Erik Nonbøl

Investigation of first mirror heating for the collective Thomson scattering diagnostic in ITER

Collective Thomson scattering (CTS) has the capabilities to measure phase space densities of fast ion populations in ITER resolved in configuration space, in velocity space, and in time. In the CTS system proposed for ITER, probing radiation at 60 GHz generated by two 1 MW gyrotrons is scattered in the plasma and collected by arrays of receivers. The transmission lines from the gyrotrons to the plasma and from the plasma to the receivers contain several quasioptical mirrors among other components. These are designed to produce astigmatic beam patterns in the plasma where the beam shapes will have a direct impact on the signal strength of the diagnostic, the spatial resolution, and the robustness of probe and receiver beam overlap against density excursions. The first mirror has a line of sight to the plasma and is thus exposed to severe neutron streaming. The present neutronics and thermomechanical modeling of a first mirror on the high field side indicates that the mirror curvature may warp due to heating. This may alter the beam quality, and therefore, thermal effects have to be accounted for during the design of the mirror. The modeling further demonstrates that thin mirrors are superior to thick mirrors from a thermomechanical point of view.

Fast ion collective Thomson scattering diagnostic for ITER: Design elements

Abstract The proposed fast ion collective Thomson scattering (CTS) diagnostic system for ITER provides the unique capability of measuring the temporally and spatially resolved velocity distribution of the confined fast ions and fusion alpha particles in a burning ITER plasma. The present paper describes the status of the iteration toward the detailed design of the ITER fast ion CTS diagnostic and explains in detail a number of essential considerations and challenges. The diagnostic consists of two separate receiving systems. One system measures the fast ion velocity component in the direction near perpendicular, and the other measures the component near parallel to the magnetic field. Each system has a high-power probe beam at an operating frequency of 60 GHz and a receiver unit. In order to prevent neutron damage to moveable parts, the geometry of the probes and receivers is fixed. An array of receivers in each receiving unit ensures simultaneous measurements in multiple scattering volumes. The latter receiving system (resolving the parallel component) is located on the high field side (HFS) of the plasma, and this constitutes a significant challenge. This HFS receiving unit has been central in the studies, and new HFS receiver mock-up measurements are presented as well as neutron flux calculations of the influence of the increased slot height.

Application of the MCNPX-McStas interface for shielding calculations and guide design at ESS

Recently, an interface between the Monte Carlo code MCNPX and the neutron ray-tracing code MCNPX was developed [1, 2]. Based on the expected neutronic performance and guide geometries relevant for the ESS, the combined MCNPX-McStas code is used to calculate dose rates along neutron beam guides. The generation and moderation of neutrons is simulated using a full scale MCNPX model of the ESS target monolith. Upon entering the neutron beam extraction region, the individual neutron states are handed to McStas via the MCNPX-McStas interface. McStas transports the neutrons through the beam guide, and by using newly developed event logging capability, the neutron state parameters corresponding to un-reflected neutrons are recorded at each scattering. This information is handed back to MCNPX where it serves as neutron source input for a second MCNPX simulation. This simulation enables calculation of dose rates in the vicinity of the guide. In addition the logging mechanism is employed to record the scatterings along the guides which is exploited to simulate the supermirror quality requirements (i.e. m-values) needed at different positions along the beam guide to transport neutrons in the same guide/source setup.

Material assessment for ITER's collective Thomson Scattering first mirror

ITERs Collective Thomson Scattering (CTS) system is a diagnostic instrument that will measure the plasma density and velocity through Thomson scattering of microwave radiation. Some of the key components of the CTS are quasioptical mirrors used to produce astigmatic beam patterns, which have impact on the strength and spatial resolution of the diagnostic signal. The mirrors are exposed to neutron radiation, which may alter the mirror properties or deform its structure. These changes may affect the collection of the scattered radiation and consequently decrease the quality of the measurements. In this work, three different materials (molybdenum (Mo), stainless steel 316L (SS-316L) and tungsten (W)) are considered for the first mirror of the CTS. The objective is to assess the suitability of these materials for this mirror and to provide a first ranking, considering the neutron radiation loads requirements defined by ITER, based on the resultant maximum Von Misses stresses and temperatures. For it, the neutron irradiation, and subsequent heat-load on the mirrors were simulated using the Monte Carlo N-Particle (MCNP) code. Based on the MCNP heat-load results, a transient thermal-structural Finite Element Analysis (FEA) of the mirror over a 400s discharge (reasonable number for computational tests, since an ITER discharge will be between 200 s and 1000 s), with and without mirror cooling, is performed. The results obtained in this preliminary analysis show that of the tested materials Mo and W are the most suitable materials for this application, being able to reliably sustain the thermal and structural stresses imposed by the neutron loads.