Experimental investigation of a high power long-pulse neutral beam profile diagnostic based on secondary electron emission
Diagnostic of neutral beam (NB) profiles in powerful long-pulse injectors intended for plasma heating and current drive in fusion devices is a problem task in consequence of very high NB power density (PD) at a level of tens MW/m2. NB profiles give an information required for determination of such important parameters as the beam divergence angle and accuracy of its axis aiming onto a tokamak entrance window. However, with so high PD use of probes introduced directly in the beam is impossible.
In the case of the ITER heating injector V-shaped calorimeter is designed for the NB commissioning and determination of its parameters. It consists of two panels, each is formed by a set of horizontally oriented cooled tubes arranged in two layers (“front” and “back”). Use of the thermocouples, placed on the tube ends, allows to measure only horizontally averaged NB vertical profile but horizontal profile remains unknown.
This report describes method of measurement of the beam detailed profiles at the calorimeter using matrix of positively biased collectors, each of them is placed between tubes of “back” layer in shadow of “front” layer tube, i.e. does not see the beam. Each collector is to collect secondary emission electrons resulting on surfaces of neighboring tubes under the beam bombardment. Such collector matrix is to provide determination of horizontal and vertical profiles in any section of the beam footprint .
Proposed method was tested on the injector test stand IREK with tubular V-shaped calorimeter. Measurements were carried out with use of positive hydrogen ion source with beam energy and current at a level of 40 keV/40 А. The calorimeter was equipped with a set of secondary-emission electron (SEE) collectors and of tungsten probes, directly entered into the beam. Initially experiments were carried out with relatively short pulses at a level of 0.4 s for comparison of normalized profiles measured by both methods and results demonstrated quite good coincidence of the profiles.
Further measurements were done with SEE collectors at pulse duration up to 5 s. An influence of secondary plasma electrons was investigated with changing its density by an increase of the gas pressure in the IREK vacuum vessel.
 A. Panasenkov, V. Smirnov. “Method of measurement of neutral beam profile in the long-pulse powerful injectors”. 28-th SOFT 2014 (Spain), Book of Abstracts, paper P4.022, p.642.