C. C. Chu

A high resolution multiposition Thomson scattering system for the Rijnhuizen Tokamak project

A high resolution multiposition Thomson scattering setup to measure the electron temperature (Te) and density (ne) of a hot plasma is described. The system is operational at the Rijnhuizen Tokamak Project. Light from a high power pulsed ruby laser is scattered by the free plasma electrons and relayed to a Littrow polychromator for spectral analysis. The spectrally and spatially resolved light is detected by a GaAsP photocathode. The two-dimensional image is intensified and recorded with a charge-coupled device camera. Te in the range of 50 eV–6 keV can be measured at 115 spatial elements of 2.6 mm length along the laser beam. The observation error is <6% at ne=2×1019  m−3 and smaller for higher ne. The high resolution and accuracy enabled the observation of small scale structures in Te and ne.

Error analysis of Rijnhuizen tokamak project Thomson scattering data

At the Rijnhuizen tokamak project a double pulse multiposition Thomson scattering diagnostic has been operational since 1996. It has been installed for the study of small scale structures in electron temperature (Te) and density (ne). This diagnostic measures Te and ne with high spatial resolution (3 mm full width at half maximum, i.e., 2% of the minor radius) and high accuracy (3%–4% of Te and 2%–3% of ne in the range of 50 eV–6 keV and ne=5×1019 m−3.) In this article an extensive error analysis is performed on both statistical and systematic deviations. It is found that the instrument function of the detection branch has a smoothing effect on the noise. This reduces the statistical error on the Te and ne measurements on each spatial position, because the resolution of the instrument is oversampled. The long tail of the instrument profile of the entire diagnostic has a significant effect on the systematic deviations in the Te and ne determination. However, it does not affect the relative size of the smal...

Calibration procedure and data processing for a TV Thomson scattering system

Calibration procedures for Thomson scattering systems based on television-like cameras, so-called TVTS systems, are described. The TVTS systems of the Rijnhuizen Tokamak Project (RTP), the TJ-II stellarator, and the Torus Experiment for Technology Oriented Research (TEXTOR) tokamak combine a 10–15 J ruby laser as a source with an intensified charge coupled device camera as a detector. A tungsten strip lamp in combination with an integrating sphere is used to calibrate all ∼105 pixels of the camera relatively to each other. Rayleigh scattering on hydrogen or nitrogen is used to perform an absolute calibration of the complete detection system. Great emphasis is placed on possible systematic errors on the determination of the electron temperature Te and density ne due to the calibration, such as tungsten lamp temperature, detoriation of the detection window, long term stability, laser beam alignment, and detector linearity. The long term stability of the system was tested by comparing different sets of calib...

Photon recycling system for multiposition tangential Thomson scattering

Multiposition tangential Thomson scattering makes it possible to measure the profiles of the local drift velocity and current density of a tokamak plasma. To achieve a reasonably small observational error (20%) a high photon yield is required. In this paper we propose an optical system to increase the photon yield by about a factor of 7. Photons from a pulsed ruby laser are captured in a ring cavity by means of an electro‐optical switch. The trapped laser beam recirculates about 14 times through the sampling volume which is part of the cavity, increasing the laser energy of 25 J to an effective energy of about 140 J.

Evidence of inhomogeneous thermal transport in RTP

There are strong indications that the radial electron thermal diffusivity is not a smooth function of the radius, but shows strong local variations. To investigate this irregularity systematically, a number of dedicated experiments have been carried out in the RTP tokamak: intense central heating, pellet ablation experiments, perturbative transport studies and steady state off-axis heating. The crucial diagnostic in these experiments is the Thomson scattering system, with a radial resolution of 1% of the minor radius. In sawtoothing plasmas a transport barrier has been identified near the sawtooth inversion radius. With steady state off-axis heating, hollow current density profiles have been sustained, and the region of reversed magnetic shear is characterized by very low thermal transport

Double pulse Thomson scattering system at RTP

In this article a double pulse multiposition Thomson scattering diagnostic, under construction at RTP, is discussed. Light from a double pulsed ruby laser (pulse separation: 10–800 μs, max. 2×12.5 J) is scattered by the free electrons of the tokamak plasma and relayed to a Littrow polychromator for spectral analysis. The spectrally resolved light is recorded by two ICCD detectors. Simulations show that the system sensitivity will be such that electron temperatures in the range of 100 eV–7 keV can be determined with an accuracy as good as 2%–3% for electron densities of 1020 m−3, with a spatial resolution down to 2.6 mm. With this diagnostic the dynamics of small scale structures in the electron temperature profile will be studied.

Evidence for small scale magnetic structures in the RTP tokamak

High resolution Thomson scattering measurements of electron temperature and density in RTP show several hot filaments in the plasma centre and sharp gradients near the sawtooth inversion radius under central ECH, and structures outside the sawtooth region both in ohmic and ECH plasmas. These measurements give evidence of a complex magnetic topology. A numerical scheme is proposed which allows computation of the magnetic equilibrium with broken surfaces. Some properties of transport in a inhomogeneous medium are discussed.

Filamentation in tokamaks

The relevance of a nest of toroidal flux surfaces as a paradigm of the magnetic topology of a tokamak plasma is challenged. High resolution Thomson scattering measurements of electron temperature and density in RTP show several hot filaments in the plasma center and sharp gradients near the sawtooth inversion radius and structures outside the sawtooth region under central ECH. In ohmic plasmas, too, the pressure and temperature profiles show significant bumps. These measurements give evidence of a complex magnetic topology. Transport in a medium with spatially strongly varying diffusivity is considered. It is shown that macroscopic transport is determined by the microscopic structure: a transport theory must predict this structure and the diffusivity in the insulating regions, while the ‘turbulent’ diffusivity is irrelevant. A numerical approach to equilibria with broken surfaces is presented.