Status and prospects for mm-wave reflectometry in ITER

Abstract

Reflectometry with wavelengths in the centimetre to millimetre-wave range will be used in ITER to measure the density profile in the main plasma and divertor regions and to measure the plasma position and shape in order to provide a reference for the magnetic diagnostics in long pulses. In addition, it is expected to provide key information for the measurement of density fluctuations. A set of reflectometers to meet the relevant ITER measurement requirements has been included in its present outline as part of the ITER design since 2001 and is being adapted to the present ITER baseline and to accommodate progress with reflectometry techniques and measurement capabilities. It comprises low and high field side (HFS and LFS, respectively) ordinary (O-) mode systems for the measurement of the density profile in the gradient regions, a LFS extraordinary (X-) mode system for the detailed study of the edge profile, an HFS X-mode system operating in the left hand cutoff to measure the core profile, a dedicated O-mode system for plasma-wall gap measurement and a multi-band, multiple line of sight O-mode system to measure divertor density profiles. This paper describes the evolution of the design, in particular some recent improvements in the engineering implementation and improvements aimed at enhancing the measurement capability. It concludes with a brief assessment of the likely measurement performance against the ITER measurement requirements for the parameters of interest and the overall confidence that the technique will be implanted on ITER.

Nuclear technology aspects of ITER vessel-mounted diagnostics

“…For weaklypeaked core profiles, measurement from the high field side (HFS) of the machine is essential to reach the core, as access to the cutoffs is impeded by shallow profiles and / or absorption for other locations [26]. For PP, measurement around the periphery at 4 locations is planned; two of these and the HFS measurements use equipment embedded behind the blanket.…”

Refractive and relativistic effects on ITER low field side reflectometer design

“…Note the radial location of the antennas is 8.7978 m, i.e. it is set back 30 cm from the wall than originally planned [6]. This arrangement provides a reasonably big antenna size for a less divergent beam for the sweeping frequency range to ensure enough returned power.…”

“…It can be clearly seen that both O-and X-mode cutoff frequencies downshift due to relativistic electrons. Relativistic electrons also broaden electron cyclotron (EC) resonant absorption region and may also cause cutoff profiles to be hollow [6], which could potentially affect core plasma access by LFS reflectometer, but this is outside the scope of this paper.…”

“…Reflectometry is well suited to the harsh fusion environment in ITER and a low field side (LFS) reflectometer including both O-and X-mode waves has been planned [6] for scrape off layer (SOL), edge, and core measurements. However, in ITER, reflectometry faces some unique design challenges.…”

“…However, in ITER, reflectometry faces some unique design challenges. First, due to high electron temperature in ITER plasmas, the relativistic electron effect [6], which strongly affects both cutoff position and beam propagation, needs to be assessed. Second, the refractive effect of the plasma on the probing wave needs to be assessed for effective receiving, due to the present requirement that multiple fixed waveguide antennas measure a variety of plasma conditions.…”