ITER Position Control Reflectometry - Conceptual Design

Abstract

The plasma position reflectometer array will supplement magnetics measurements for plasma position control in ITER by estimating the position and shape of the flux surface near the plasma boundary. Reflectometry can measure the location of a specified density layer near the separatrix and is expected to be especially useful in controlling the location of the plasma boundary during the long equilibrium phase of the discharge when magnetics signals may accumulate offset errors. A poloidal array of eight swept frequency, ordinary mode reflectometers operating between 15 and 60 GHz has been evaluated. This frequency range corresponds to cutoff densities between 0.028 and 0.45x10²⁰ m^-3 [f ₀ = 9.0n ^1/2 _e (m ^-3) Hz] which are estimated to encompass the scrape off density for projected ITER plasma scenarios. The position measurement relies on a time delay technique and, thus, measures the distance from from the plasma surface to a physical reference point near the wall during each frequency sweep. Reflectometry systems for profile measurements using both the ordinary and extraordinary mode cutoffs are used widely in fusion plasmas and are reviewed by Laviron.¹