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With a high resolution visible light spectrometer the Hα emission at the RTP tokamak has been studied. Monitoring the Hα line (6562.8 A) with a toroidal line of sight into the plasma shows spectra with intricate Hα line structures. These line profiles can be explained by the occurrence of three different velocity groups of excited hydrogen with relative line shifts of the order of 2 A. The origin of these three groups results from collisional processes with species of hydrogen, namely neutral hydrogen, H+ and also H2+ molecules.

The results of a series of experiments done in RTP which had the purpose to avoid or ameliorate radiative density-limit disruptions, will be reported. Avoidance of disruptions was achieved by stabilizing the m = 2 radiation induced tearing mode (RTM) with electron cyclotron resonance heating (ECRH). Both continuous and modulated power deposition was studied. It was found that stabilization with modulated ECRH in phase with the O-point was less efficient than continuous ECRH, contrary to theoretical expectations. Detailed scans of the EC power deposition and of the power intensity were in agreement with the assumption that radiative heat loss is the driving mechanism of this m = 2 mode. Amelioration of disruptions was achieved, in a pilot experiment, by applying ECRH at the end of the energy quench. In this way, the current decay that follows a major density-limit disruption could be reversed.

The local measurement of the island width $w$, around the resonant surface, allowed a direct test of the extended Rutherford model [P. H. Rutherford, PPPL Report-2277 (1985)], describing the evolution of radiation-induced tearing modes prior to disruptions of tokamak plasmas. It is found that this model accounts very well for the observed exponential growth and supports radiation losses as being the main driving mechanism. The model implies that the effective perpendicular electron heat conductivity in the island is smaller than the global one. Comparison of the local measurements of $w$ with the magnetic perturbed field $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{B}$ showed that $w\ensuremath{\propto}{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{B}}^{1/2}$ was valid for widths up to $18%$ of the minor radius.

In the Rijnhuizen Tokamak Project (RTP) plasmas with electron cyclotron heating (ECH), a transient rise of the core T-e is observed when hydrogen pellets are injected tangentially to induce fast cooling of the peripheral region. The core T-e rise is a sharp function of the normalized power deposition radius (rho (dep)). The strongest effect (deltaT(e) up to 80%) is observed when rho (dep) is just inside the q = 1 surface, whereas only modest effects are seen with larger or smaller rho (dep). The rise is due to both a steepening of the T-e profile between the q = 1 and q = 2 surfaces, and a peaking inside rho (dep).

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A 'non-local' response of the plasma core triggered by peripheral plasma perturbations other than laser ablation is found in the RTP tokamak. Oblique pellet injection (OPI) has been used to induce fast cooling of the peripheral plasma. In response, an inward cold pulse (T-e drop) and a slightly delayed core T-e rise are observed. A somewhat similar 'non-local' response is observed when the peripheral plasma is heated by modulated electron cyclotron heating or by fast current ramps, i.e. the core temperature drops in response to the peripheral heating. The plasma conditions for the occurrence of the 'non-local' response have been investigated. The core T-e rise following OPI is associated with the formation of a large temperature gradient in the region 1 < q < 2. The T-e rise is largest at low electron density and for large pellet deposition radii. Above a critical density the T-e rise disappears and only the (weaker) drop in core T-e is observed. Time dependent transport simulations show that the propagation of the inward cold pulse is consistent with local transport, while the core T-e rise is a slower phenomenon requiring a large transient drop of chi(e) in the region 1 < q < 2. A 'non-local' response of the plasma core triggered by peripheral plasma perturbations other than laser ablation is found in the RTP tokamak. Oblique pellet injection (OPI) has been used to induce fast cooling of the peripheral plasma. In response, an inward cold pulse (Te drop) and a slightly delayed core Te rise are observed. A somewhat similar 'non-local' response is observed when the peripheral plasma is heated by modulated electron cyclotron heating or by fast current ramps, i.e. the core temperature drops in response to the peripheral heating. The plasma conditions for the occurrence of the 'non-local' response have been investigated. The core Te rise following OPI is associated with the formation of a large temperature gradient in the region 1 < q < 2. The Te rise is largest at low electron density and for large pellet deposition radii. Above a critical density the Te rise disappears and only the (weaker) drop in core Te is observed. Time dependent transport simulations show that the propagation of the inward cold pulse is consistent with local transport, while the core Te rise is a slower phenomenon requiring a large transient drop of χe in the region 1 < q < 2.

In the Rijnhuizen tokamak project, the double pulse multiposition Thomson scattering diagnostic is in full operation. Its high spatial resolution enables the measurement of small scale structures in T-e, n(e), and p(e). Thomson scattering profiles during an ordinary sawtooth crash show the displacement of the hot core in great detail. Measurements on off-axis sawtoothing plasmas show that a small central part remains unaffected. Filaments are observed in plasmas which show a transient central temperature rise in response to fast edge cooling. (C) 1999 American Institute of Physics. [S0034-6748(99)67601-8].

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In the Rijnhuizen Tokamak Project plasmas, a transient rise of the core electron temperature is observed when hydrogen pellets are injected tangentially to induce fast cooling of the peripheral region. High-resolution Thomson scattering measurements show that the T-e rise is associated with large temperature gradients in the region 1 < q < 2. This region acts as a layer of transiently increased thermal resistivity (transport barrier) when probed by fast heat pulses from modulated electron cyclotron heating.

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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.

Transport studies in the RTP tokamak are presented. A Z(eff) scan shows a weak dependence of chi(e) on collisionality. Studies of the progagation of heat pulses, induced by both modulated ECRH and sawteeth, show that chi(inc) is independent of input power. Moreover, these perturbative studies indicate a transport barrier related to the q = 1 surface. Independent evidence for the existence of such a transport barrier comes from high-resolution multiposition Thomson Scattering measurements.

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