Design advances of the Core Plasma Thomson Scattering diagnostic for ITER

Abstract

The Core Plasma Thomson Scattering (CPTS) diagnostic on ITER performs measurements of the electron temperature and density profiles which are critical to the understanding of the ITER plasma. The diagnostic must satisfy the ITER project requirements, which translate to requirements on performance as well as reliability, safety and engineering. The implications are particularly challenging for beam dump lifetime, the need for continuous active alignment of the diagnostic during operation, allowable neutron flux in the interspace and the protection of the first mirror from plasma deposition. The CPTS design has been evolving over a number of years. One recent improvement is that the collection optics have been modified to include freeform surfaces. These freeform surfaces introduce extra complexity to the manufacturing but provide greater flexibility in the design. The greater flexibility introduced allows for example to lower neutron throughput or use fewer surfaces while improving optical performance. Performance assessment has shown that scattering from a 1064 nm laser will be sufficient to meet the measurement requirements, at least for the system at the start of operations. Optical transmission at λ < 600 nm is expected to degrade over the ITER lifetime due to fibre darkening and deposition on the first mirror. For this reason, it is proposed that the diagnostic should additionally include measurements of TS 'depolarised light' and a 1319 nm laser system. These additional techniques have different spectral 1Corresponding author.

Design of Thomson scattering diagnostics for the Divertor Tokamak Test (DTT) facility

“…The use of such a short laser pulse has an impact also on other aspects of the system. First it will restrict the choice of the APDs to those with a sufficiently fast response time [12]. In addition it may help to mitigate the measurement noise due to the laser stray light.…”

“…A second TS system is under study for DTT whose main purpose is to measure the T e and n e profiles in the central region of the plasma and, as in many other fusion experiments, provide also the information necessary to resolve the edge pedestal. For this system we have again followed a conventional approach, as for the divertor TS system, and have chosen to use the equatorial diagnostic port both for the input laser and for the collection optics, in a set-up similar to that foreseen for the ITER core TS system [12]. This set-up provides good accessibility to the edge region, where the highest resolution is required and the plasma conditions make the measurement more critical.…”

“…Finally the plasma light has been calculated by spatially integrating the plasma bremsstrahlung emissivity along each viewing chord. We have also included an enhancement factor ξ = 2 for a preliminary estimate of the contribution of line emission [12]. Then the error on each measured TS signal has been determined including the signal shot noise, the plasma light background and the detector dark current and excess noise.…”

Polarimetric Thomson scattering for high Tefusion plasmas

“…We now calculate the expected performance of this hybrid TS set-up. We assume for the polychromator a set of spectral channels corresponding to the first six channels of the 8-channel set described in [14]. With this arrangement the low wavelength cut-off is at λ C = 587 nm and it has been shown that in these conditions the accuracy of the TS measurements obtained by the usual spectral analysis is somewhat limited at high T e .…”

1319 nm single-frequency injection seeded Q-switched laser based on ramp-hold-fire

“…The 1319 nm lasers play a crucial role in laser medicine, optical communication, laser display, plasma research, nonlinear frequency and sodium atomic fluorescence detection at 80-110 km [1][2][3][4][5][6][7] . The 1319 nm laser is mainly generated by the transition between the R2 stark sublevel of the upper level 4 F3/2 and the X1 stark sublevel of the 4 I13/2 level (R 2 →X 1 ) of the Nd: YAG crystal, and the effective stimulated emission cross-section of the 1319 nm laser is about 0.95×10 -19 cm 2 [8] , which is one third of the 1064 nm laser transition.…”