F. Orsitto

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).

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.

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.

Energy confinement and plasma heating during lower hybrid experiments

RF power up to 450 kW has been injected into the plasma of FT in the electron heating regime (ne<or=5*1013 cm-3) producing electron and ion temperature increases of about 1 keV and .5 keV respectively without significant enhancement of Zeff. A density increase is observed due to an improvement of particle confinement time. An energy balance at intermediate power levels (PRF=300 kW) is carried out for two different types of discharges, one without sawteeth the other with sawteeth. The balance shows that in order to account for the total injected power one has to assume equal electron thermal conductivity for the OH and the OH+RF phase. The energy confinement time does not vary from its ohmic value for both discharges. Finally an investigation, at ne above the density limit, of the radial source of the fast ion tails and of the characteristics of the parametric decay instability is presented.

Production and Diagnosis of Energetic Particles in FAST

The Fusion Advanced Study Torus (FAST) has been proposed as a possible European satellite facility to study fast-ion physics in deuterium plasmas under conditions relevant to a burning plasma. Energetic minority ions (H or 3He) accelerated by ion cyclotron resonance heating (ICRH), with dimensionless parameters close to those of fusion-born alphas in ITER, can be produced in FAST via 30 MW power ICRH minority heating. This work provides a first assessment of the extent to which the 3He fast-ion population can be diagnosed in FAST with a set of dedicated diagnostics for confined fast particles. Neutron emission spectroscopy (NES), gamma-ray spectroscopy (GRS) and collective Thomson scattering (CTS) diagnostics have been reviewed with a description of the state-of-the-art hardware and a preliminary analysis of the required lines of sight. The results of the analysis, based on numerical simulations of the spatial and energetic particle distribution function of the ICRH-accelerated ions for the standard FAST H-mode scenario, suggest that NES and GRS measurements can provide information on the fast 3He population effective tail temperature, with time resolutions in the range 20–100 ms. The proposed CTS diagnostic can measure the fast-ion parallel and perpendicular temperature with a spatial resolution of 5–10 cm and a time resolution of 10 ms. The paper provides a scientific basis for the prediction of the production and diagnosis of energetic ions in FAST.

Analysis of JET Polarimeter With a Propagation Code Based on the Stokes Formalism

Internal magnetic measurements are essential to obtaining reliable and accurate magnetic reconstructions in the interior of the plasma column in Tokamaks. In the last years, polarimetry has been increasingly used to provide global constraints to equilibrium codes. Joint European Torus (JET) polarimeter has four lateral channels, whose arrangement is similar to the topology of the diagnostic foreseen in ITER. A statistical analysis of JET polarimetry measurements have been provided in the past for only one vertical channel (CH3) using a polarimetry propagation code based on the Stokes vector formalism. A new propagation code has therefore been developed for the lateral channels to simulate and interpret the measurements of the Faraday rotation (FR) and Cotton-Mouton (CM) phase shift in JET. In this paper, a complete analysis of the integral form of the Stokes equation for the lateral channels is presented. In particular, the analysis shows that there is a strong interaction between the FR and CM effect even at low currents and low magnetic fields. The code results of both FR and CM have been used to estimate the line-integrated density and a proper benchmarking with experimental data has been performed. Two approximations to obtain the electron density in real time, using only plasma current in case of FR measurements and also toroidal magnetic fields in case of CM measurements, are proposed.

EFD-C(13)03/32 ITER Like Wall Impact on MHD Instabilities in JET Discharges

1Consorzio RFX, EURATOM-ENEA Association, Corso Stati Uniti 4, 35127 Padova, Italy 2Euratom/CCFE Fusion Association, Culham Science Centre, Abingdon, OX14 3DB, UK 3Associazione EURATOM-ENEA sulla Fusione, C.R. Frascati, Roma, Italy 4Association EURATOM-CEA, CEA/DSM/IRFM, Cadarache 13108 Saint Paul Lez Durance, France 5Associacao EURATOM/IST, Instituto de Plasmas e Fusao Nuclear, Instituto Superior Tecnico, Av Rovisco Pais, 1049-001 Lisbon, Portugal 6FOM institute DIFFER, EURATOM association, P.O. Box 1207, Nieuwegein, Netherlands 7Max-Planck-Institut fur Plasmaphysik, EURATOM-Assoziation, D-85748 Garching, Germany 8Associazione EURATOM-ENEA sulla Fusione, Universita di Roma, Italy

New Approximations and Calibration Methods to Provide Routine Real-Time Polarimetry on JET

The increasing importance of providing reliable polarimetric measurements in real time, for both machine protection and plasma control, has motivated the development of a quick version of the calibration algorithms for JET polarimeter. This new code, which interprets the calibration procedure performed before each shot, is based on a physical equivalent model of the diagnostic optical path and is valid for any operational regime of JET. It provides results before the plasma breakdown, and, with its estimates of the optical paths parameters, the polarimetric measurements have an accuracy more than sufficient for real-time purposes. New approximate equations have been validated in order to obtain the line integrated density from the newly calibrated horizontal chords, so that also these polarimetric measurements can also be used for density feedback and machine protection. The availability of reliable polarimetric measurements in real time opens new perspectives also to the determination of the plasma boundary, the magnetic equilibrium, and their use in advanced feedback control schemes.

Transmission and coupling at high power density of 8 GHz lower hybrid on FT

The possibility of injecting RF power into the plasma with a grill structure for high field tokamak is considered. System description and technical problems are all incorporated into.

DIOSCUR-divertor optimization and steady current study of a tokamak aimed at steady state operation with reactor relevant plasma parameters

The DIOSCUR tokamak is a superconducting machine using Nb/sub 3/Sn CIC coils and having a size similar to JET (Joint European Torus), although with a toroidal magnetic field of 7 T and a sustainable plasma current of 10 MA for long pulses (1000 s). The machine will have a divertor and great configurational flexibility in plasma aspect ratio up to A=5.2, current, up to 18 MA, and attainable poloidal beta , to allow the study of stationary regimes and also the influence of bootstrap current. For CD and profile control, 50 MW of LH and negative ion beam power are foreseen. ICRF (ion cyclotron resonant frequency) heating and fast wave current drive have also been considered.<<ETX>>