M. Gilmore

Investigation of rescaled range analysis, the Hurst exponent, and long-time correlations in plasma turbulence

A detailed investigation of rescaled range (R/S) analysis to search for long-time correlations (via the Hurst exponent, H) in plasma turbulence is presented. In order to elucidate important issues related to R/S analysis, structure functions (SFs), one of several techniques available for calculating H, are also applied, and comparisons between the two methods are made. Time records of both simulated data and fluctuation reflectometry data from the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] are analyzed. It is found that the R/S method can be used to accurately determine H, provided a long enough data record is used, and that H is an indicator of persistence in the data. In addition, subtleties of the correct application of both methods are discussed, and potential advantages of SFs are pointed out.

Structure function analysis of long-range correlations in plasma turbulence

Long-range correlations (temporal and spatial) have been predicted in a number of different turbulence models, both analytical and numerical. These long-range correlations are thought to significantly affect cross-field turbulent transport in magnetically confined plasmas. The Hurst exponent, H—one of a number of methods to identify the existence of long-range correlations in experimental data—can be used to quantify self-similarity scalings and correlations in the mesoscale temporal range. The Hurst exponent can be calculated by several different algorithms, each of which has particular advantages and disadvantages. One method for calculating H is via structure functions (SFs). The SF method is a robust technique for determining H with several inherent advantages that has not yet been widely used in plasma turbulence research. In this article, the SF method and its advantages are discussed in detail, using both simulated and measured fluctuation data from the DIII-D tokamak [J. L. Luxon and L. G. Davis, ...

Experimental Characterization of the Stagnation Layer between Two Obliquely Merging Supersonic Plasma Jets

We present spatially resolved measurements characterizing the stagnation layer between two obliquely merging supersonic plasma jets. Intrajet collisionality is very high, but the interjet ion-ion mean free path is of the order of the stagnation layer thickness of a few centimeters. Fast-framing camera images show a double-peaked emission profile transverse to the stagnation layer, with the central emission dip consistent with a density dip in the interferometer data. We demonstrate that our observations are consistent with collisional oblique shocks.

The HelCat dual-source plasma device

The HelCat (Helicon-Cathode) device has been constructed to support a broad range of basic plasma science experiments relevant to the areas of solar physics, laboratory astrophysics, plasma nonlinear dynamics, and turbulence. These research topics require a relatively large plasma source capable of operating over a broad region of parameter space with a plasma duration up to at least several milliseconds. To achieve these parameters a novel dual-source system was developed utilizing both helicon and thermionic cathode sources. Plasma parameters of n(e) approximately 0.5-50 x 10(18) m(-3) and T(e) approximately 3-12 eV allow access to a wide range of collisionalities important to the research. The HelCat device and initial characterization of plasma behavior during dual-source operation are described.

Signal amplitude effects on reflectometer studies of density turbulence in tokamaks

The effect of amplitude fluctuations on reflectometer measurements of density turbulence has been investigated through comparison of reflectometry and Langmuir probes on the CCT and DIII-D tokamaks. Power spectra, turbulent radial correlation lengths, and root-mean square magnitude variations (at the H-mode transition) of the homodyne reflectometer signal (given by , which depends strongly on the amplitude E and nonlinearly on the phase ), show good agreement with Langmuir probes. The homodyne signal is found to be dominated by the amplitude fluctuations and not by the phase for high density fluctuation levels. Correspondingly, power spectra and correlation lengths deduced from the phase data alone show agreement with the homodyne signal only at low density fluctuation levels. It is concluded that for these plasma parameters the homodyne signal is closely representative of the density fluctuation behaviour and that this response is related to the reflectometer amplitude E. This correspondence of the homodyne signal and density fluctuations is in contrast to most theoretical/modelling work which has typically concentrated on the phase. A one-dimensional simulation of resonance absorption effects upon the amplitude and phase of a reflectometer is presented as an example of how amplitude fluctuations might arise due to processes internal to the plasma. The implications of these results and the connection to theory are discussed.

Experimental evidence for collisional shock formation via two obliquely merging supersonic plasma jetsa)

We report spatially resolved measurements of the oblique merging of two supersonic laboratory plasma jets. The jets are formed and launched by pulsed-power-driven railguns using injected argon, and have electron density ∼1014 cm−3, electron temperature ≈1.4 eV, ionization fraction near unity, and velocity ≈40 km/s just prior to merging. The jet merging produces a few-cm-thick stagnation layer, as observed in both fast-framing camera images and multi-chord interferometer data, consistent with collisional shock formation [E. C. Merritt et al., Phys. Rev. Lett. 111, 085003 (2013)].

Recent results from the National Spherical Torus Experiment

The National Spherical Torus Experiment (NSTX) is a low aspect-ratio fusion research facility whose research goal is to make a determination of the attractiveness of the spherical torus concept in the areas of high-β stability, confinement, current drive, and divertor physics. Remarkable progress was made in extending the operational regime of the device in FY 2002. In brief, βt of 34% and βN of 6.5 were achieved. H-mode became the main operational regime, and energy confinement exceeded conventional aspect-ratio tokamak scalings. Heating was demonstrated with the radiofrequency antenna, and signatures of current drive were observed. Current initiation with coaxial helicity injection produced discharges of 400 kA, and first measurements of divertor heat flux profiles in H-mode were made.

Detailed comparison of plasma turbulence correlation length measurements using microwave reflectometry and a Langmuir probe array

Detailed comparisons of reflectometer and Langmuir probe measurements of turbulent radial correlation lengths in the Large Plasma Device (LAPD) at UCLA are presented. The LAPD is a magnetized linear plasma device where detailed measurements of electrostatic turbulence with probe arrays are possible. Furthermore, density profiles are well known and experiments have been conducted over a large number of repeatable discharges, resulting in low levels of measurement and statistical error. It has been found that homodyne reflectometer and probe correlation length measurements are in good agreement, to within 10-15%, over a wide range of density gradient scale lengths and relative fluctuation levels.

Secondary electron yield measurements from materials with application to collectors of high-power microwave devices

An experimental test facility has been established for measuring the secondary electron yield (SEY) of materials thought to be suitable for low yield vacuum electronic applications such as collectors in high-power microwave (HPM) tubes. Experiments can be broadly divided into two energy-regimes: a high-energy (1-50 keV) and a low-energy (10 eV-1 keV) regime. Measurements of SEY at high energies are presented for the following materials: copper, titanium, and Poco graphite. Observation of time-dependent SEY behavior in these samples suggests that surface processes play an important role during measurements. In addition, SEY at low energies and as a function of the angle of incidence of primary electrons has been measured for plasma sprayed boron carbide (PSBC). The experimental results presented here are benchmarked with existing SEY data in the literature, empirically and to first principle formulae

Ion‐space‐charge initiation of gated field emitter failure

Failures of individual micron‐scale gated field emitters are observed to be similar in many respects to cathodic arcs. The initiation of a failure event at elevated pressures by ion‐space‐charge enhancement of the electric field at the emitter tip is simulated and compared with experimental results. The experimental results show a significantly lower pressure failure threshold than that predicted by the simulation. This discrepancy may indicate the presence of additional processes.