DITE Tokamak Research Articles

On attenuation of electron current of flush-mounted probes and electrodes in magnetised plasmas

The ratio of electron and ion saturation currents measured by probes embedded in the plasma-facing components (PFCs) in magnetised plasmas is typically lower than predicted by Langmuir theory. In the past, several works targeted various effects influencing the ion saturation current, which can be larger than expected, especially if the magnetic field is at a grazing angle with respect to the probe surface. In this contribution, we focus on the electron current, which can be reduced when the magnetic field line intercepting the probe passes through the magnetic pre-sheath of the nearby PFC. In such a case an effective potential well forms and repels a fraction of the electrons incoming from the plasma. Experimental results obtained by a tiltable limiter at the DITE tokamak are revisited and reproduced by means of 2D3V particle-in-cell simulations. The reduction of the electron current is indeed observed both in simulations and in experiment. This mechanism may also explain why some divertor biasing experiments did not produce flows in the divertor region.

Retarding Field Energy Analysis for Ion Temperature Measurement in the Tokamak Edge

At the energies characteristic of the tokamak edge plasma, the Retarding Field Analyser (RFA) constitutes probably the simplest method of directly measuring the ion velocity distribution parallel to the magnetic field and the magnitude of the sheath potential fall. For the case of Maxwellian velocity distributions, the technique provides an estimate of the edge ion temperature. Despite their relative simplicity, care must be taken both to operate these devices correctly in the rather unique tokamak edge environment and to consider carefully the interpretation of data thus obtained. This contribution addresses some aspects of both of these issues using the results of RFA experiments conducted on the DITE tokamak.

Propagation characteristics and relation between electrostatic and magnetic fluctuations in DIT

An account is given of experiments on the DITE tokamak, using a fast reciprocating set of combined Langmuir and magnetic probes in the same discharge to compare the propagation characteristics of the turbulent electrostatic and magnetic perturbations with those expected from the measured local electric field and pressure gradient, and to investigate the effect of shear on cross-field transport. In addition, the relation between the electrostatic and magnetic aspects of the turbulence is investigated using direct cross-spectral measurements. The results are consistent with a picture in which magnetic fluctuations in the edge are due to the currents driven by the basically electrostatic turbulence, whose own propagation behaviour is dominated by the effects of the sheared E*B/B2 velocity

Ion velocity distributions at the tokamak edge

This paper compares the form of f(v∥) arising from the warm-ion, kinetic models of Emmert et al. [Phys. Fluids 23, 803 (1980)] and Bissel and Johnson [Phys. Fluids 30, 779 (1987)] with experimentally measured distributions from the DITE tokamak obtained with a retarding field analyzer (RFA). The results show that the commonly adopted assumption of a distribution accelerated through the sheath potential and approximating to a Maxwellian in the high-energy tail is reasonable. In this respect, Emmert’s distribution is more appropriate. In addition, the common practice of neglecting the presheath effect on the distribution is generally justified in analyzing the RFA characteristic, but may incur large error under some circumstances. While the comparison also illustrates how differing assumptions regarding the source function can have a reasonably strong influence on the ion velocity distribution at walls and limiters, the integrated distributions required for comparison with RFA data are sufficiently similar that distinguishing between the models experimentally is not generally possible. The problems of resolving these differences experimentally are discussed and suggestions are proposed for further experiments.

Edge ion analysis in DITE

Recent advances in electrostatic particle analysers enable a detailed analysis of impurity and fuel ions in the tokamak boundary. The paper presents measurements of impurity content, charge state mix, ion temperature, ion energy distribution functions, sheath voltage and secondary electron yields obtained in the DITE tokamak. The consequences of these results for the understanding of impurity production processes are discussed.

Description of neutral gas in a divertor under conditions of strong interaction with plasma

A model for hydrogen neutral gas description in a tokamak divertor is proposed. The model includes hydrodynamic equations for cold neutrals and a kinetic equation for the hot atoms entering the gas from the plasma layer. The results of the gas parameter calculation are presented for the experimental conditions prevailing in the DITE tokamak. An explanation is given for the experimental observation that the D α line radiation intensity increases more slowly than the gas pressure. The gas target efficiency in a reactor scale divertor is calculated.

Investigation of the fluxes to a surface at grazing angles of incidence in the tokamak boundary

The paper describes an experimental investigation of the effects of grazing angles of incidence on the fluxes of power and particles to a surface inserted into the boundary of the DITE tokamak. Related thermographic data from the TFTR tokamak are also presented. A summary is given of the important issues relating to angle of incidence which are relevant to large tokamaks such as JET, NET and ITER and which require experimental confirmation. An array of Langmuir probes has been constructed to carry out experiments as a function of angle on DITE. Results from this tilting probe array (TPA) are described, which show how the ion saturation current, electron saturation current, floating potential and the fitted electron temperature vary with surface angle. It is shown that when the magnetic field is at grazing angles of incidence to the surface a conventional interpretation of the probe characteristics fails completely. Power flux distributions from infra-red thermography of the TFTR moveable limiter are presented, which show a substantially higher heat flux at the tangency point than is calculated conventionally.

Feedback stabilization of disruption precursors in a tokamak.

Magnetic feedback control has been used in the DITE tokamak to substantially reduce saturated m=2, n=1 instabilities in both Ohmic discharges and discharges with lower hybrid current drive (LHCD). Feedback has been used for the first time to significantly increase the disruptive density limit in a tokamak. LHCD on DITE stabilizes the sawtooth instability but generates a large m=1,2, n=1 instability. Both components of this mode have been controlled for the first time with feedback. Open loop experiments with LHCD show that mode locking occurs and allow detailed study of this phenomenon.

Impurity production at graphite limiters in the DITE tokamak

Through a series of controlled experiments on the DITE tokamak the main impurity production mechanisms at graphite limiters are investigated. Three principal processes can release carbon impurities: deuterium chemical sputtering, physical sputtering and oxygen chemical erosion. By varying, in turn, the limiter temperature, the bombarding particle energies and the local oxygen flux the rate of each erosion process is changed independently. From these experiments it is found that oxygen erosion dominates. Hence, carbon impurities leave the graphite surface primarily in the form of CO gas. The observed radial transport of intrinsic carbon and oxygen originating from the limiter is confirmed to be similar to that resulting from the localized puffing of CO. Metal impurities produced at the graphite limiter are physically sputtered, predominantly by carbon and oxygen impurity ions. The observed transport of chromium from the limiter has been modelled with a physical sputtering transport code.

The parallel transport of ions in a magnetized plasma

Analytic and Monte Carlo modelling of the parallel transport of ions along magnetic field lines is presented. The processes considered include the initial ionization of a neutral atom and the subsequent heating, parallel dispersal and further ionization of the resulting ion. The model predictions for the spatial distributions of dispersing ions are compared with experimental results from an impurity gas puffing experiment on the DITE Tokamak. The good agreement obtained between theory and experiment suggests that the ionization, heating and diffusion processes are correctly modelled assuming classical processes.

Measurements of cross-field plasma transport to an object in a tokamak edge plasma

When a material object is inserted into the scrape-off layer of a tokamak, it creates a wake which extends parallel to the magnetic field. Experiments are described which are the first direct test of the concept of a collection length, i.e the parallel scale length for this disturbance. The disturbed zone is a consequence of the equilibrium between parallel and cross-field particle transport. Two adjacent probes on the DITE tokamak were used. The large 'disturber' probe is a rectangular graphite plate which creates the main particle flow pattern and it can be rotated to change its projected width normal to the magnetic field. The small 'search' probe is located 0.5 m away on the same magnetic field line. It consists of a back-to-back pair of single probes which are used to measure the poloidal variations of the ion saturation current and Mach number in the wake of the disturber probe. Results are presented from experiments in which the disturber probe is rotated to change its natural collection length. The disturber probe was also biased with respect to the torus. The data obtained demonstrate the importance of considering non-ambipolar transport processes which are seldom discussed in the literature.

Plasma ion mass spectrometry in the boundary of the DITE Tokamak

Very limited diagnostic techniques are available for the study of flux and charge state distribution of impurities and plasma ions in the Tokamak scrape-off layer. The author describes an in situ plasma ion mass spectrometer (PIMS) which provides real time information on the charge-to-mass ratio of the ions in the scrape-off layer. The instrument, which is being developed on the DITE Tokamak, is based on the cycloidal focusing system and achieves a mass resolution of approximately=20. Results are presented which demonstrate the performance of the device. The observed charge state distributions are compared with theoretical predictions and an explanation for the near equality of the oxygen and carbon fluxes is presented.

Carbon impurity transport around limiters in the DITE tokamak

The transport of impurity ions originating at the limiter in a tokamak is critically dependent on the location of the ion in the boundary plasma. In the confined plasma, just inboard of the limiter, impurity ions will disperse freely into the discharge whilst in the scrape-off layer the pre-sheath plasma flow and the associated ambipolar electric field may tend to sweep impurities back to the limiter surface. In this paper we have studied, both by experiment and by theory, the transport of carbon impurity ions in the vicinity of the limiter. By comparing experimental measurements of the spatial distributions of impurities around the limiter with that predicted from a Monte Carlo computer code it appears that the parallel dispersal on closed field lines in the confined plasma is consistent with classical transport processes and that in the scrape-off layer the dispersal is indeed impeded by the pre-sheath plasma flow.

Impurity production and transport at limiters

This paper concentrates on the description and evaluation of experiments on the DITE tokamak. These are designed to characterise the processes involved in the production and transport of neutral and ionised impurities near carbon limiters. The need for good diagnostics in the scrape-off layer is highlighted. Langmuir probes are used to provide input data for models of impurity production at limiters. Observations of the radial profiles of carbon and oxygen impurities are compared with the code predictions. Changeover experiments involving hydrogen and helium plasmas are used as a means for investigating the role of the atomic physics and chemistry. The impurity control limiter (ICL) experiment is described which shows how geometry plays an important role in determining the spatial distributions of the neutral and ionised carbon. New diagnostics are required to study the flux and charge state distribution of impurities in the boundary. Preliminary results from an in-situ plasma ion mass-spectrometer are presented. The role of oxygen and the importance of evaluating the wall sources of impurity are emphasised.

Results from the dite pump-limiter experiment

Experiments have been carried out on the DITE tokamak with a large poloidal pump limiter of a relatively simple design. A particle exhaust into the pump limiter of 6% of the total particle flux leaving the plasma has been achieved.

Electron density and temperature-sensitive x-ray-emission-line ratios for heliumlike Si xiii in the DITE tokamak

Recent calculations of electron impact excitation rates in heliumlike Si xiii are used to derive the electron-density-sensitive emission-line ratio R (=f/i) and temperature-sensitive ratio G[=(f+i)/r], where f, i, and r are the forbidden 1${s}^{2}$ $^{1}s$ $^{3}S$, intercombination 1${s}^{2}$ $^{1}p$ $^{3}\mathrm{P}_{1}$,2 and resonance 1${s}^{2}$ 1${s}^{2}$${}^{1}$S--1s2p $^{1}P$ transitions, respectively. A comparison of these with R and G ratios measured from x-ray spectra of the divertor injected tokamak experiment, for which the electron density and temperature have been well determined, reveals good agreement between theory and observation, with discrepancies of typically 8% in R and 5% in G. This provides experimental support for the accuracy of the atomic data adopted in the line ratio calculations. The theoretical results may be applied to the analysis of remote sources for which no independent electron density and temperature estimates exist (such as solar flares), in order to derive values of ${N}_{e}$ and ${T}_{e}$ which, on the basis of the above comparison between theory and observation, should be accurate to \ifmmode\pm\else\textpm\fi{}0.3 and \ifmmode\pm\else\textpm\fi{}0.1 in the log, respectively.

The effect of ecrh on the dite plasma boundary

During the past year electron cyclotron resonance heating (ECRH) has been applied for the first time to the DITE tokamak. A new edge diagnostic, operable both as a Langmuir probe and a retarding field analyser, has been used extensively to investigate the variation of plasma boundary parameters during ECRH heated discharges. Arrays of Langmuir probes embedded in a pumped and inner bumper limiter have also been operated during additionally heated discharges. Measurements have been made over a wide range of ECRH input powers, as a function of global plasma density and for many values of the toroidal magnetic field. The behaviour of the edge plasma in response to the additional heating is observed to be strongly dependent on the major radial position at which resonant absorption of the ECRH pulse occurs. Radial profiles of electron density and temperature have been obtained, together with observations of the time response of these edge parameters to the ECRH pulse. Characteristic lengths in the boundary are observed to remain approximately unchanged throughout heated discharges. In general, the edge particle flux exhibits a well defined, reproducible behaviour, increasing during ECRH to ~1.5 times its ohmic value with rise times ≤ 2 ms. The percentage increase in the edge temperature is smaller with rise times ≤ 10 ms. Reasonable global power balance is obtained using data from the limiter Langmuir probes in both ohmic and ECRH phases of a typical discharge.

The impurity control limiter experiment

The paper presents results from the impurity control limiter (ICL) experiment, which was installed in the DITE tokamak. The ICL was designed so that the majority of ions leaving the plasma strike the undersides of its graphite tiles, which are angled towards the wall. These experiments confirm that the impurities sputtered from the ICL are ionized predominantly in the scrape-off layer and that there is significant deposition of neutral carbon on the walls. Both the spatial distribution of ionization and the toroidal distribution of carbon on the walls are in good agreement with code predictions. In helium discharges, the insertion of the ICL resulted in a 50% reduction of the total radiation and a 37–66% reduction in the impurity contribution to Zeff compared with the values obtained with rail limiters of conventional geometry. In deuterium discharges, the reduction in the total radiation was only 20%; this is attributed to the greater importance of the wall sources of carbon and oxygen in deuterium plasma relative to those in helium.

Broadband (1-25Å) X-ray Crystal Spectroscopy of Impurity Ions in the Dite Tokamak

AbstractWe present results from a Bragg rotor spectrometer and refer to wide–ranging studies of intrinsic and injected impurity ions in the DITE Tokamak. The instrument has absolute calibration for flux and wavelength and can detect impurities at the part per million level in the band from 1 to 24Å. Monochromator and fast spectral survey modes, combined with a spatial scan facility, result in a versatile and powerful diagnostic tool.

High Resolution Spectroscopy of Dite Tokamak in the 10Å Region

AbstractAn account is given of the X-Ray emission from the DITE tokamak in the spectral region 4Å–10Å measured by a Johann curved crystal photographic spectrometer. This high resolution instrument has been used for detailed line identifications eg. of L-shell transitions in the Fe spectrum at wavelengths ∼8Å and of the He-like ion spectra of Al, Si and Ar. The fine structure has been resolved for H-like Mg, Al and Si. Departures from the statistical value of 0.5 forβ, the intensity ratio of the Lyman-αdoublet, have been observed. Experimental values for the fine structure separation are presented for these medium Z metals.