The MPR concept

A neutron spectrometer of a new type for fusion diagnostics has been constructed at the Dept. of Neutron Research at Uppsala University. The
spectrometer combines high sensitivity with good energy
resolution and high count rate capability. The instrument is based on making a neutron beam manifest itself through proton recoils which are subsequently measured in a magnetic spectrometer. This is referred to as the magnetic proton recoil (MPR) method.

The MPR spectrometer is installed as a diagnostic on JET and has operated since 1996, before the deuterium-tritium experimental campaign (DTE1) in 1997. It is also a prototype for an ITER diagnostic, aimed at providing information on a range of plasma quantities, such as ion temperature, fuel ion densities, total neutron yield, and a-particle effects. 

The MPR principle

A collimated beam of neutrons from the plasma hits a polyethylene target (CH2).
Some neutrons scatter elastically on the target hydrogen nuclei; the recoil protons knocked out within a narrow cone in the forward direction, after being selected by a circular aperture (proton collimator), are focused, deflected, momentum analyzed by the magnet (dipole D1 and dipole D2), and transported to the focal plane where they are registered. This technique involves no active (analog) energy determination and the only live function is the proton counting (digital) in the focal plane. This makes
possible to have a high count rate, since no complicated signal processing or data analysis is necessary.
 

The MPR at JET

The MPR neutron spectrometer is installed at JET with a "quasi"-tangential line of sight slightly tilted upwards. The Vacuum vessel with port and wall are marked in the figure below (A-C) as well as the main parts of the MPR described above (1-6). The MPR position with respect to the JET torus.