles using a polarimeter/interferometer (POINT) for long pulse operation in EAST (invited).

A multichannel far‐infrared interferometer used on the Joint European Torus (JET) is described. The light source is a 195‐μm DCN laser. The instrument is of the Mach–Zehnder type, with a heterodyne detection system. The modulation frequency (100 kHz) is produced by diffraction from a rotating grating. There are six vertical and two oblique channels. The latter rely on retroreflection from mirrors mounted on the vessel wall. Their vibration is compensated by a second wavelength interferometer at 118.8 μm. The various subsystems are described, with emphasis on features necessitated by (a) large path lengths, (b) remote handling requirements, (c) fluctuations in atmospheric humidity, and (d) unmanned automatic operation. Typical measurements, along with real‐time and off‐line data analysis, are presented. The phase‐shift measurement is made with an accuracy of (1)/(20) of a fringe, corresponding to a line‐integrated electron density of 5×1017 m−2. Comparison with other electron density diagnostics are shown. The introduction of additional optics allows measurements of the Faraday effect and a determination of the poloidal magnetic field distribution. The signal processing and data analysis are described. Errors introduced by the calibration procedure, birefringence of the probing beams, toroidal field pickup, the flux geometry, and the density profile are considered. The Faraday angle is measured with an accuracy of 5% and a time resolution of 1–10 ms. The poloidal magnetic field is deduced with an accuracy of ±15%.

D. Véron

Papers

JET polari-interferometer