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Analysis of the evolution of plasma parameters during sawteeth in Ohmically- and electron cyclotron-heated plasma in the T-10 Tokamak (a_L=0.3 m, R₀=1.5 m) reveals that the sawtooth crashes are associated with values of the electron temperature gradient d(ln T_c)/dr at the q=1 surface in excess of a critical value.

The author investigates the space-time dynamics of electromagnetic fields generated by the sharp leading edge of a high-current flux of electron oscillators in a plasma waveguide under the development of a Cherenkov-type beam instability.

Far-forward scattering measurements from a cylindrical magnetized arc plasma are reported. A well-stabilized CW CO₂ laser serves as a light source. Self-excited electron density fluctuations with frequencies in the range 40 kHz to 1 MHz have been observed. Three frequency peaks can be distinguished from a broad background signal. The fluctuation level is maximal near the cathode, where the mean electron density approaches N_c=2*10¹⁴ cm^-3 on axis. In this region a relative fluctuation amplitude of 5% has been measured. In addition to the beam profile measurements, which are necessary in far-forward scattering experiments, a spatial profile of the modulated light intensity has been obtained. It is found that the scattered power is maximal in the region of the largest electron density gradient. The fluctuations are described as azimuthal propagating waves. In contrast to the plane-wave model used in other publications this model takes the spatial limits of the fluctuation field into account. The spatial distribution of the modulated light intensity at the detection plane is calculated by using the well-known diffraction theory. The theoretical model is in good agreement with the experimental data.

It is shown that these two models are closely related. The 'Tangled Discharge Model' (TDM) necessarily involves a non-vanishing <v*b>, the so-called 'dynamo' effect, while the effective functioning of this term in the 'Dynamo' models is shown to require a stochastic magnetic field structure. There is no inconsistency between this conclusion and the fact that the fully-relaxed state may be described by analytical forms such as the 'Bessel Function Model' which apparently contain good flux surfaces. The TDM is not a satisfactory description of the behaviour of present relaxed-state systems, but might be appropriate in the limit of large Lundquist number S.

The electron distribution function is statistically examined from measurements by Thomson scattering at +or-7 cm off-axis points (about half of the minor radius, 13.5 cm) from the center of the vacuum vessel of the TPE-1RM15 reversed field pinch (RFP) machine. Four different RFP discharge conditions are examined at both ports on a shot-by-shot basis. The wavelength shift of the distribution function and corresponding poloidal current density are deduced so that the same bulk electron temperatures at both the inside (-7 cm) and outside (+7 cm) ports are obtained. The deviation of the distribution function from the shifted Maxwellian is examined from the difference in fitted temperature with different weighting functions. The results indicate that in two of the experimental groups, the plasma current density carried by the tail electrons above 2.6 keV exceeds that carried by the shifted bulk Maxwellian distribution. For all of the data groups, the ratio of the current density carried by tail electrons to the total current density has a positive correlation with both E/E_C (=0.14-0.22) and eta _k/ eta _S, where E_C is the critical electric field for thermal electrons to run away, and eta _k and eta _S, are the plasma resistivity estimated from the helicity balance equation and Spitzer's formula, respectively. These tail electrons observed in the core plasma of an RFP can be attributed to the origin of the high-energy electrons recently observed at the edge region of RFPs.

The authors report on an experiment in a double-plasma machine where a positively-biased plane Langmuir probe situated in the target chamber, can excite a high-amplitude, coherent instability in the ion-frequency range when an ion beam from the source chamber is directed to it without reaching it. The ratio of ion-beam to plasma density is around 0.3. In this case the electron branch of the probe characteristic 'breaks off' after the second knee due to the ion beam, and saturates, By increasing the probe bias further an ion instability appears, which is associated with a further drop in the averaged probe current, Under certain conditions the instability shows an intermittent behaviour, i.e. fluctuates without any obvious external influence.

The behaviour of the (2, 1) tearing mode in lower hybrid current drive (LHCD) discharges in ASDEX is examined. Due to the change in the current profile j(r), this mode can be destabilized during LHCD. This often leads to mode locking and a disruption, thereby limiting the amount of LH power that can be transferred to the plasma during current drive. A statistical analysis for two different current drive spectra shows that this power limit is dependent on both plasma current I_p and line-averaged electron density n_c. The authors discuss the possibilities of disruption control and show how tailoring of the LHCD spectra might avoid the occurrence of the (2, 1) mode.

Some opportunities for measuring n_c, T_e, T_i, Z_eff and background plasma drift velocities in the edge region of magnetic confinement devices based on impurity injection are described. The technique would be based on spatially resolving (along B) the distributions of successive charge states and the measurement of (Doppler) temperatures of the states. Some simple analytic relations can be obtained to permit the extraction of first-order estimates of the plasma parameters, one at a time, from the experimental data; however, in practice, it will generally be necessary to use an impurity transport code with the input plasma parameters varied to obtain a best fit to all of the available experimental data simultaneously.