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Register a new userHelicon plasma in a magnetic shuttle
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The definition of magnetic shuttle is introduced to describe the magnetic space enclosed by two tandem magnetic mirrors with the same field direction and high mirror ratio. Helicon plasma immersed in such a magnetic shuttle which can provide the confinement of charged particles is modeled using an electromagnetic solver. The perpendicular structure of wave field along this shuttle is given in terms of stream vector plots, showing significant change from midplane to ending throats, and the vector field rotates and forms a circular layer that separating plasma column radially into core and edge regions near the throats. The influences of driving frequency, plasma density and field strength on the wave field and power absorption are computed in detail. It is found that the wave magnitude and power absorption decrease for increased driving frequency and reduced field strength, and maximize around a certain level of plasma density. The axial standing-wave feature always exists, due to the interference between forward and reflected waves from ending magnetic mirrors, while the radial wave field structure largely stays the same. Distributions of wave energy density and power absorption density all show shrinking feature from midplane to ending throats, which is consistent with the nature of helicon mode that propagating along field lines. Theoretical analysis based on a simple magnetic shuttle and the governing equation of helicon waves shows consistency with computed results and previous studies.
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The ionization efficiency of helicon plasma discharge is explored by changing the low axial magnetic field gradients near the helicon antenna. The highest plasma density is found for a most possible diverging field near the antenna by keeping the other operating condition constant. Measurement of axial wave number together with estimated radial wavenumber suggests the oblique mode propagation of helicon wave along the resonance cone boundary. Propagation of helicon wave near the resonance cone angle boundary can excite electrostatic fluctuations which subsequently can deposit energy in the plasma. This process has been shown to be responsible for peaking in density in low field helicon discharges, where the helicon wave propagates at an angle with respect to the applied uniform magnetic field. The increased efficiency can be explained on the basis of multiple resonances for multimode excitation by the helicon antenna due to the availability of a broad range of magnetic field values in the near field of the antenna when a diverging magnetic field is applied in the source.
Measurement of radial density profile in both the source and expansion chambers of a helicon plasma device have revealed that it is always centrally peaked in the source chamber, whereas in the expansion chamber near the diverging magnetic field it becomes hollow above a critical value of the magnetic field. This value corresponds to that above which both electrons and ions become magnetized. The temperature profile is always peaked off- axis and tail electrons are found at the peak location in both the source and expansion chambers. Rotation of the tail electrons in the azimuthal direction in the expansion chamber due to gradient-B drift produces more ionization off-axis and creates a hollow density profile; however, if the ions are not magnetized, the additional ionization does not cause hollowness.
Large volume helicon plasma sources are of particular interest for large scale semiconductor processing, high power plasma propulsion and recently plasma-material interaction under fusion conditions. This work is devoted to studying the coupling of four typical RF antennas to helicon plasma with infinite length and diameter of $0.5$~m, and exploring its frequency dependence in the range of $13.56-70$~MHz for coupling optimization. It is found that loop antenna is more efficient than half helix, Boswell and Nagoya III antennas for power absorption; radially parabolic density profile overwhelms Gaussian density profile in terms of antenna coupling for low-density plasma, but the superiority reverses for high-density plasma. Increasing the driving frequency results in power absorption more near plasma edge, but the overall power absorption increases with frequency. Perpendicular stream plots of wave magnetic field, wave electric field and perturbed current are also presented. This work can serve as an important reference for the experimental design of large volume helicon plasma source with high RF power.
Radio Frequency (RF) driven helicon plasma sources are commonly used for their ability to produce high-density argon plasmas (n > 10^19/m^3) at relatively moderate powers (typical RF power < 2 kW). Typical electron temperatures are < 10 eV and typical ion temperatures are < 0.6 eV. A newly designed helicon antenna assembly (with concentric, double-layered, fully liquid-cooled RF-transparent windows) operates in steady-state at RF powers up to 10 kW. We report on the dependence of argon plasma density, electron temperature and ion temperature on RF power. At 10 kW, ion temperatures > 2 eV in argon plasmas are measured with laser induced fluorescence, which is consistent with a simple volume averaged 0-D power balance model. 1-D Monte Carlo simulations of the neutral density profile for these plasma conditions show strong neutral depletion near the core and predict neutral temperatures well above room temperatures. The plasmas created in this high-power helicon source (when light ions are employed) are ideally suited for fusion divertor plasma-material interaction studies and negative ion production for neutral beams.
We measure the expansion of an ultracold plasma across the field lines of a uniform magnetic field. We image the ion distribution by extracting the ions with a high voltage pulse onto a position-sensitive detector. Early in the lifetime of the plasma ($< 20$ $mu$s), the size of the image is dominated by the time-of-flight Coulomb explosion of the dense ion cloud. For later times, we measure the 2-D Gaussian width of the ion image, obtaining the transverse expansion velocity as a function of magnetic field (up to 70 G). We observe that the expansion velocity scales as B$^{-1/2}$, explained by a nonlinear ambipolar diffusion model with anisotropic diffusion in two different directions.
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