A. Boboc

ELM pacing investigations at JET with the new pellet launcher

A new pellet injection system was installed at JET designed for both fuelling and ELM pacing. The purpose of the pacing section was to validate pellet ELM pacing as a suitable tool for ELM mitigation in ITER. Pellet pacing was confirmed at the large size scale of JET. The dynamics of triggered ELMs was investigated with respect to their spontaneous counterparts. Triggered ELMs show features also typical for spontaneous ELMs in several operational regimes. Since none of these regimes was unsettled by the pellets this is a strong hint for compatibility with other plasma control tools. Observations and modelling results indicate the ELM triggering occurs by the evolution of the pellet ablation plasmoid into the first ELM filament followed by a poloidal spread of the instability. An ELM obviously can be forced by a pellet due to the strong local perturbation imposed already under unusual onset conditions but then evolves like any ELM typical for the corresponding plasma regime. For tool optimization the pellet mass and hence the convective confinement losses imposed have to be minimized. In our experiments, a lower mass threshold was observed for the first time. It has been found that to reliably trigger an ELM the pellet needs to be sufficiently large (and fast) to penetrate close to the pedestal top. Recent investigations are clear steps forward to validate the pellet pacing approach for ITER.

Runaway electron beam generation and mitigation during disruptions at JET-ILW

Disruptions are a major operational concern for next generation tokamaks, including ITER. They may generate excessive heat loads on plasma facing components, large electromagnetic forces in the machine structures and several MA of multi-MeV runaway electrons. A more complete understanding of the runaway generation processes and methods to suppress them is necessary to ensure safe and reliable operation of future tokamaks. Runaway electrons were studied at JET-ILW showing that their generation dependencies (accelerating electric field, avalanche critical field, toroidal field, MHD fluctuations) are in agreement with current theories. In addition, vertical stability plays a key role in long runaway beam formation. Energies up to 20 MeV are observed. Mitigation of an incoming runaway electron beam triggered by massive argon injection was found to be feasible provided that the injection takes place early enough in the disruption process. However, suppressing an already accelerated runaway electron beam in the MA range was found to be difficult even with injections of more than 2 kPa.m3 high-Z gases such as krypton or xenon. This may be due to the presence of a cold background plasma weakly coupled to the runaway electron beam which prevents neutrals from penetrating in the electron beam core. Following unsuccessful mitigation attempts, runaway electron impacts on beryllium plasma-facing components were observed, showing localized melting with toroidal asymmetries.

Modelling of polarimetry measurements at JET

This paper presents a study aimed at validating the ability of the presently available models to predict the Cotton–Mouton (C–M) effect. The Faraday rotation and the C–M phase shift angle can be calculated by means of a rigorous numerical solution of Stokes equations. Numerical and approximated solutions are presented and compared with experimental data. A detailed comparison is done with the time traces of measurements, inside a limited dataset representative of JET regimes. A statistical analysis is then carried out on a dataset including data from 300 discharges. In general the C–M measurements are in agreement with the numerical model, and the line integral of plasma density deduced by the C–M measurements is in agreement with that measured by LIDAR Thomson scattering (well inside the experimental error, which is close to two fringes for the polarimetry measurements, 1 fringe = 1.14 × 1019 m−2).

Analysis of Faraday rotation in JET polarimetric measurements

ThepaperdealswithJETpolarimetermeasurementsandinparticularitpresents a study of the Faraday rotation angle, which is used as a constraint in equilibrium codes. This angle can be calculated by means of the rigorous numerical solution of Stokes equations. A detailed comparison of calculations is carried outwiththetimetracesofmeasurements, insidealimiteddatasetrepresentative of JET discharges: in general, it is found that the Faraday rotation angle and Cotton‐Mouton phase shift measurements can be represented by the numerical solution to Stokes equations. To obtain this agreement in particular for Faraday rotation, a shift of the magnetic surfaces must be included. This results in an improvement of the position of the magnetic surfaces as calculated by the EFIT equilibrium code. The approximated linear models normally used can be applied only at low density and current. The Cotton‐Mouton is calculated at high plasma density including the contribution by the Faraday rotation angle. For highplasmacurrentthenon-lineartermsinthepropagationequationscanbeimportant. These conclusions have some impact on the mathematical form of the polarimetric constraints (Faraday and Cotton‐Mouton) in equilibrium codes. (Some figures in this article are in colour only in the electronic version)

A new calibration code for the JET polarimeter

An equivalent model of JET polarimeter is presented, which overcomes the drawbacks of previous versions of the fitting procedures used to provide calibrated results. First of all the signal processing electronics has been simulated, to confirm that it is still working within the original specifications. Then the effective optical path of both the vertical and lateral chords has been implemented to produce the calibration curves. The principle approach to the model has allowed obtaining a unique procedure which can be applied to any manual calibration and remains constant until the following one. The optical model of the chords is then applied to derive the plasma measurements. The results are in good agreement with the estimates of the most advanced full wave propagation code available and have been benchmarked with other diagnostics. The devised procedure has proved to work properly also for the most recent campaigns and high current experiments.

Real-time recovery of the electron density from interferometric measurements affected by fringe jumps

Optical interferometers are normally used in magnetically confined plasmas to measure the refractive index of the plasma by comparing the phase shift variation between a reference and a probe laser beam, from which the line-integrated electron density can be derived. Unfortunately, interferometric measurements are affected by fringe jumps, which are basically the erroneous phase difference determination due to the loss of signals or a phase difference bigger than 2π. The multiple causes include the refraction, wavelength of the laser radiation used, sensitivity, and time resolution of the measurements. On the other hand, the plasma density has become an essential piece of information for many real-time control schemes, which can therefore be completely jeopardized by fringe jumps. To overcome this problem at JET two main approaches can be adopted. The first approach consists of performing a real-time correction of the affected chords, eliminating the spurious effect of the fringe jumps, and providing a co...

Recent developments of the JET far-infrared interferometer-polarimeter diagnostic.

The far-infrared diagnostic provides essential internal measurements of the plasma density and magnetic field topology (q-profile via Faraday rotation angle) in real-time. The diagnostic capabilities have recently been extended in a number of key areas. Fast interferometer data, with 10 μs time resolution, and a new MATLAB code have allowed improved analysis of the evolution of density profiles during fast events such as vertical plasma displacements, edge localized mode, pellet fuelling, and disruptions. Using the polarimeter measurements in real-time, a new calibration procedure has been developed based on a propagation code using the Mueller matrix formalism. A further major upgrade of the system is presently underway: adding a second color laser to the vertical channels and implementing a new phase counter based on analog zero crossing and field-programmable gate array boards.

The multichord far infrared polarimeter of the RFX experiment

A multichannel far infrared (FIR) polarimeter has been installed on the reversed field experiment (RFX) to measure the poloidal magnetic field profile, BΘ. The polarimeter uses a FIR laser (λ=118.8 μm) whose beam is transmitted to the machine via a 31 m long, nitrogen filled beam line. Faraday rotation measurements are reported for five of six parallel chords crossing the 1 m diameter RFX plasma on a poloidal cross section. The density profile is known from a CO2 interferometer. Low noise deuterated L-alaine doped triglycene sulphate (DLATGS) detectors are used. An accuracy of about 0.2° is obtained in the measured Faraday rotation angle, which is sufficient to estimate the characteristics of the BΘ profile. Removal of the noise induced by mechanical vibrations allowed acceptable measurements of Faraday rotation angles.

Mutual interaction of Faraday rotation and cotton-mouton phase shift in jet polarimetric measurements

The paper presents a study of Faraday rotation (FR) angle and Cotton–Mouton (CM) phase shift measurements to determine their mutual interaction and the validity of the linear models presently used in equilibrium codes. Comparison between time traces of measurements and model calculations leads to the result that only an exact numerical solution of Stokes equations can reproduce in all the experimental data. As a consequence, approximated linear models can be applied only in a limited range of plasma parameters. In general, the nonlinear coupling between FR and CM is important for the evaluation of polarimetry parameters.

Systematic comparison between line integrated densities measured with interferometry and polarimetry at JET

A systematic comparison between the line integrated electron density derived from interferometry and polarimetry at JET has been carried out. For the first time the reliability of the measurements of the Cotton-Mouton effect has been analyzed for a wide range of main plasma parameters and the possibility to evaluate the electron density directly from polarimetric data has been studied. The purpose of this work is to recover the interferometric data with the density derived from the measured Cotton-Mouton effect, when the fringe jump phenomena occur. The results show that the difference between the line integrated electron density from interferometry and polarimetry is with one fringe (1.143 x 10(19) m(-2)) for more than 90% of the cases. It is possible to consider polarimetry as a satisfactory alternative method to interferometry to measure the electron density and it could be used to recover interferometric signal when a fringe jumps occurs, preventing difficulties for the real-time control of many experiments at the JET machine.