JET is home to a Thomson scattering LIDAR system, a unique forerunner for one of ITER’s key diagnostic. Recently, the system was enhanced with modern, highly sensitive and ultra-fast detectors. The measurement results demonstrate that the chosen detector technology delivers highly accurate data and is well suited for ITER.

Thomson scattering (TS) diagnostic systems are one of the most robust and reliable methods of measuring plasma temperature and density in a fusion device. Using intense bursts of laser light, these diagnostics remotely interrogate the thermal motion of the plasma electrons by analysing the scattered light signal. These measurements require powerful laser systems and highly sensitive detectors since the scattering process is very weak (electrons are small!). For every gigawatt of incident optical power, only a few hundred nanowatt return as Thomson scattered light.

The advantages of LIDAR

Although TS systems are employed on nearly every large fusion device, JET is the only machine to have a LIDAR variant of this diagnostic. It registers the distance between detector and scattering location by measuring the travelling time of the light signal (time-of-flight), in much the same way that RADAR does with radio-waves. Because it delivers density and temperature information together with the location in just one signal, a LIDAR system is simpler than a conventional TS system. It requires only a single access port into the plasma chamber, whereas the latter needs a second port for a large lens to collect the scattered light separately for each plasma location. For these reasons LIDAR is particularly attractive for ITER, where space is at a premium because of the myriad systems that will surround the vessel.

Improved resolution

LIDAR systems rely on short laser pulses and fast detector response times to achieve high spatial resolution – the upgraded system at JET features pulse lengths as low as 300 picoseconds, or one third of one billionth of a second. One of the fundamental challenges for ITER’s LIDAR system is developing and proving fast detector technology featuring high sensitivity for a wide wavelength range, as spectrum of the scattered light is broadened by the Doppler effect. Over the recent shutdown, the TS team at JET has been busy optimising the LIDAR collection optics and replacing the detectors, originally installed in 1986, with ultra-fast and highly sensitive Gallium-Arsenide-Phosphide detectors. These enhancements have led to significant improvements in the signal-to-noise ratio and temperature and density measurements are much more accurate. Scientists can now, for instance, measure at much lower plasma densities. Over JET’s next intervention period the TS team also plan to upgrade the digitising electronics, whose technology is now lagging 30 years behind the new detectors. Making full use of the fast detectors will improve the spatial resolution of the measured profile from 12 cm to the ITER requirement of 7 cm.

These enhancements offer more than just excellent data for JET experiments; the technical knowledge and experience gained throughout this process is invaluable for the ITER LIDAR development; JET is the only place in the world where this measurement technique and its associated technologies can be tested.
The JET Thomson Scattering team