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Gekelman, W.

Publications and source records attributed to Gekelman, W..

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Experimental observation of a field-aligned ion beam produced by magnetic reconnection of two flux ropes

An ion beam field-aligned to the background guide field ($B$ 0 = $330$ G) was observed in a reconnection experiment on the Large Plasma Device and, to the authors' knowledge, is the first experimental observation of its kind. Two kink-unstable flux ropes ( L = 11 m, d = 7.6 cm) were made to collide, which allows magnetic reconnection to occur. Sub-Alfvénic ion beams with energies of up to 15 eV were then observed from measurements of the local ion energy distribution function. The beam ions do not appear to be heated. Further, they were correlated with the collision of the ropes and appear to be energized by magnetic reconnection. The results and interpretation of the measurements are supported by three-dimensional gyrokinetic particle simulations of the merging flux ropes and electric field measurements from previous experiments [W. Gekelman et al. , Astrophys. J. 853, 33 (2018)]. The mechanism behind the acceleration appears to be non-local.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Thomson scattering on the large plasma device

We have developed a non-collective Thomson scattering diagnostic for measurements of electron density and temperature on the Large Plasma Device. A triple grating spectrometer with a tunable notch filter is used to discriminate the faint scattering signal from the stray light. In this paper, we describe the diagnostic and its calibration via Raman scattering and present the first measurements performed with the fully commissioned system. Depending on the discharge conditions, the measured densities and temperatures range from 4.0 × 10 12 to 2.8 × 10 13 cm -3 and from 1.2 to 6.8 eV, respectively. The variation of the measurement error with plasma parameters and discharges averaged is also discussed.

47 OTHER INSTRUMENTATION↗
DOE OSTI2018

Spiky electric and magnetic field structures in flux rope experiments

Magnetic flux ropes are structures that are common in the corona of the sun and presumably all stars. They can be thought of as the building blocks of solar structures. They have been observed in Earth’s magnetotail and near Mars and Venus. When multiple flux ropes are present magnetic field line reconnection, which converts magnetic energy to other forms, can occur when they collide. The structure of multiple magnetic ropes, the interactions between multiple ropes, and their topological properties such as helicity and writhing have been studied theoretically and in laboratory experiments. In this paper, we report on spiky potential and magnetic fields associated with the ropes. We show that the potential structures are chaotic for a range of their temporal half-widths and the probability density function (PDF) of their widths resembles the statistical distribution of crumpled paper. The spatial structure of the magnetic spikes is revealed using a correlation counting method. Computer simulation suggests that the potential structures are the nonlinear end result of an instability involving relative drift between ions and electrons.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Nonlocal Ohms Law, Plasma Resistivity, and Reconnection During Collisions of Magnetic Flux Ropes

The plasma resistivity was analyzed in an experiment on the collision of two magnetic flux ropes. Whenever the ropes meet, some magnetic energy is lost as a result of reconnection. Volumetric data, in which all the relevant time-varying quantities were recorded in detail, are introduced. Ohm's law is shown to be nonlocal and cannot be used to evaluate the plasma resistivity. The resistivity was instead calculated using the AC Kubo resistivity and shown to be anomalously high in certain regions of space.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Non-local Ohm's law during collisions of magnetic flux ropes

Two kink unstable magnetic flux ropes are produced in a carefully diagnosed laboratory experiment. Using probes, the time varying magnetic field, plasma potential, plasma flow, temperature, and density were measured at over 42 000 spatial locations. These were used to derive all the terms in Ohm's law to calculate the plasma resistivity. The resistivity calculated by this method was negative in some spatial regions and times. Ohm's law was shown to be non-local. Instead, the Kubo resistivity at the flux rope kink frequency was calculated using the fluctuation dissipation theorem. In conclusion, the resistivity parallel to the magnetic field was as large as 40 times the classical value and peaked where magnetic field line reconnection occurred as well as in the regions of large flux rope current.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Generation of shear Alfvén waves by repetitive electron heating

Abstract ELF/ULF waves are powerful tools for submarine communication, geophysical mapping, and radiation belt remediation. However, due to their large wavelength (on the order of 10 2 –10 4 km or 0.1–10 R E ) it is difficult to launch them using ground‐based antennas. Alternatively, these waves can be generated by modulating the temperature of the ionosphere using ground‐based HF transmitters. The paper reports a detailed laboratory study on the generation of shear Alfvén waves by repetitive electron heating. The experiments were conducted on the large plasma device at University of California, Los Angeles. In the experiment, 10 pulses of high‐power microwaves (250 kW, 1 µs each) near the plasma frequency modulated at a variable fraction between 0.1 and 1.0 of f ci are launched transverse to the background field. In addition to bulk electron heating the interaction generates a population of fast electrons in the tail of the distribution function. The field‐aligned current carried by the fast electrons acts as an antenna that radiates shear Alfvén waves. It is demonstrated that a shear Alfvén wave at a controllable, arbitrary frequency ( f < f ci ) can be coherently driven by the repetitive microwave pulses. The radiation pattern and power dependence of the virtual antenna are also presented. The experiments provide a novel virtual antenna concept relevant to the equatorial region where the Earth's magnetic field is horizontal and the field‐aligned plasma density gradient is small. The results are important to design of new mobile ionospheric heaters for equatorial and middle latitude locations.

Wang, Y.↗
DOE OSTI2015

Electron beam generated whistler emissions in a laboratory plasma

Naturally occurring whistler mode emissions in the magnetosphere, are important since they are responsible for the acceleration of outer radiation belt electrons to relativistic energies and also for the scattering loss of these electrons into the atmosphere. Recently, we reported on the first laboratory experiment where whistler waves exhibiting fast frequency chirping have been artificially produced [1]. A beam of energetic electrons is launched into a cold plasma and excites both chirping whistler waves and broadband waves. Here we extend our previous analysis by comparing the properties of the broadband waves with linear theory.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Experimental study of subcritical laboratory magnetized collisionless shocks using a laser-driven magnetic piston

Recent experiments at the University of California, Los Angeles have successfully generated subcritical magnetized collisionless shocks, allowing new laboratory studies of shock formation relevant to space shocks. The characteristics of these shocks are compared with new data in which no shock or a pre-shock formed. The results are consistent with theory and 2D hybrid simulations and indicate that the observed shock or shock-like structures can be organized into distinct regimes by coupling strength. With additional experiments on the early time parameters of the laser plasma utilizing Thomson scattering, spectroscopy, and fast-gate filtered imaging, these regimes are found to be in good agreement with theoretical shock formation criteria.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

A resistively heated CeB{sub 6} emissive probe

The plasma potential, V{sub p}, is a key quantity in experimental plasma physics. Its spatial gradients directly yield the electrostatic field present. Emissive probes operating under space-charge limited emission conditions float close to V{sub p} even under time-varying conditions. Throughout their long history in plasma physics, they have mostly been constructed with resistively heated tungsten wire filaments. In high density plasmas (>10{sup 12} cm{sup −3}), hexaboride emitters are required because tungsten filaments cannot be heated to sufficient emission without component failure. A resistively heated emissive probe with a cerium hexaboride, CeB{sub 6}, emitter has been developed to work in plasma densities up to 10{sup 13} cm{sup −3}. To show functionality, three spatial profiles of V{sub p} are compared using the emissive probe, a cold floating probe, and a swept probe inside a plasma containing regions with and without current. The swept probe and emissive probe agree well across the profile while the floating cold probe fails in the current carrying region.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Enhanced loss of magnetic-mirror-trapped fast electrons by a shear Alfvén wave

Laboratory observations of enhanced loss of magnetic mirror trapped fast electrons irradiated by a shear Alfvén Wave (SAW) are reported. The experiment is performed in the quiescent after-glow plasma in the Large Plasma Device [Gekelman et al., Rev. Sci. Instrum. 62(12), 2875–2883 (1991)]. A trapped energetic electron population (>100 keV) is generated in a magnetic mirror section (mirror ratio ≈ 2, length = 3.5 m) by an X-mode high power microwave pulse, and forms a hot electron ring due to the grad-B and curvature drift. SAWs of arbitrary polarization are launched externally by a Rotating Magnetic Field source (δB/B{sub 0} ≈ 0.1%, λ{sub ∥} ≈ 9 m). Irradiated by a right-handed circularly polarized SAW, the loss of electrons, in both the radial and the axial direction of the mirror field, is significantly enhanced and is modulated at f{sub Alfvén}. The periodical loss continues even after the termination of the SAW. Experimental observations suggest that a spatial distortion of the ring is formed in the SAW field and creates a collective mode of the hot electron population that degrades its confinement and leads to electron loss from the magnetic mirror. The results could have implications on techniques of radiation belt remediation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗