Evidence of Electron Heating by Alpha Particles in JET Deuterium-Tritium Plasmas.

The fusion-born alpha particle heating in magnetically confined fusion machines is a high priority subject for studies. The self-heating of thermonuclear fusion plasma by alpha particles was observed in recent deuterium-tritium (D-T) experiments on the joint European torus. This observation was possible by conducting so-called "afterglow" experiments where transient high fusion yield was achieved with neutral beam injection as the only external heating source, and then termination of the heating at peak performance. This allowed the first direct evidence for electron heating of plasmas by fusion-born alphas to be obtained. Interpretive transport modeling of the relevant D-T and reference deuterium discharges is consistent with the alpha particle heating observation.

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.

In situ wavelength calibration of the edge CXS spectrometers on JET.

A method for obtaining an accurate wavelength calibration over the entire focal plane of the JET edge CXS spectrometers is presented that uses a combination of the fringe pattern created with a Fabry-Pérot etalon and a neon lamp for cross calibration. The accuracy achieved is 0.03 Å, which is the same range of uncertainty as when neglecting population effects on the rest wavelength of the CX line. For the edge CXS diagnostic, this corresponds to a flow velocity of 4.5 km/s in the toroidal direction or 1.9 km/s in the poloidal direction.

Stationary Zonal Flows during the Formation of the Edge Transport Barrier in the JET Tokamak.

High spatial resolution Doppler backscattering measurements in JET have enabled new insights into the development of the edge Er. We observe fine-scale spatial structures in the edge Er well with a wave number krρi≈0.4-0.8, consistent with stationary zonal flows, the characteristics of which vary with density. The zonal flow amplitude and wavelength both decrease with local collisionality, such that the zonal flow E×B shear increases. Above the minimum of the L-H transition power threshold dependence on density, the zonal flows are present during L mode and disappear following the H-mode transition, while below the minimum they are reduced below measurable amplitude during L mode, before the L-H transition.

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.

Method to obtain absolute impurity density profiles combining charge exchange and beam emission spectroscopy without absolute intensity calibration.

Investigation of impurity transport properties in tokamak plasmas is essential and a diagnostic that can provide information on the impurity content is required. Combining charge exchange recombination spectroscopy (CXRS) and beam emission spectroscopy (BES), absolute radial profiles of impurity densities can be obtained from the CXRS and BES intensities, electron density and CXRS and BES emission rates, without requiring any absolute calibration of the spectra. The technique is demonstrated here with absolute impurity density radial profiles obtained in TEXTOR plasmas, using a high efficiency charge exchange spectrometer with high etendue, that measures the CXRS and BES spectra along the same lines-of-sight, offering an additional advantage for the determination of absolute impurity densities.

Measurements of intrinsic ion Bernstein waves in a tokamak by collective Thomson scattering.

In this Letter we report measurements of collective Thomson scattering (CTS) spectra with clear signatures of ion Bernstein waves and ion cyclotron motion in tokamak plasmas. The measured spectra are in accordance with theoretical predictions and show clear sensitivity to variation in the density ratio of the main ion species in the plasma. Measurements with this novel diagnostic demonstrate that CTS can be used as a fuel ion ratio diagnostic in burning fusion plasma devices.

Development of novel fuel ion ratio diagnostic techniques.

To overcome the challenge of measuring the fuel ion ratio in the core (ρ<0.3) of ITER, a coordinated effort aiming at developing diagnostic techniques has been initiated. The investigated techniques are novel uses or further development of existing methods such as charge exchange recombination spectrometry, neutron spectrometry, and collective Thomson scattering. An overview of the work on the three diagnostic techniques is presented.

Charge exchange spectroscopy as a fast ion diagnostic on TEXTOR.

An upgraded charge exchange spectroscopy diagnostic has been taken into operation at the TEXTOR tokamak. The angles of the viewing lines with the toroidal magnetic field are close to the pitch angles at birth of fast ions injected by one of the neutral beam injectors. Using another neutral beam for active spectroscopy, injected counter the direction in which fast ions injected by the first beam are circulating, we can simultaneously measure a fast ion tail on the blue wing of the D(alpha) spectrum while the beam emission spectrum is Doppler shifted to the red wing. An analysis combining the two parts of the spectrum offers possibilities to improve the accuracy of the absolute (fast) ion density profiles. Fast beam modulation or passive viewing lines cannot be used for background subtraction on this diagnostic setup and therefore the background has to be modeled and fitted to the data together with a spectral model for the slowing down feature. The analysis of the fast ion D(alpha) spectrum obtained with the new diagnostic is discussed.

Validation of the ITER CXRS design by tests on TEXTOR.

The charge exchange recombination spectroscopy system (CXRS) for ITER is designed to measure the core helium concentration, and in addition, profiles of ion temperature and rotation. This highly demanding task, due to the huge background radiation (bremsstrahlung) and the high attenuation of the dedicated diagnostic neutral beam, requires high throughput spectrometers with high resolution. On TEXTOR, a CXRS system has been developed with the aim to test the physics implications of these specifications. (i) A relevant spectrometer has been tested. (ii) A method to determine the helium concentrations from the CXRS intensity, using the beam emission has been evaluated. A 20% discrepancy in beam emission was revealed. (iii) The determination of the magnetic pitch angle by the ratio of Balmer lines showed qualitatively the right behavior, although the accuracy was limited by the polarization sensitivity of the first mirror. (iv) The simulation code developed for the prediction of the CXRS spectra was quantitatively confronted with experimental data.

Review of atomic data needs for active charge-exchange spectroscopy on ITER.

The quantitative exploitation of active beam spectra is largely based on an advanced atomic modeling. Under the ITER operating conditions the penetration depth of a diagnostic beam into the plasma core crucially affects the intensities of spectral lines and hence the uncertainties of derived plasma parameters. A critical review of atomic data and an assessment of its error margins are, therefore, urgently needed. The aim of the present work is to verify the existing beam-stopping and beam-emission data for hydrogen beam in fusion plasmas. The agreement between the ADAS database and the present calculations is found to be within 5% for the beam-stopping data in a H-plasma. The calculation of beam attenuation in the presence of He-ash (4%) and Be ions (2%) demonstrates the agreement between the present data and the ADAS database within 10%. Finally, the maximum deviation of 15% is found only for beam-emission data at the electron density of 1x10(12)-2x10(12) cm(-3), which is significantly below the ITER density of 10(14) cm(-3).