N.J. Lopes Cardozo

Disruptions in JET

In JET, both high density and low-q operation are limited by disruptions. The density limit disruptions are caused initially by impurity radiation. This causes a contraction of the plasma temperature profile and leads to an MHD unstable configuration. There is evidence of magnetic island formation resulting in minor disruptions. After several minor disruptions, a major disruption with a rapid energy quench occurs. This event takes place in two stages. In the first stage there is a loss of energy from the central region. In the second stage there is a more rapid drop to a very low temperature, apparently due to a dramatic increase in impurity radiation. The final current decay takes place in the resulting cold plasma. During the growth of the MHD instability the initially rotating mode is brought to rest. This mode locking is believed to be due to an electromagnetic interaction with the vacuum vessel and external magnetic field asymmetries. The low-q disruptions are remarkable for the precision with which they occur at qψ = 2. These disruptions do not have extended precursors or minor disruptions. The instability grows and locks rapidly. The energy quench and current decay are generally similar to those of the density limit.

Extreme hydrogen plasma densities achieved in a linear plasma generator

A magnetized hydrogen plasma beam was generated with a cascaded arc, expanding in a vacuum vessel at an axial magnetic field of up to 1.6T. Its characteristics were measured at a distance of 4cm from the nozzle: up to a 2cm beam diameter, 7.5×1020m−3 electron density, ∼2eV electron and ion temperatures, and 3.5km∕s axial plasma velocity. This gives a 2.6×1024H+m−2s−1 peak ion flux density, which is unprecedented in linear plasma generators. The high efficiency of the source is obtained by the combined action of the magnetic field and an optimized nozzle geometry. This is interpreted as a cross-field return current that leads to power dissipation in the beam just outside the source.

Perturbative Transport Studies in Fusion Plasmas

Studies of transport in fusion plasmas using perturbations of an equilibrium state are reviewed. Essential differences between steady-state and perturbative transport studies are pointed out. Important transport issues that can be addressed with perturbative experiments are identified as: (i) Are the transport relations linear (or nearly so)? (ii) What are the dominant dependences on plasma parameters; can they be understood from theory? (iii) Are there significant off-diagonal terms in the transport matrix? If so, how important are these for global confinement? (iv) Do the data obtained with perturbative experiments indicate that tokamak performance can be optimized along lines different from those presently explored? The theoretical framework for perturbative transport experiments is given. It is shown that perturbative experiments yield transport coefficients that are essentially different from the steady-state transport coefficients. In particular, when transport can be described by a transport matrix with off-diagonal elements, a perturbative experiment yields (one or more of) the eigenvalues of the matrix. In contrast, the coefficients obtained by steady-state analysis are linear combinations of the matrix coefficients, with the actual values of the various gradients as multipliers. Hence, the outcome of a steady-state transport evaluation depends on the actual values of the gradients, whereas a perturbative experiment measures the underlying transport matrix. Experimental perturbation techniques and techniques for data analysis are reviewed. Perturbations include the sawtooth instability, oscillatory gas feed, modulated power input, pellet injection etc. Data analysis techniques range from the time-to-peak analysis employed in sawtooth pulse propagation through Fourier or Laplace transforms, to direct numerical modelling. A review of the most important sources of systematic error is given.

Large amplitude quasi-stationary MHD modes in JET

Oscillating MHD modes in JET are often observed to slow down as they grow and generally stop rotating (lock) when the amplitude exceeds a critical value, then continue to grow to large amplitudes (br/Bθ ~ 1%). The mode can grow early in the current rise or after perturbations, such as a pellet injection or a large sawtooth collapse, and maintain a large amplitude throughout the remainder of the discharge. Such large amplitude quasistationary MHD modes can apparently have profound effects on the plasma, including stopping central ion plasma rotation, reducing the amplitude and changing the shape of sawteeth, flattening the temperature profile around resonant q surfaces and reducing the stored energy. Perhaps most important, large amplitude locked modes are precursors to most disruptions. Some large amplitude modes can be avoided by proper programming of the q evolution. The apparent reasons for the mode locking in a particular location are discussed and a comparison with theory is made.

Disruption generated runaway electrons in TEXTOR and ITER

Runaway generation during a major disruption has been observed in TEXTOR. Measurements of the synchrotron radiation yielded number, energy and pitch angle of the runaways. A simple model, which assumes that the runaways take over the current density in the centre of the discharge, successfully describes these measurements. This model is applied to JET and ITER. One interesting result of the model is that it could be favourable for ITER to have a high runaway production. This leads to a lower runaway energy and less runaway damage. Quantitative predictions are sensitive to the value of the runaway parameter = E/Ecrit, which is determined by the post-disruption temperature. The present estimate for ITER gives = 0.02, which results in a maximum runaway energy of 300 MeV in a runaway beam with a total energy of 500 MJ. However, if is enhanced, these values will be reduced. An increase to = 0.04 is sufficient to decrease the maximum runaway energy to 55 MeV and the total beam energy to 130 MJ. Secondary generation plays an important role in these predictions

Tearing mode stabilization by electron cyclotron resonance heating demonstrated in the TEXTOR tokamak and the implication for ITER

Controlled experiments on the suppression of the m/n = 2/1 tearing mode with electron cyclotron heating and current drive in TEXTOR are reported. The mode was produced reproducibly by an externally applied rotating perturbation field, allowing a systematic study of its suppression. Heating inside the island of the mode is shown to be the dominant suppression mechanism in these experiments. An extrapolation of these findings to ITER indicates that the projected system for suppression of the tearing mode could be significantly more effective than present estimates indicate, which only consider the effect of the current drive but not of the heating inside the island.

Optimization of the output and efficiency of a high power cascaded arc hydrogen plasma source

The operation of a cascaded arc hydrogen plasma source was experimentally investigated to provide an empirical basis for the scaling of this source to higher plasma fluxes and efficiencies. The flux and efficiency were determined as a function of the input power, discharge channel diameter, and hydrogen gas flow rate. Measurements of the pressure in the arc channel show that the flow is well described by Poiseuille flow and that the effective heavy particle temperature is approximately 0.8eV. Interpretation of the measured I-V data in terms of a one-parameter model shows that the plasma production is proportional to the input power, to the square root of the hydrogen flow rate, and is independent of the channel diameter. The observed scaling shows that the dominant power loss mechanism inside the arc channel is one that scales with the effective volume of the plasma in the discharge channel. Measurements on the plasma output with Thomson scattering confirm the linear dependence of the plasma production on...

A synchrotron radiation diagnostic to observe relativistic runaway electrons in a tokamak plasma

In present day tokamaks runaway electrons can be confined long enough to gain energies in the order of several tens of megaelectron volts. At these energies synchrotron radiation is emitted in the infrared wavelength range which can easily be detected by thermographic cameras. The spectral features of this synchrotron radiation are reviewed. On TEXTOR-94 a diagnostic exploiting this synchrotron radiation has been developed and is presented here. It is shown how to deduce the runaway parameters like runaway energy, pitch angle, runaway current and beam radius from the measurements. Based on the experience at TEXTOR-94 the feasibility of a similar synchrotron diagnostic on the International Thermonuclear Experimental Reactor is discussed. The maximum emission is expected in the wavelength range from 1–5 μm. A beam of 10 MeV runaway electrons with a current of about 15 kA will already be detectable.

Tokamak heat transport ― a study of heat pulse propagation in JET

The propagation of heat pulses originating from sawtooth activity in JET has been investigated in a series of limiter discharges with the following parameters: plasma current, Ip 3 MA, toroidal magnetic field, BT 3 T and elongation, κ = 1.45. The auxiliary power was varied such that the total power ranged from 2 to 13.5 MW. Electron temperature perturbations in a 20 cm region around a minor radius of r = (2/3)a were recorded with high time resolution, using a 12 channel electron cyclotron emission polychromator. From these measurements the electron heat diffusivity was derived. Over the whole range of powers considered, was found to be independent of power and to lie in the range of 2.5 ± 0.5 m2s−1. The quantity is compared to as derived from global power balance analysis. For Ohmic heating, the latter is lower than by a factor of 2.5. For increasing auxiliary power, approaches . A model for the dependence of the local χe n the temperature gradient is presented; it permits a unified description of the heat pulse behaviour, the deterioration of confinement and a certain degree of profile consistency. The model does not invoke non-local parameters such as the total power input. It is shown that the present heat pulse data, subject to this interpretation, contradict the τE scaling laws of the typical form τE ∝ P−0.5.

A model for electron transport barriers in tokamaks, tested against experimental data from RTP

Recent experiments in the RTP tokamak have shown that the electron temperature, T-e, profile in EC heated L mode discharges can have only a small number of distinct profile shapes, depending on the location of the additional heating. The sharp transitions between the profile shapes are associated with the loss or gain of a low rational q surface. These observations suggest that the electron thermal diffusivity chi(e) is a direct function of the safety factor q, with alternating layers of low and high chi(e). It is shown that such a model not only excellently describes the distinct profile shapes but also predicts other, observed but so far unexplained, phenomena, such as sharp off-axis maxima of T-e observed with off-axis heating at specific radii.