M. Bornatici

Electron cyclotron radiative transfer in fusion plasmas

The radiative transfer in a system at local thermodynamic equilibrium is investigated on the basis of the solution of the (geometrical optics) transfer equation, accounting for the non-local nature of the radiative process due to both re-absorption of the emitted radiation and reflectivity of the walls of the system. The specific case of the electron cyclotron (EC) radiation in a cylindrical fusion plasma with specularly reflecting walls for which an analytical solution can be derived, is addressed and, in particular, the radial profile of the net power density radiated is evaluated by making use of an improved expression for the EC absorption coefficient. A detailed numerical analysis, carried out by varying both the wall reflection coefficient and the radial profile of the plasma temperature, reveals that a reversal of the net power density profile can occur on the plasma outboard for sufficiently high wall reflectivity. From a comparison with bremsstrahlung radiation profiles it is apparent that a local treatment of EC power emission is needed for sufficiently hot plasmas as expected, e.g. in the so-called advanced regimes of DT tokamak reactors. Furthermore, an exact approach is used to check the accuracy of approximate EC net power density profiles as calculated with the CYTRAN code showing that the latter provides a globally reasonable approximation. Evaluating the total EC radiated power from the exact local approach shows that its scaling with the reflection coefficient is very well described by a scaling following from a recently established global model for the EC radiation, which improves the well-known Trubnikov scaling. The results obtained are discussed in view of their possible relevance to affecting the plasma temperature profile.

RAYTEC: a new code for electron cyclotron radiative transport modelling of fusion plasmas

As it was recognized that local electron cyclotron (EC) wave power losses can be a competitive contribution to the 1D electron power balance for reactor-grade tokamak plasmas in regimes as anticipated for steady-state operation, a systematic effort is ongoing to improve the modelling capability for the radial profile of EC wave emission. This effort aims at generating a hierarchy of codes that cover the non-local behaviour of EC wave transport for inhomogeneous plasmas and in the presence of reflecting walls with increasingly improved accuracy and also provide sufficient computational efficiency for being usable in 1D transport studies. The recently developed code RAYTEC, which explicitly addresses the geometrical effects present in toroidal plasmas with arbitrary cross-section, is described and used to investigate the impact of elongation of the plasma cross-section and of toroidicity on the angular dependence of the EC radiation field, on the profile of the net EC wave power density lost from the plasma and on the total EC power loss for ITER-like plasma conditions. Furthermore, a comparison is made with the results of simpler models in use to describe both local and total EC power losses as well as with ones obtained from analytical formulae that are introduced on the basis of Trubnikovs formula for EC power emission.

Benchmarking of codes for calculating local NET EC power losses in fusion plasmas

Abstract The codes SNECTR, CYTRAN, CYNEQ, and EXACTEC are compared in view of the calculation of the profile of the net electron cyclotron (EC) wave power density emitted for different electron temperature profiles and average temperatures of relevance for reactor-grade magnetoplasmas. The effects of either specularly or diffusely reflecting walls are assessed for a cylindrical plasma with circular cross-section, specular reflection, as assumed in EXACTEC, providing a lower bound to the net EC wave power losses in the hot plasma core (and therefore, as a rule, also to the total EC power loss) as well as to reabsorption in the edge plasma. The assumption of isotropy of the radiation intensity in the plasma that is adopted in both CYTRAN and CYNEQ (which cannot be justified a priori) is discussed and found to be adequate for strong diffuse reflection. However, it overestimates the net EC power loss in the plasma core for weakly as well as for specularly reflecting walls by up to 20%. The full transport code SNECTR (no longer in active use), for specular reflection, and the exact cylindrical code EXACTEC are in excellent agreement with each other while for strong diffuse reflection EXACTEC is found to underestimate the net EC power loss typically by 20%. EXACTEC, CYTRAN, and CYNEQ are confirmed to be well suited for use in systematic transport simulations of fusion plasmas.

Electron cyclotron wave power loss in fusion plasmas: a model comparison ∗

For the modelling of hot reactor-grade plasmas a way to compute local electron cyclotron wave losses efficiently and with good accuracy is required. Thereby the approach to calculating the electron cyclotron absorption coefficient is an essential element. A comparison of three available analytical forms (a quasi-exact one and the asymptotic forms due to Trubnikov and to Robinson) shows that, overall, the use of Robinsons formula is the most suitable choice.

Electron cyclotron radiative transfer in the presence of polarization scrambling in wall reflections

An exact analytical solution of the equation of radiative transfer is obtained for a cylindrical system with specularly reflecting walls, accounting for the presence of polarization scrambling in the reflection process. The effects of polarization scrambling on the specific intensity of the radiation can be described via an effective wall reflection coefficient for the extraordinary (x) and ordinary (o) mode. For the special case of electron cyclotron radiation in a fusion plasma, a numerical analysis of the impact of polarization scrambling on both the specific intensity of the radiation and the radial profile of the net power radiated as well as on the total power loss is carried out for ITER-like parameters in steady-state operation.

ECE Diagnostic on ITER in the Presence of Superthermals

The Effects of superthermals on ECE T.-l1leasurfments ill t.he mre plasma of ITER are investixad gat.ed on the basis of a model distribution function fo which a generalized Kirchhoffs law is valid. On varying both t.he spatial profiles and the position of t.he superthermals. the feasibility of T.xad measurements using the first harmonic ordinary-mode as well as the sEcond harmonic extraordinaryxad modE is assessed in respect to the accuracy requirement. set by ITER.

ECE Diagnostic on ITER

On the basis of a detailed numerical investigation of the ECE spectra for flat as well as peaked model profiles for temperature and density, it is shown that for ITER conditions with central temperatures ≳20keV the first harmonic ordinary mode is the most suitable choice for localized measurements of the temperature over a significant portion of the plasma cross-section. For the plasma core and flat profiles the corresponding spatial resolution is 10 cm, typically, improving somewhat for peaked profiles. The second harmonic extraordinary mode, instead, is more strongly affected by harmonic overlap, which spoils its utilization for measurements of the central temperature profile, its diagnostic potential being restricted at best to the outboard plasma region.