Plasma instability of a vacuum arc centrifuge

Hole, Matthew; Dallaqua, R.S.; Simpson, S W; Bosco, E. Del

doi:10.1103/physreve.65.046409

Cited by 19 publications

(24 citation statements)

References 30 publications

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“…In this paper, a two-fluid model, which was developed originally for the vacuum arc centrifuge by Hole et al 33,34 to explain oscillations observed in the density and electric potential, is applied to study low frequency oscillations observed in WOMBAT. We show that the equilibrium and perturbed density profiles and the space potential profile from the model and data are consistent.…”

Section: Introductionmentioning

confidence: 99%

A flowing plasma model to describe drift waves in a cylindrical helicon discharge

2011

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A two-fluid model developed originally to describe wave oscillations in the vacuum arc centrifuge, a cylindrical, rapidly rotating, low temperature and confined plasma column, is applied to interpret plasma oscillations in a RF generated linear magnetised plasma (WOMBAT), with similar density and field strength. Compared to typical centrifuge plasmas, WOMBAT plasmas have slower normalised rotation frequency, lower temperature and lower axial velocity. Despite these differences, the two-fluid model provides a consistent description of the WOMBAT plasma configuration and yields qualitative agreement between measured and predicted wave oscillation frequencies with axial field strength. In addition, the radial profile of the density perturbation predicted by this model is consistent with the data. Parameter scans show that the dispersion curve is sensitive to the axial field strength and the electron temperature, and the dependence of oscillation frequency with electron temperature matches the experiment. These results consolidate earlier claims that the density and floating potential oscillations are a resistive drift mode, driven by the density gradient. To our knowledge, this is the first detailed physics model of flowing plasmas in the diffusion region away from the RF source. Possible extensions to the model, including temperature non-uniformity and magnetic field oscillations, are also discussed.

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Section: Introductionmentioning

confidence: 99%

A flowing plasma model to describe drift waves in a cylindrical helicon discharge

2011

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“…Figure 2 also shows that the current in the longitudinal cup is not altered during the high voltage pulses. An oscillation of about 25 kHz persisting throughout the discharge is attributed to drift waves, 12 with no significant negative perturbation being seen beyond the amplitude of this oscillation. The numerical simulations done so far by Rej et al 8 and Kostov et al 9 assumed low density ͑n ϳ 10 9 cm −3 ͒ plasmas with sheath thicknesses of several centimeters, in which the confined electron layer is formed a few millimeters from the electrode's surface.…”

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confidence: 96%

Magnetic suppression of secondary electrons in plasma immersion ion implantation

Tan

Ueda

Dallaqua

et al. 2005

Applied Physics Letters

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In this work, magnetic suppression of secondary electrons in plasma immersion ion implantation is demonstrated experimentally in a vacuum arc system. Secondary electrons emitted normally to a copper sample surface were detected by a Faraday cup, whose signal exhibited large negative spikes coincident with high voltage pulses when aluminum ions of an unmagnetized plasma were implanted. When a 12.5 mT magnetic field parallel to the sample's surface is applied, these spikes are not seen, showing that secondary electrons were magnetically suppressed. Another cup, oriented to detect electrons that flow along the field lines, does not exhibit such negative spikes in either unmagnetized or magnetized plasmas, indicating that a virtual cathode was formed by the trapped secondary electrons. Plasma immersion ion implantation (PIII) is a non-lineof-sight technique that eliminates the necessity of target manipulation. It consists of applying negative high voltage pulses to a target immersed in a plasma medium, from where ions are extracted directly and accelerated into the target, resulting in a homogeneous surface implantation from all sides. Several works have demonstrated the successful metallurgical and semiconductor implantation using this technique.1,2 The use of ion implantation using plasmas made of metallic ion specie 3,4 has also been demonstrated to have numerous applications, when vacuum arc sources proved to be excellent devices for the production of metallic plasmas. 5Secondary electrons emitted due to high energy ion bombardment of material surfaces in PIII can decrease significantly the efficiency of this technique, since a large portion of power is lost into electron energy. Secondary electron emission coefficient can be as large as 20 in metal surfaces, reducing efficiency to values as low as 5%. Furthermore, for electron energies higher than about 40 keV, electron impact with chamber walls produces hazardous x-ray radiation. 6 One technique used to suppress x-ray generation is based on electrostatic confinement of secondary electrons, which are trapped within a metal enclosure biased to the same potential as the target.7 Secondary electrons are repeatedly reflected within this enclosure, and are prevented from impacting the chamber walls. Efficiency could be improved if electrons dissipate their energy into the plasma during reflections. Despite the success in reducing x-ray levels, to our knowledge efficiency improvement was not demonstrated.Suppression of secondary electrons has been proposed by Rej et al. 8 using an externally applied magnetic field, parallel to the target surface. Secondary electrons would be confined by the field forming an electron layer near the surface, which acts as a virtual cathode, reducing the local electric field so that subsequent emitted electrons can be reabsorbed by the surface. Since electrons are free to move in the direction parallel to the magnetic field lines, the virtual cathode will be maintained if it is formed faster than the axial transit time. If this condit...

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“…5, with the highest values occurring at r 10 mm. This motivates comparison with the theoretical eigenmode potential V th r [15]. The finite spatial resolution of the measurements, primarily determined by the Langmuir probe radial extent of 2 mm, was accounted for by convolving V th r with a Gaussian kernel function K r of corresponding width.…”

Section: Prl 99 205004 (2007) P H Y S I C a L R E V I E W L E T T E mentioning

confidence: 99%

“…The most striking feature is a 28 kHz quasiperiodic oscillation, modulated by a slowly varying envelope, whose ÿ3 dB bandwidth is 10 kHz. By comparing field profiles with a linear analysis based on a steady state model in rigid body motion, the oscillation was identified as a density gradient driven drift wave [15], propagating axially and azimuthally. The azimuthal velocity was approximately the same as that of the plasma rotation.…”

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confidence: 99%

“…Figure 1 shows a schematic of the VAC used here, the Plasma Centrifuge at the Laboratório Associado de Plasma of Brazil's Instituto Nacional de Pesquisas Espaciais [14]. Data for this work are from an extensive set of experiments designed to explore properties of the electrostatic oscillations observed in VACs [15]. A Langmuir probe was inserted 150 mm downstream of the anode mesh to measure the floating potential.…”

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confidence: 99%

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Laboratory Evidence for Stochastic Plasma-Wave Growth

Austin

Hole²,

Robinson³

et al. 2007

Phys. Rev. Lett.

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The first laboratory confirmation of stochastic growth theory is reported. Floating potential fluctuations are measured in a vacuum arc centrifuge using a Langmuir probe. Statistical analysis of the energy density reveals a lognormal distribution over roughly 2 orders of magnitude, with a high-field nonlinear cutoff whose spatial dependence is consistent with the predicted eigenmode profile. These results are consistent with stochastic growth and nonlinear saturation of a spatially extended eigenmode, the first evidence for stochastic growth of an extended structure. Statistical plasma theories, such as stochastic growth theory [1,2] (SGT) and self-organized criticality (SOC) [3,4], have enjoyed wide success in describing nonuniform fluctuations in complex systems. In particular, complex initial and/or boundary conditions, sources, and sinks of energy, and nonlinear evolution can lead to a highly nonuniform plasma structure in a state of dynamical equilibrium. In these systems, textbook models of wave growth that assume simply structured initial conditions offer little insight into emergent properties such as marginal stability. In contrast, statistical plasma theories connect the salient features of the wave physics to characteristic distributions of wave properties. For example, in SGT a randomly varying wave growth rate leads to a lognormal distribution, in SOC ''sandpile avalanche'' type behavior yields scalefree power-law distributions, while Gaussians are expected in many contexts. SGT has wide applicability [5,6] including type III solar radio bursts [5], magnetospheric Langmuir, beam and z-mode waves [7], and pulsar emissions [8], plus a range of quasilinear, particle-in-cell, and other simulations [6]. Evidence for other statistical plasma theories, such as SOC, has been found in solar flares and the intermittent turbulence of the Earth's magnetotail [9][10][11] and it has been invoked to describe confinement and transport phenomena in magnetically confined plasmas [12,13].These successes motivate searching for new applications of statistical plasma theories. In this Letter we perform a statistical analysis of fluctuations in a laboratory plasma and find lognormal distributions over a wide range of field strength, the first laboratory evidence for an SGT state. Also, a departure from lognormality at high-field strengths is shown to be consistent with nonlinear saturation of a spatially extended eigenmode structure.The device used to obtain the data is a vacuum arc centrifuge (VAC), a magnetized plasma device originally designed to separate nuclear isotopes. It uses an arc discharge to form a multiply ionized (Z > 1) low temperature (T 6 10 4 K) high density (n i 10 18 m ÿ3 ) Mg plasma. The plasma is situated in an axial magnetic field (B 0:1 T) and is formed by drawing a 1 kA arc between a metal cathode and a wire mesh anode 60 mm distant. As the return current passes through the mesh, resistive losses generate a radial electric field. The interaction of the resultant radial current with the axial magne...

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Plasma instability of a vacuum arc centrifuge

Cited by 19 publications

References 30 publications

A flowing plasma model to describe drift waves in a cylindrical helicon discharge

A flowing plasma model to describe drift waves in a cylindrical helicon discharge

Magnetic suppression of secondary electrons in plasma immersion ion implantation

Laboratory Evidence for Stochastic Plasma-Wave Growth

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