Instrumentation for the upgrade to the JET core charge-exchange spectrometers

Charge-exchange spectroscopy on JET has become particularly challenging with the introduction of the ITER-like wall. The line intensities are weaker and contaminated by many nuisance lines. We have therefore upgraded the instrumentation to improve throughput and allow the simultaneous measurement of impurity and fuel-ion charge exchange by splitting the light between two pairs of imaging spectrometers using dichroic beam splitters. Imaging instruments allow us to stack 11 × 1 mm diameter fibres on the entrance slits without cross talk. CCD cameras were chosen to have 512 × 512 pixels to allow frame transfer times <0.2 ms which with minimum exposure times of 5 ms give tolerable smearing even without a chopper. The image plane is optically demagnified 2:1 to match the sensor size of these cameras. Because the image plane of the spectrometer is tilted, the CCD must also be tilted to maintain focus over the spectrum (Scheimpflug condition). To avoid transverse keystoning (causing the vertical height of the spectra to change across the sensor), the configuration is furthermore designed to be telecentric by a suitable choice of the lens separation. The lens configuration is built almost entirely from commercial off-the-shelf components, which allowed it to be assembled and aligned relatively rapidly to meet the deadline for in-vessel calibration in the JET shutdown.

Carbon charge exchange analysis in the ITER-like wall environment

Charge exchange spectroscopy has long been a key diagnostic tool for fusion plasmas and is well developed in devices with Carbon Plasma-Facing Components. Operation with the ITER-like wall at JET has resulted in changes to the spectrum in the region of the Carbon charge exchange line at 529.06 nm and demonstrates the need to revise the core charge exchange analysis for this line. An investigation has been made of this spectral region in different plasma conditions and the revised description of the spectral lines to be included in the analysis is presented.

The MAST motional Stark effect diagnostic

A motional Stark effect (MSE) diagnostic is now installed and operating routinely on the MAST spherical tokamak, with 35 radial channels, spatial resolution of ∼2.5 cm, and time resolution of ∼1 ms at angular noise levels of ∼0.5°. Conventional (albeit very narrow) interference filters isolate π or σ polarized emission. Avalanche photodiode detectors with digital phase-sensitive detection measure the harmonics of a pair of photoelastic modulators operating at 20 and 23 kHz, and thus the polarization state. The π component is observed to be significantly stronger than σ, in reasonably good agreement with atomic physics calculations, and as a result, almost all channels are now operated on π. Trials with a wide filter that admits the entire Stark pattern (relying on the net polarization of the emission) have demonstrated performance almost as good as the conventional channels. MSE-constrained equilibrium reconstructions can readily be produced between pulses.

Accuracy of EFIT equilibrium reconstruction with internal diagnostic information at JET

In tokamak experiments, equilibrium reconstruction codes are used to calculate the location of the last closed flux surface, to map diagnostic information, and to derive important properties like current density and safety factor. At JET, the equilibrium code EFIT is automatically executed after each discharge. For speed and robustness, intershot EFIT is based on magnetic probe measurements only. As a consequence, the intershot profiles of the safety factor can be wrong for a variety of plasma scenarios. Internal diagnostic information, the pitch angle as measured with the motional stark effect, Faraday rotation angles, as well as pressure profile information can increase the accuracy of the EFIT equilibrium. In this paper, the accuracy of the internal diagnostics at JET and their impact on the EFIT results are discussed in detail. The influence of control parameters like the form of the test functions for ff' and p' on the equilibrium is investigated. The q(min) from this analysis agrees with information from magnetohydrodynamics analysis (e.g., Alfvén cascades and sawtooth analysis) to within 10%-15%.

Ab initio modeling of the motional Stark effect on MAST

A multichord motional Stark effect (MSE) system has recently been built on the MAST tokamak. In MAST the pi and sigma lines of the MSE spectrum overlap due to the low magnetic field typical for present day spherical tokamaks. Also, the field curvature results in a large change in the pitch angle over the observation volume. The measured polarization angle does not relate to one local pitch angle but to an integration over all pitch angles in the observation volume. The velocity distribution of the neutral beam further complicates the measurement. To take into account volume effects and velocity distribution, an ab initio code was written that simulates the MSE spectrum on MAST. The code is modular and can easily be adjusted for other tokamaks. The code returns the intensity, polarized fraction, and polarization angle as a function of wavelength. Results of the code are presented, showing the effect on depolarization and wavelength dependence of the polarization angle. The code is used to optimize the design and calibration of the MSE diagnostic.

Poloidal rotation dynamics, radial electric field, and neoclassical theory in the jet internal-transport-barrier region

Results from the first measurements of a core plasma poloidal rotation velocity (upsilontheta) across internal transport barriers (ITB) on JET are presented. The spatial and temporal evolution of the ITB can be followed along with the upsilontheta radial profiles, providing a very clear link between the location of the steepest region of the ion temperature gradient and localized spin-up of upsilontheta. The upsilontheta measurements are an order of magnitude higher than the neoclassical predictions for thermal particles in the ITB region, contrary to the close agreement found between the determined and predicted particle and heat transport coefficients [K.-D. Zastrow, Plasma Phys. Controlled Fusion 46, B255 (2004)]. These results have significant implications for the understanding of transport barrier dynamics due to their large impact on the measured radial electric field profile.

Monitoring Alfvén cascades with interferometry on the JET Tokamak

A microwave interferometry technique is applied for the first time for detecting a discrete spectrum of Alfvén cascade (AC) eigenmodes excited with fast ions in reversed magnetic shear plasmas of the Joint European Torus. The interferometry measurements of plasma density perturbations associated with ACs show an unprecedented frequency and time resolution superior to that obtained with external magnetic coils. The measurements of ACs are used for monitoring the evolution of the safety factor and density of rational magnetic surfaces in the region of maximum plasma current.

First gamma-ray measurements of fusion alpha particles in JET trace tritium experiments

Gamma-ray spectra from nuclear reactions between fusion-born alpha (alpha) particles and Be impurities were measured for the first time in deuterium-tritium plasmas in the Joint European Torus. The time dependence of the measured spectra allowed the determination of the density evolution of fast alpha particles. Correlation between the decay time of the gamma-ray emission and the plasma parameters in different plasma scenarios was established. Results are consistent with classical slowing down of the alpha particles in discharges with high plasma currents and monotonic q-profiles. In low plasma current discharges and in the discharges with large on-axis current holes (extreme reversal central magnetic shear), the gamma-ray emission decay times are shorter than the classical slowing down times, indicating an alpha-particle confinement degradation in such discharges in line with theoretical predictions.

MHD stability of advanced tokamak scenarios with reversed central current: an explanation of the "current hole"

A region of zero current density in the plasma center has been observed in the advanced tokamak scenarios with off-axis lower-hybrid current drive in the JET and JT-60U tokamak experiments. Significantly, the central current density does not become negative, although this is expected based on conventional current diffusion. In this paper, it is shown that the zero central current density and the absence of negative central current can be explained by the influence of a resistive kink magnetohydrodynamic instability.

Observation of zero current density in the core of jet discharges with lower hybrid heating and current drive

Simultaneous current ramping and application of lower hybrid heating and current drive (LHCD) have produced a region with zero current density within measurement errors in the core ( r/a< or =0.2) of JET tokamak optimized shear discharges. The reduction of core current density is consistent with a simple physical explanation and numerical simulations of radial current diffusion including the effects of LHCD. However, the core current density is clamped at zero, indicating the existence of a physical mechanism which prevents it from becoming negative.