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For arbitrary wave propagation angle with respect to the magnetic field the wave cut-off and in particular, its dependence on the particle temperature are investigated. The obtained results demonstrate that when the wave propagation near the true cut-off (N = N = 0) as well as the cut-off of waves propagating at fixed, real N-value (N = 0, N0) are considered, it is essential to take relativistic effects into account. A simple expression is given for the cut-off density which is acceptable for most applications below T = 20 keV. In this temperature range it predicts values of the cut-off density of the ordinary mode which are close to the relativistic ones.

A high spatial resolution (3 mm full width half maximum, i.e. 2% of the minor radius) double-pulse multiposition Thomson scattering system was in operation at the Rijnhuizen tokamak project RTP from March 1996 until September 1998. It upgrades the previously installed single-pulse Thomson scattering system. Two measurements of full electron temperature, T_e, and density, n_e, profiles can be performed in rapid succession during one plasma discharge, using a double-pulsed laser and a dual charge-coupled-device camera system. The temporal resolution is determined by the separation time between the two subsequent laser pulses and can be tuned in the range 20-800 µs. Typical relative experimental errors are 3-5% for T_e and 2-4% for n_e at n_e = 5 × 10¹⁹ m^-3. Plasma experiments show the application of the diagnostic; double-pulse Thomson scattering measurements have been performed on high-n_e ohmic plasmas with large m/n = 2/1 magnetohydrodynamic activity, centrally heated plasmas with filaments (i.e. high T_e peaks) applying electron cyclotron heating, plasmas with a transient central T_e-rise after oblique pellet injection showing central filaments, and off-axis heated plasmas with off-axis sawtooth activity. The double-pulse feature of the diagnostic enables the study of the dynamics of T_e and n_e profiles in these plasmas.

In the HT-7 superconducting tokamak, the onset of a multifaceted asymmetric radiation from the edge (MARFE) usually occurs in the early ohmic discharges of each experimental campaign before wall conditioning. The occurrence and location of a MARFE is identified by different diagnostic systems. An improved confinement mode plasma which was induced by the MARFE is observed, and the global particle confinement time increases 1.9 times. The relaxation time between the MARFE event trigger and the L-H transition is about 1.4 ms, the following L-H transition time is 1.9 ms, and the improved confinement mode phase is maintained for about 40 ms. The MARFE cools the plasma edge, and the electron density profile is observed to become more narrow and peaked. The occurrence of a MARFE is strongly correlated with Z_eff but not with the density, and it always occurs at Z_eff = 3-8 ohmic discharges. In the case of a good wall condition (Z_eff = 1-2), the onset of MARFEs has not been observed before reaching the Greenwald density limit.

Highly coherent low-frequency fluctuations are studied experimentally in the low electron temperature plasma in the H-1 toroidal heliac. The fluctuations, presumably pressure-gradient-driven resistive MHD modes, produce considerable radial particle transport. A build-up of strong sheared radial electric field leads to dramatic modifications in the fluctuation-driven transport. These modifications correlate with sudden changes in the plasma confinement, resembling low-to-high transitions in other machines. Strong negative shear in the radial electric field eventually leads to the suppression of the fluctuations, while the onset of the strong positive shear correlates with the reversal of the fluctuation-driven particle flux, leading to the inward-directed pinch. It is concluded that the radial electric field modifies radial profiles of the fluctuation propagation velocity due to the Doppler shift, and it is this velocity shear which is important for the transport modifications.

In JT-60U ELMy H-mode plasmas, the width of the edge pedestal _ped observed in the ion temperature profiles is two to three times larger than that in the edge localized modes (ELM)-free plasmas. At high triangularity, _ped reaches 8-15 cm (9-16% of the minor radius) at the plasma current of 1 MA. In the high triangularity ELMy H-mode, the ion temperature at the pedestal shoulder and _ped can increase gradually with a time constant of ~2-3 s, which is longer than the global energy confinement time by one order of magnitude. In addition, pressure gradient in the pedestal layer increases simultaneously. The width _ped scales linearly with the poloidal gyro radius of thermal ions _pi at a fixed peripheral safety factor q₉₅ (_ped~3-4_pi at q₉₅ = 3-4), and has a weak dependence on q₉₅; _ped~5_piq₉₅^-0.3. The shrinkage of _ped observed at a high density can be explained by a decrease in the edge ion temperature with respect to a decrease in _pi. At a higher q₉₅(~6) a further increase in _ped was observed after the disappearance of ELMs.

A recent numerical study has found that, contrary to conventional theoretical and experimental expectations, reversed-shear plasmas are unstable primarily because the term proportional to the shear in the Mercier criterion is destabilizing. In the present study, the role of the magnetic shear, both local and global, is examined for various tokamak configurations with monotonic and non-monotonic safety factor profiles. The enhancement of the local shear due to the outward shift of the magnetic axis suggests that the latter are less susceptible to interchanges. Furthermore, by regrouping the terms in the criterion, the V´´ term when differentiated instead with respect to the toroidal flux, is shown to absorb the dominant shear term. No Mercier instability is found for similar profiles as in the previous study.

A comparison between two different quantitative spectroscopic measurements of the ion density of He-like carbon C⁴⁺ has been performed and used as a consistency check of a collisional-radiative model. The C⁴⁺ ion density was calculated from a zero-dimensional excitation and ionization balance for a resonant vacuum ultraviolet transition in Li-like carbon C³⁺. This was compared with a one-dimensional excitation balance for a non-resonant ultraviolet transition in C⁴⁺. At stable plasma conditions the two methods give consistent results.

Identification of coherent waves from fluctuating tokamak plasmas is important for the understanding of magnetohydrodynamics (MHD) behaviour of the plasma and its control. Toroidicity, plasma shaping, uneven distances between the resonant surfaces and detectors, and non-circular conducting wall geometry have made mode identification difficult and complex, especially in terms of the conventional toroidal and poloidal mode numbers, which we call (m,n)-identification. Singular value decomposition (SVD), without any assumption of the basis vectors, determines its own basis vectors representing the fluctuation data in the directions of maximum coherence. Factorization of a synchronized set of spatially distributed data leads to eigenvectors of time- and spatial-covariance matrices, with the energy content of each eigenvector. SVD minimizes the number of significant basis vectors, reducing noise, and minimizes the data storage required to restore the fluctuation data. For sinusoidal signals, SVD is essentially the same as spectral analysis. When the mode has non-smooth structures the advantage of not having to treat all its spectral components is significant in analysing mode dynamics and in data storage. From time SVD vectors, we can see the evolution of each coherent structure. Therefore, sporadic or intermittent events can be recognized, while such events would be ignored with spectral analysis.

We present the use of SVD to analyse tokamak magnetic fluctuation data, time evolution of MHD modes, spatial structure of each time vector, and the energy content of each mode. If desired, the spatial SVD vectors can be least-square fit to specific numerical predictions for the (m,n) identification. A phase-fitting method for (m,n) mode identification is presented for comparison. Applications of these methods to mode locking analysis are presented.

Hydromagnetic surface waves along a moving compressible plasma surrounded by a stationary plasma medium are investigated. The surface waves propagating along a cylindrical column should decay in amplitude, in a direction normal to the interface, i.e radially. Waves with amplitude proportional to the Bessel function J₁(x) and Hankel function H₁(x) do not, generally, represent a surface wave as reported by Satyanarayanan. The correct dispersion relation for a cylindrical plasma column with steady flow is obtained and the results are discussed for the isolated photospheric flux tubes, coronal streamers and solar wind flow tubes. It was found that while flux tubes support surface waves, the coronal streamers and solar wind flow tubes do not support surface waves for the parameters we have chosen.

The MHD stability in a tokamak, especially the Mercier criterion, is quite sensitive to the equilibrium parameters. In the paper, we solved the free-boundary equilibrium with hollow current profile and, in usual cases, the code gives the solution with enough accuracy. Through a careful re-examination of the equilibrium, however, it was found that the equilibrium accuracy was reduced for increasing q-profile hollowness and, near the magnetic axis, the plasma occasionally tended to the inverse D shape, which reduces the Mercier stability index. This resulted in the misleading output of the paper; that is, the Mercier criterion can be violated for the high inverse q profile even for q₀ greater than 1. By the re-calculation of the Mercier criterion for equilibria with high accuracy, we conclude that, although the stability index reduces with increasing q₀, the Mercier criterion is not violated. We acknowledge many colleagues who pointed out the importance of the equilibrium accuracy and suggested the re-examination of the stability calculation.

The received date for this paper was misquoted. It should have read `Received 17 March 1999, in final form 7 June 1999'.

The received date was inadvertently omitted. It should have read `Received 23 February 1999, in final form 24 May 1999'.