nGEM neutron diagnostic concept for high power deuterium beams

Abstract

The ITER neutral beam test facility under construction in Padova will host two experimental devices: SPIDER, a 100 kV negative H/D RF source, and MITICA, a full scale, 1 MeV deuterium beam injector. Detection of fusion neutrons will be used as a means to resolve the horizontal beam intensity profile. The neutron detection system will be placed right behind the SPIDER beam dump, as close to the neutron emitting surface as possible thus providing the map of the neutron emission on the beam dump surface. The system uses nGEM neutron detectors. These are Gas Electron Multiplier detectors equipped with a cathode that also serves as neutron-proton converter foil. The cathode is designed to ensure that most of the detected neutrons at a point of the nGEM surface are emitted from the corresponding 40 × 22 mm 2 beamlet footprint on the dump front surface. The nGEM readout pads (area 20 × 22 mm 2 ) will record a useful count rate of 5 kHz providing a time resolution of temporal structures in the neutron fluency rate to be measured

Neutron measurements from beam-target reactions at the ELISE neutral beam test facility

“…6, which also shows that C S should not depart significantly from 20%. This result provides additional confidence on the assumptions used to design neutron diagnostics planned for SPIDER and MITICA 6 . A consideration must however be made on the uncertainties affecting the calculations, particularly those on the deposited power P. There are two sources of uncertainty on P. The first one is on the profile of the power deposition, for which we had to rely completely on a calculation, which however showed that the beam deposition involves more than 90% of the beam dump.…”

“…This corresponds to the transition to the plateau phase in the neutron measurements. In order to interpret quantitatively the data, we have performed a calculation of the neutron yield expected from the target as a function of time, following the model described in [6,12,13].…”

“…In these devices, neutron emission will be used to measure the footprint of the deuterium beam when it hits the dump. 6 Unlike tokamak plasmas, neutrons are here due to deuterium-deuterium (dd) reactions between the beam and the adsorbed deuterons in the target and their level thus depend on deuteron absorption in the beam dump.…”

Progress on development of SPIDER diagnostics

“…Hence information about the beam intensity profile and its uniformity can be retrieved from the neutrons flux density distribution over the dump. To this purpose a Gas Electron Multiplier detectors equipped with a cathode that also serves as neutron-proton converter foil (nGEM) is used, positioned just behind the dump at about 3 cm from the dump surface [37]. A 230x352 mm 2 nGEM with 256 pixels has been built and will be installed in SPIDER to measure the footprint of a beamlet group with an expected resolution of about 3 cm [38].…”

A wire calorimeter for the SPIDER beam: Experimental tests and feasibility study

“…Best spatial uniformity will be achieved by the instrumented calorimeter STRIKE [5], which intercepts the beam with unidirectional CFC tiles observed with a thermocamera from behind: this is the only method capable of measuring the intensity profile of the individual beamlets, but it can bear the full beam power of SPIDER only for few seconds, because it is inertially cooled. A neutron imaging diagnostic can discriminate the single beamlet, but only works with deuterium [6] and a poor resolution can be achieved with the thermocouples installed on the water cooled copper beam dump. Other optical diagnostics do not intercept the beam, but use line-integrated measurements, like beam emission spectroscopy [7] or visible tomography [8], the latter capable of achieving a resolution of a group of few beamlets through tomographic inversion techniques.…”