Bolometry provides absolute measurements of total radiation losses of a plasma discharge, regardless the radiation wavelengths. A bolometer is just a tiny piece of metal with precisely defined thermal properties that heats up due to plasma radiation. The radiation comes through a narrow slit, named pinhole that defines a “viewing line” of each bolometer. Plasma radiation losses along the viewing line are then derived from the increase in the bolometer temperature. With a set of suitably positioned bolometers and along with them a number of viewing lines, it is possible to estimate the radiation emissivity pattern on a plasma cross-section. AT JET, several sets of bolometers allow precise mapping of plasma emissivity and surface radiation. High spatial resolution is required in the divertor region in order to localise large radiation losses caused by particle exhaust. That is why the array of viewing lines is denser in the divertor region and why four other bolometric cameras dedicated purely to divertor observations were also refurbished.
Neutron/gamma profile monitor
A single vertical cross-section of the plasma is sufficient to learn about the state of the whole plasma volume as the cross-section does not vary significantly around the tokamak, in its toroidal direction. As a matter of fact, any local disturbance is immediately spread along the magnetic field lines – plasma particles move freely in this direction. Consequently, only very few fast diagnostic systems (eg magnetic diagnostics) monitor the toroidal irregularities. On the contrary, it is essential to measure the plasma’s vertical cross-section in as much detail as possible, to determine plasma profiles in the direction perpendicular to the toroidal magnetic field. The limiting factor in this is the number and position of available ports (windows into the plasma). Due to this limitation, a number of diagnostics have very similar geometrical set-ups e.g. the JET gamma-ray profile monitor, the soft X-ray diagnostics and the JET main bolometer system that measures total plasma radiated power.
The neutron and gamma-ray profile monitor represents just one of tens of passive diagnostic methods applied at JET. The monitor has two cameras that allow observations of plasma radiation from ten horizontal and nine vertical directions. In this way we can localise the source of the radiation, in this case the neutrons produced by fusion or gamma-rays produced by nuclear reactions. The latter can serve us to trace the presence of fast-ions, in particular helium nuclei (alpha particles).