Cryogenic Plasma Research Articles

Very slowly decaying afterglow plasma in cryogenic helium gas

A late afterglow period of cryogenic plasma with density less than 10/sup 8/ cm/sup -3/ in 4.2-K helium gas is measured, where the plasma is lost mainly through ambipolar diffusion. We fabricate a large stainless-steel cylindrical discharge vessel of 16.6 cm in diameter and 8.2 cm in length. The vessel is a TE/sub 011/ mode cavity with resonant frequency 2.85 GHz and Q-value larger than 3000. The diffusion length of the cavity, 2.1 cm, is much larger than those previously reported on cryogenic plasmas. A high-voltage pulse of 15 kV, 600 A with duration 2.5 /spl mu/ s is applied between tungsten needle electrodes to produce a plasma repeatedly. Gas pressure is varied from 0.08 to 1.1 torr. Temporal changes in plasma density and electron-atom momentum transfer collision frequency are measured by an improved method of microwave interferometer including the cavity. The plasma decay with time constant on the order of 1 s is observed. We can see very slowly fading fluorescent light with our naked eyes. The decay time is increased, if gas temperature is cooled below 4.2 K.

Temperature influence on etching deep holes with SF6/O2 cryogenic plasma

A cryogenic SF6/O2 plasma process has been used to investigate the etching of deep holes in silicon wafers. The influence of crystallographic and aspect ratio dependence of the etch rate on the holes profile have been explored. It was found that wafer temperature, during the etching process, played a crucial role in controlling the anisotropy and deteriorative faceting due to crystal orientation dependent etching. High anisotropy and switching of the process to crystallographic independent etching was achieved by controlling the temperature. Aspect ratio dependent etching was also identified as a serious limitation for the required homogeneity in etched depth.

Proposal of Cryogenic Plasmas in Liquid Helium II

Novel possibilities to create cryogenic plasma states in liquid helium II below 2.16 K at 1 atm are proposed. Below 0.4 K, a mixture of positive and negative ions in the superfluid can be collision-free in the sense that the momentum relaxation collision frequencies between ions and neutral atoms, impurity atoms and excitons such as phonons, rotons and vortices are less than the plasma frequencies. In particular, positive ions in the plasmas are analogous to free electrons in conventional gaseous discharges at low pressures, although the plasma parameters are quite different from each other. A possible cryogenic plasma is proposed to be produced as a transient state. Preliminary experiments for ion mixture left after pulsed discharges in liquid helium II are demonstrated.

Development of a pure cryogenic positron plasma using a LINAC positron source

The development of a high density cryogenic pure positron plasma trap at the LLNL positron beam facility opens new possibilities for antihydrogen research. We discuss a planned measurement of the three-body collisional recombination rate in magnetized plasmas, a possible antihydrogen atomic beam experiment, and other applications of pure positron plasmas.

On anomalous vacuum-UV emission by a cryogenic plasma in a krypton-xenon mixture

The anomalous emission (λ~146 nm) from a Kr–Xe mixture observed in a previous study is interpreted as emission in the course of optical collisions. A kinematic model is constructed which explains the dependence of the emission intensity on the composition and temperature of the gas mixture.

Anomalous amplification of vacuum ultraviolet radiation from a cryogenic plasma in a krypton–xenon mixture

Experimental investigations were made of the vacuum ultraviolet spectrum of a discharge in a cryogenic plasma formed from a mixture of krypton and xenon. An increase in the continuum intensity was observed when the discharge in krypton was cooled. Anomalous amplification of the vacuum ultraviolet continuum of krypton was observed when a small amount of xenon was added.

Guiding center atoms: Three-body recombination in a strongly magnetized plasma

The three-body recombination rate is calculated for an ion introduced into a magnetically confined, weakly correlated, and cryogenic pure electron plasma. The plasma is strongly magnetized in the sense that the cyclotron radius for an electron rce=(kBTe/me)1/2/Ωce is small compared to the classical distance of closest approach b=e2/kBTe, where Te is the electron temperature and Ωce=eB/mec is the electron cyclotron frequency. Since the recombination rate is controlled by a kinetic bottleneck a few kBTe below ionization, the rate may be determined by considering only the initial cascade through states of electron-ion pairs with separation of order b. These pairs may be described as guiding center atoms since the dynamics is classical and treatable with the guiding center drift approximation. In this paper, an ensemble of plasmas characterized by guiding center electrons and stationary ions is described with the BBGKY hierarchy. Under the assumption of weak electron correlation, the hierarchy is reduced to a master equation. Insight to the physics of the recombination process is obtained from the variational theory of reaction rates and from an approximate Fokker–Planck analysis. The master equation is solved numerically using a Monte Carlo simulation, and the recombination rate is determined to be 0.070(10)n2eveb5 per ion, where ne is the electron density and ve=(kBTe/me)1/2 is the thermal velocity. Also determined by the numerical simulation is the transient evolution of the distribution function from a depleted potential well about the ion to its steady state.

Elastic electron-neutral interaction measurements in neon at ultralow energies

The cross section ${\ensuremath{\sigma}}_{\mathrm{MT}}$ for the elastic momentum transfer of electrons with neon atoms in the electron energy range $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\ensuremath{\le}u(\mathrm{eV})\ensuremath{\le}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}1}$ has been deduced from measurements of the microwave absorptivity of a transient cryogenic afterglow plasma. A least-squares fit of our experimental data by the modified effective range formula gives a zero-energy scattering length of $(0.24\ifmmode\pm\else\textpm\fi{}0.015){a}_{0}$, where ${a}_{0}$ is the Bohr radius.

Elastic Electron-Neutral-Atom Interaction Measurements in Helium at Ultralow Energies

The cross section ${\ensuremath{\sigma}}_{\mathrm{MT}}$ for the elastic momentum transfer of electrons with helium atoms at ultralow energies has been deduced from measurements of the microwave absorptivity of a transient cryogenic afterglow plasma. The electrons were selectively heated by an adjustable-microwave electromagnetic field. This cross section can be approximated as a function of the incident electron kinetic energy ${u}_{e}$ between 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ and ${10}^{\ensuremath{-}1}$ eV by ${\ensuremath{\sigma}}_{\mathrm{MT}}({\mathrm{cm}}^{2})=5.15\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}{[{u}_{e}\frac{(\mathrm{eV})}{2}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}]}^{\ensuremath{-}0.02}$.