Very intense hydrogen Lyman-α (2p 2 P 0 → 1s 2 S) light emission (λ = 121.6 nm) is observed from neon gas near atmospheric pressure containing small admixtures (per mil) of hydrogen when this gas mixture is excited by ionizing particle beams. A DC beam of 15 keV electrons or a pulsed beam of 100 MeV 32 S ions were used in different experiments for excitation. A collisional energy transfer rate constant from neon to H * of (3±1)×10 −11 cm 3 s −1 has been measured using time-resolved optical spectroscopy on the Lyman-α line. Conversion efficiencies of particle beam power into Lyman-α light of the order of 10% have been observed. No other significant radiation was emitted in the entire VUV, UV and visible spectral region. In particular, no other hydrogen lines are observed under these conditions. The selective excitation of the H(2p) level is interpreted as arising from a resonant energy transfer between Ne * 2 excimers and hydrogen molecules.

The experimental observation of nonlinear parallel viscosity in the Interchangeable Module Stellarator is reported. The damping of plasma flows, by poloidal viscosity and ion-neutral collisions, is determined from the dependence of the radial current on the radial electric field in the plasma. At low neutral density, the radial current drops and a jump in the radial electric field occurs at a poloidal Mach number in the range 210 to 215, in reasonable agreement with a model based on nonlinear viscosity. The nonlinearity of viscosity diminishes as the neutral density is increased. [S0031-9007(98)05995-X] PACS numbers: 52.55.Hc, 52.25.Fi Improving the confinement in toroidal magnetic devices has been a central focus of the fusion program since the discovery of the L-H transition in ASDEX [1] in the early 1980s to the more recent dramatic reduction in core turbulent transport with reverse magnetic shear in the TFTR [2] and DIII-D [3] tokamaks. In the past few years, spontaneous L-H transitions have also been reported in the Wendelstein VII-AS [4] and CHS [5] stellarators, and recently in the H-1 heliac [6]. Pioneering efforts to improve confinement in toroidal machines using biased electrodes dates back to 1966 and the B-3 stellarator [7]. Biased electrode experiments in tokamaks [8-10], meanwhile, have shown that there is a sudden jump in the radial electric field during the L-H transition about a poloidal Mach number (M p 2E r ͞B p n t where E r is the radial electric field, B p is poloidal magnetic field, and n t is the thermal velocity of ions) of unity. It has been suggested that the jump in the radial electric field is a result of the nonlinearity of viscosity at high E 3 B flow speeds [11].In this Letter, we show the first results of a sudden jump in the radial electric field at a poloidal Mach number much greater than one, in reasonable agreement with an expression by Shaing for the parallel viscosity in stellarators [12]. The unity poloidal Mach number in tokamaks occurs because of the purely toroidal modulation of the magnetic field; the higher poloidal Mach number in stellarators occurs because of the additional helical modulation of the magnetic field. Furthermore, we show that the bifurcation of the radial electric field due to the nonlinear viscosity can be suppressed by an increase in the neutral density. The competition between neoclassical parallel viscosity and ion-neutral collisions in a stellarator has been described theoretically for low speed [13] when the viscosity is linear with the flow, and for high rotation speed [12] when the viscosity becomes nonlinear. Previous experiments [14] in the Interchangeable Module Stellarator (IMS) [15] measuring the radial conductivity, flow speed, and damping rate confirmed the neoclassical model at low rotation speeds. The work described here confirms the model at high rotation speed. A higher power plasma source, higher plasma densities, and larger bias voltages contributed to the higher rotation speeds observed in this paper compared to previous experi...

Observations of a performance limiting feedback phase instability in the HBT-EP tokamak are reported. The phase instability consists of a rapid growth of the phase difference between an m/n = 2/1 tearing mode and an external resonant magnetic perturbation. Observations of mode angular dynamics during phase instability test discharges show good agreement with theoretical estimates of the phase instability timescale. The phase instability limits feedback performance in HBT-EP by decreasing the feedback loop's phase accuracy as gain increases.

Abstract-A numerical code is developed to study the effect of ion density and temperature (as well as ion pressure and temperature gradients) on the amplitude of the local maxima of the poloidal viscosity with respect to the poloidal Mach number using the model developed by Shaing [1]. The sum of the poloidal viscosity and ion-neutral collisions is determined from the dependence of the plasma radial current on the radial electric field in a biased electrode experiment. The experimental results in the interchangeable module stellarator agree qualitatively with the predictions of the numerical calculation regarding the effects of variation of ion density and temperature on viscosity.

Efficient energy transfer between neon excimer molecules and hydrogen has been found. A small, high gas density light source has been developed, emitting entirely on the hydrogen 2p-l s transition at I 21 .567nm (Lyman-a). Light output densities of lOW/cm2 are obtained. Electron beam energy conversion efficiencies of lO% have been measured.