A Multichannel Neutron Collimator for the ITER Tokamak

Abstract

A concept for the multichannel neutron collimator of the ITER is presented. The design of the collimator is based on the use of a 12-collimator radial neutron camera that was developed earlier and two complementary compact neutron cameras. It is proposed that the compact neutron cameras be placed inside the shielding blocks located in the equatorial (nine collimators) and diverter diagnostic (seven collimators) ITER ports. The plasma would thereby be fully covered in the vertical direction, and the total number of collimator channels would be 21 (12 channels in the radial camera and nine channels in the compact camera). The collimator length and diameters, as well as the optimal materials for the shielding blocks and the inner walls of the collimators, were determined using the MCNP code. It is shown by simulation that, for an adequate collimation of neutron fluxes to be achieved, the collimators should be 1.0–1.5 m long, have an i.d. of 4–5 cm, and be enclosed in a water–iron shielding. It is proposed that threshold fission chambers based on ²³⁸U and natural- and CVD-diamond detectors be used as the sensors. The computations are presented to demonstrate that this composition and arrangement of the channels in the radial and compact cameras makes it possible to measure the two-dimensional spatial distribution of the neutron source and the total thermonuclear power to an accuracy of ∼10%.