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Digitale Medien

Intense on-axis plasma production and associated relaxation oscillations in a large volume helicon source (1999)

Degeling, A. W. ; Sheridan, T. E. ; Boswell, R. W.

[S.l.] : American Institute of Physics (AIP)

Physics of Plasmas 6 (1999), S. 3664-3673

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ISSN: 1089-7674

Quelle: AIP Digital Archive

Thema: Physik

Notizen: A helicon wave mode with a peak downstream density of greater than 1018 m−3 in argon that exhibits bright ArII emission along the axis has been characterized. The experimental conditions are: Ar gas pressure of 1–5 mTorr, external magnetic field of 70–150 G and radio frequency (rf) power input between 2 and 4 kW a 13.56 MHz using a double half-turn antenna into a source of 9 cm inner radius and 50 cm length that opens into a diffusion chamber 45 cm radius and 200 cm length. Radial profiles of the density in the source and downstream show that plasma production is strongly concentrated on axis. B-dot probe measurements indicate that the wave phase velocity in this discharge mode is between 2 and 2.5×106 m/s, which has been shown previously to be the optimum velocity for resonant wave heating of electrons to increase the ionization rate. An interesting property of the high-density mode is that it is unstable on timescales of a few milliseconds and that a relaxation oscillation occurs between the high- and low-density modes. It is believed that this is driven by the depletion of neutrals in the source region due to ionization and momentum exchange with ions leaving the source. © 1999 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.873624

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2

Digitale Medien

Absolute measurements and modeling of radio frequency electric fields using a retarding field energy analyzer (2000)

Charles, C. ; Degeling, A. W. ; Sheridan, T. E. ; [weitere]

[S.l.] : American Institute of Physics (AIP)

Physics of Plasmas 7 (2000), S. 5232-5241

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ISSN: 1089-7674

Quelle: AIP Digital Archive

Thema: Physik

Notizen: Measurements of the rf electric field have been made along the z axis of a helicon reactor using a retarding field energy analyzer. A fluid code and a simple analytical model have been developed to analyze the ion energy distribution functions, especially in the case of bimodal distributions where the ion transit time through the sheath in front of the analyzer is comparable to the rf period. A generalized curve (and an analytical approximation to that curve) has been developed from the analytical model and confirmed by the self-consistent fluid model for high, low, and intermediate ion transit time, which can be used by experimenters to quickly convert the experimental results (energy peak separation, plasma potential and density, electron temperature), which are related to rf sheath oscillations, to absolute values of the rf electric field. An analysis of the errors involved in the derivation of the field is given. The results agree qualitatively with rf pickup measured with a floating Langmuir probe. © 2000 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.1322557

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3

Digitale Medien

Modeling ionization by helicon waves (1997)

Degeling, A. W. ; Boswell, R. W.

[S.l.] : American Institute of Physics (AIP)

Physics of Plasmas 4 (1997), S. 2748-2755

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ISSN: 1089-7674

Quelle: AIP Digital Archive

Thema: Physik

Notizen: The response of the electron distribution function in one dimension to a traveling wave electric field is modeled for parameters relevant to a low-pressure helicon wave plasma source, and the resulting change in the ionization rate calculated. This is done by calculating the trajectories of individual electrons in a given wave field and assuming no collisions to build up the distribution function as the distance from the antenna is increased. The ionization rate is calculated for argon by considering the ionization cross section and electron flux at a specified position and time relative to the left-hand boundary, where the distribution function is assumed to be Maxwellian and the wave travels to the right. The simulation shows pulses in the ionization rate that move away from the antenna at the phase velocity of the wave, demonstrating the effect of resonant electrons trapped in the wave's frame of reference. It is found that the ionization rate is highest when the phase velocity of the wave is between 2 and 3×106 m/s, where the electrons interacting strongly with the wave (i.e., electrons with velocities inside the wave's "trapping width") have initial energies just below the ionization threshold. Results from the model are compared with experimental data and show reasonable qualitative agreement. © 1997 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.872143

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4

Digitale Medien

Plasma production from helicon waves (1996)

Degeling, A. W. ; Jung, C. O. ; Boswell, R. W. ; [weitere]

[S.l.] : American Institute of Physics (AIP)

Physics of Plasmas 3 (1996), S. 2788-2796

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ISSN: 1089-7674

Quelle: AIP Digital Archive

Thema: Physik

Notizen: Experimental measurements taken in a large magnetoplasma show that a simple double half-turn antenna will excite m=1 helicon waves with wavelengths from 10–60 cm. Increased ionization in the center of the downstream plasma is measured when the axial wavelength of the helicon wave becomes less than the characteristic length of the system, typically 50–100 cm. A sharp maximum in the plasma density downstream from the source is measured for a magnetic field of 50 G, where the helicon wave phase velocity is about 3×108 cm s−1. Transport of energy away from the source to the downstream region must occur to create the hot electrons needed for the increased ionization. A simple model shows that electrons in a Maxwellian distribution most likely to ionize for these experimental conditions also have a velocity of around 3×108 cm s−1. This strong correlation suggests that the helicon wave is trapping electrons in the Maxwellian distribution with velocities somewhat slower than the wave and accelerating them into a quasibeam with velocity somewhat faster than the wave. The nonlinear increase in central density downstream as the power is increased for helicon waves with phase velocities close to the optimum electron velocity for ionization lends support to this idea. © 1996 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.871712

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5

Digitale Medien

Model for relaxation oscillations in a helicon discharge (1999)

Degeling, A. W. ; Sheridan, T. E. ; Boswell, R. W.

[S.l.] : American Institute of Physics (AIP)

Physics of Plasmas 6 (1999), S. 1641-1648

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ISSN: 1089-7674

Quelle: AIP Digital Archive

Thema: Physik

Notizen: Relaxation oscillations observed in the large-volume, helicon plasma experiment WOMBAT (Waves on Magnetized Beams and Turbulence) [R. W. Boswell and R. K. Porteous, Appl. Phys. Lett. 50, 1130 (1987)] are modeled. These oscillations have a period of several milliseconds and have been identified as transitions between a low-density, inductive discharge and a high-density, helicon-wave discharge. In the model, it is assumed that the mode transitions are triggered by variations in the neutral density in the source region. The neutral density decreases due to ionization augmented by ion pumping and increases due to refilling of the source chamber from the much larger diffusion chamber. The system is modeled using two, coupled, nonlinear, ordinary differential equations that describe the neutral and plasma densities in the source chamber. Ionization by inductively-coupled fields and ionization due to electrons accelerated by helicon waves with phase velocities near the threshold electron velocity for ionization are considered. The model is found to reproduce experimentally measured variations of the plasma density and helicon wave phase velocity with rf power, neutral pressure and magnetic field. The negative impedance needed for the existence of a relaxation oscillation is provided by the helicon-wave coupling mechanism. © 1999 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.873419

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