Gradient driven electrostatic instabilities are investigated in TORPEX ͓A. Fasoli, B. Labit, M. McGrath, S. H. Müller, M. Podestà, and F. M. Poli, Bull. Am. Phys. Soc. 48, 119 ͑2003͔͒, a toroidal device ͑R =1 m, a = 0.2 m͒ in which plasmas are produced by microwaves ͑P ഛ 20 kW͒ with f rf = 2.45 GHz, in the electron cyclotron frequency range. Typical density and temperature are n e ഛ 10 17 m −3 and T e Ӎ 5 eV, respectively. The magnetic field is mainly toroidal ͑ഛ0.1 T͒, with a small vertical component ͑ഛ4 mT͒. Instabilities that can be generally identified as drift-interchange waves are observed and characterized for different levels of collisionality with neutrals. The frequency spectrum and the spatial profile of the fluctuation-induced flux are measured. An 86-tip probe is used to reconstruct the spatio-temporal evolution of density structures across the plasma cross section. The measured structures are characterized statistically, and related quantitative observables are constructed.

The Space Telescope Imaging Spectrograph (STIS) was successfully installed into the Hubble Space Telescope (HST) in 1997 February, during the second HST servicing mission, STS-82. STIS is a versatile spectrograph, covering the 115-1000 nm wavelength range in a variety of spectroscopic and imaging modes that take advantage of the angular resolution, unobstructed wavelength coverage, and dark sky offered by the HST. In the months since launch, a number of performance tests and calibrations have been carried out and are continuing. These tests demonstrate that the instrument is performing very well. We present here a synopsis of the results to date.

The mechanisms leading to ionization of neutral gas by means of microwaves in the electron cyclotron range of frequencies in a simple toroidal plasma configuration are investigated in the TORPEX device (Fasoli A et al 2003 Bull. Am. Phys. Soc. 48 119). The role of the upper hybrid resonance, where most of the ionization events occur once the plasma is formed, is clearly demonstrated. The fast phenomena related to ionization are discriminated from the slower mechanisms leading to the stationary background profiles by measuring the density perturbation caused by a modulation of the injected microwave power. The same method permits reconstruction of the spatial profile of the particle source. The ionization processes are then related to experimental control parameters such as the injected microwave power.

TORPEX [A. Fasoli, B. Labit, M. McGrath, S. H. Müller, M. Podestà, and F. M. Poli, Bull. Am. Phys. Soc. 48, 119 (2003)] is dedicated to the study of electrostatic instabilities, turbulence, and transport. Plasmas are produced by waves in the electron cyclotron frequency range and are confined by a toroidal magnetic field of about 0.1T to which a small vertical component Bz is added. The crucial role of Bz for the basic confinement scheme through the generation of parallel flows has been studied previously. This paper focuses on the effects of Bz on turbulence. The observed strong dependence indicates an intrinsic coupling between average profiles, confinement, and turbulence regulated by the action of Bz. Two approaches to characterize turbulence are adopted, via time series statistics and via the direct measurement of spatiotemporal structures, made possible by the novel hexagonal turbulence imaging probe diagnostic, which is described in detail. Analysis methods to condense the large amount of data of such imaging diagnostics are proposed.

The low aspect ratio of the mega amp spherical tokamak (MAST) allows differentiation between different forms of the H-mode threshold scaling. With optimized fuelling using inboard puffing, and a connected double null divertor (DND) magnetic configuration, the H-mode power threshold data lie about 1.7 times higher than recent scaling laws. Slight magnetic configuration changes, of the order of the ion Larmor radius, around a connected DND significantly influence H-mode access. H-mode confinement in discharges with low frequency edge localized modes (ELMs) is generally consistent with international scaling laws, e.g. IPB98(y,2). Strong indications of both particle and energy internal transport barriers have been seen. Normalized beta values β N > 5 have been obtained, approaching the ideal n = 1 no wall external kink stability limit. Sawtooth triggered neo-classical tearing modes have been observed; numerical modelling of the island evolution reproduces mode behaviour well and confirms the significance of stabilizing field curvature effects. Divertor power loading studies, including transient effects due to ELMs, show a strong bias of power efflux to the outboard targets, where it is more easily handled. ELM energy losses, W ELM , are less than 4% of the stored energy in all regimes explored so far, but ELM effluxes extending 30 cm outside the outboard separatrix have been measured. Toroidally asymmetric divertor biasing resulted in significant broadening of the D α profile on the biased components and a reduction in the total power to the unbiased components. Halo current magnitudes and asymmetries are generally small compared with conventional tokamaks;