W. M. Solomon

Zonal flow generation in the improved confinement mode plasma and its role in confinement bifurcations

Unstable fluctuations develop in the initially quiescent plasma in the improved confinement mode of the H-1 heliac when the radial electric field (Er) shear exceeds some critical value. These unstable Er shear-driven modes are shown to generate zonal-flow-like poloidally symmetric potential structures, similar to those generated in the low confinement mode (Shats M G and Solomon W M 2002 Phys. Rev. Lett. 88 045001). The structures modulate their parent waves, the background Er shear and the fluctuation-driven radial transport. The onset of zonal flows is observed as a precursor to the plasma confinement bifurcation to an even higher confinement regime.

Fluctuations and stability of plasmas in the H-1NF heliac

The H-1NF heliac is a medium-sized heliac stellarator experiment with major radius R = 1 m, and average plasma minor radius a = 0.15–0.2 m. Its ‘flexible-heliac’ coil set permits precise variation in the value and shape of the rotational transform (ι) profile, with regions of both positive and negative shear. Operation at low fields ( B< 0. 2T ) with argon plasmas heated by helicon waves produces plasmas that have large ion Larmor radii (ρi/a ∼ 0.4) and show confinement transitions at low power like those in the edge of large devices, yielding fundamental measurements concerning electric fields and zonal flows. At a higher field (0.5 T), precise rotational transform scans with H–He plasmas heated by ICRF show resonant equilibrium and stability phenomena which depend on the value of the rotational transform at the radius of zero shear.

Multichannel visible spectroscopy diagnostic for particle transport studies in the H-1 heliac

A multichannel spectroscopy diagnostic has been developed to study cross-field particle transport in the radiation-dominated low-temperature plasmas (Te<100u2009eV) in the H-1 heliac. The optical setup covers the full plasma minor radius in the poloidal plane collecting light from ten parallel chords arranged tangentially to the flux surfaces. The light collected from the plasma is coupled into optical fibers and through interference filters into photomultipliers. Two such ten-fiber arrays are aligned parallel to one another to allow the simultaneous monitoring of two different spectral lines. The net radial electron particle flux is determined from the continuity equation by integrating over the ionization source term in the steady-state partially ionized plasma. The diagnostic measures the neutral line intensities and their ratios (in case of helium using the line ratio technique) and also measures excited neutral and ion spectral lines (in case of the argon plasma transport studies). A comparative analysis...

Fluctuation studies using combined Mach/triple probe

A probe consisting of two poloidally separated triple probes and a Mach probe (TMT probe) has been designed and installed on the H-1 heliac to study fluctuations. Mach probes are shown to be sensitive to the fluctuations in the electron density, electron and ion temperatures, and ion drift velocity. If the ion Larmor radius is much larger than the characteristic probe dimension, then the Mach probe is insensitive to the magnetic field. When the Mach probe is oriented such that the two tips are separated radially, it becomes sensitive to the radial velocity of the ions. A model has been devised to allow the above mentioned time-resolved plasma parameters to be reconstructed from the data obtained using the TMT probe. One of the important results of these studies is that ion temperature fluctuations cannot be neglected.

Measurements of poloidal rotation velocity using cross-correlation spectroscopy in the H-1 heliac

A correlation spectroscopy diagnostic [M.G. Shats and J. Howard, Fusion Eng. Des. 34–35, 271 (1997)] measures fluctuation spectra and local fluctuation intensities in a radiation-dominated plasma, such as the low-temperature plasma in the H-1 heliac (Te<50u2009eV, ne<2×1018u2009m−3). When the fluctuation coherence lengths in the poloidal and radial directions are shorter than the plasma radius, the cross-correlation function of the two crossed-sightline fluctuating intensities contains information about the fluctuations amplitude and their phase in the intersection volume. The optical setup on the H-1 heliac uses two nearly orthogonal views to image 20 optical fibers arranged into two linear arrays in the plasma poloidal cross section. A matrix of 10×10 cross-correlation functions is then analyzed to determine the poloidal phase velocity of the fluctuations, poloidal and radial correlation lengths, and the radial profiles of the fluctuations intensity. The results on the poloidal propagation velocity measured usi...

Overview and Results from the H‐1 National Facility

The H‐1 heliac has been substantially upgraded under the Australian Government’s Major National Research Facility Program. Enhancements include precision magnet power supplies, a 250kW RF source, and a 200kW 28GHz ECH system in collaboration with NIFS and Kyoto University. The power supply allows operation to fields ∼1 T with computer‐controlled configurations, and ripple < 0.01% to eliminate induced currents. Up to 200kW of RF power at 7 MHz is used at present to produce plasmas using helicon waves, and to produce target plasma for second harmonic ECH at ∼100kW. Local and remote data access methods include MDSPlus, a Java control interface and an advanced SQL‐based electronic log. At low fields in argon, tomographic interferometry and probe studies show low‐mode‐number coherent oscillations in electron density and electron and ion temperatures. These are suppressed when the plasma enters a higher confinement mode. Multiple diagnostic studies and comparison experiments with a linear helicon device suggest...

Experimental Evidence of Nonlinear Spectral Power Transfer In Zonal Flow Generation

Strong fluctuations observed in the low‐temperature plasma in the H‐1NF heliac are analyzed using the power transfer function in order to understand energy transfer mechanisms between short and long wavelengths, including zonal flows.

Multichannel spectroscopy diagnostic for helium line intensity ratio measurements (abstract)

A multichannel spectroscopy diagnostic has been designed to measure the electron temperature and density in radiation-dominated helium plasma in the H-1 heliac. The diagnostic principle relies on the well-known technique of measuring neutral helium line intensity ratios. A 2×10 optical fiber matrix (1 mm fiber diameter) is imaged into the plasma and collects light from ten chords. Two sets of ten-fiber arrays are coupled to the input slits of two monochrometers. The output slits are coupled to photomultiplier tubes through another two ten-fiber arrays. Several pairs of neutral helium spectral lines are used to measure time and spatially resolved Te and ne. The effect of structures and magnetic islands on the plasma profiles is investigated using this diagnostic.

Collective microwave scattering diagnostic on the H-1 heliac

A multichannel microwave scattering diagnostic has been developed and installed on the H-1 heliac. The purpose of the new diagnostic is to study small-scale plasma fluctuations in H-1, which are believed to be responsible for the loss of particles and energy from the plasma. The diagnostic is a 132 GHz, four-channel superheterodyne system. The transmitter and receiver antennas (consisting of horns and focusing bispherical mirrors) are located inside the vacuum vessel of H-1. A radial resolution of Δr/a∼0.2 is achieved. The scattering volume is positioned in the density gradient region at r/a∼0.6. At present, the system is aligned to measure fluctuations in the poloidal wave number range from approximately 10 to 25 cm−1. The use of the heterodyne detection system allows the fluctuation propagation direction to be determined. The low frequency bandwidth of the system is 1 MHz. The instrument sensitivity is about Ps/Pi∼10−6.