Weak Axial Magnetic Field Research Articles

Effects of a Preembedded Axial Magnetic Field on the Current Distribution in a Z -Pinch Implosion

The fundamental physics of the magnetic field distribution in a plasma implosion with a preembedded magnetic field is investigated within a gas-puff Z pinch. Time and space resolved spectroscopy of the polarized Zeeman effect, applied for the first time, reveals the impact of a preembedded axial field on the evolution of the current distribution driven by a pulsed-power generator. The measurements show that the azimuthal magnetic field in the imploding plasma, even in the presence of a weak axial magnetic field, is substantially smaller than expected from the ratio of the driving current to the plasma radius. Much of the current flows at large radii through a slowly imploding, low-density plasma. Previously unpredicted observations in higher-power imploding-magnetized-plasma experiments, including recent, unexplained structures observed in the magnetized liner inertial fusion experiment, may be explained by the present discovery. The development of a force-free current configuration is suggested to explain this phenomenon.

Embedded ferromagnetic microwires for monitoring tensile stress in polymeric materials

Considerable efforts have been made to develop testing non-destructive methods for polymer composite materials. We would like to introduce researchers in the field of smart materials to a new method of monitoring internal stresses. The method can be classified as an embedded sensing technique, where the sensing element is a glass-coated ferromagnetic microwire with a specific magnetic anisotropy. With a diameter 10–100μm, the microwire impedance acts as the controlled parameter which is monitored for a weak alternating current (AC) in the MHz range. The microwire impedance becomes stress sensitive in the presence of a weak constant axial bias magnetic field. This external parameter allows the impedance stress sensitivity to be easily tuned. In addition, a local bias field may also allow the reconstruction of stress profile when it is scanned along the microwire. The experimental results are analysed using simple magnetostatic and impedance models.

Thermal diffusion in a binary fluid mixture flows due to a rotating disc of uniform high suction in presence of a weak axial magnetic field

The effect of a weak uniform axial magnetic field on separation of a binary mixture of incompressible viscous thermally and electrically conducting fluids flowing due to a rotating disc of uniform high suction is examined. Neglecting the induced electric field the equations governing the motion, temperature and concentration are solved in cylindrical polar coordinate by expanding the flow parameters as well as the temperature and the concentration in powers of suction parameter. The solution obtained for concentration distribution is plotted against the different axial distances from the disc for various values of non-dimensional parameters. It is found that the temperature gradient, axial magnetic field, Reynolds number, Schmidt number, Prandtl number and suction parameter effect the species separation significantly.

Solute segregation during modified vertical gradient freezing of alloyed compound semiconductor crystals with magnetic and electric fields

Single crystals of gallium–aluminum–antimonide are solidified from a solution of molten gallium–antimonide and aluminum–antimonide. Electromagnetic stirring can be induced in the melt by applying a weak electric field together with a weak axial magnetic field. This paper presents a numerical model which uses a Chebyshev spectral collocation method with a second-order implicit time integration scheme with Gauss–Lobatto collocation points. This investigation models the unsteady motion and solute transport during vertical gradient freezing by submerged heater growth with electromagnetic stirring. The radial homogeneity in the crystal improves as the solute’s concentration increases.

Vertical gradient freezing using submerged heater growth with rotation and with weak magnetic and electric fields

Investigations for the melt growth gallium-antimonide (GaSb) semiconductor crystals are underway at the US Air Force Research Laboratory at Hanscom Air Force Base by the vertical gradient freeze (VGF) method utilizing a submerged heater. Electromagnetic stirring can be induced in the gallium-antimonide melt just above the crystal growth interface by applying a weak radial electric current in the melt together with a weak axial magnetic field. A force convection in the melt can be induced by rotating the heater. This paper uses a Chebyshev spectral collocation method and investigates the effects of heater rotation and electromagnetic stirring on the melt motion during the VGF process using a submerged heater. Clockwise and counterclockwise rotations of the heater augment and opposes the flow induced by the electromagnetic stirring, respectively. The radial velocity along the crystal–melt interface is proportional to both the strength of the electromagnetic stirring and the clockwise rotation of the heater. A relatively low rotation rate in the clockwise direction significantly increases this radial velocity.

Characteristics of Silicon Carbide Etching Using Magnetized Inductively Coupled Plasma

In this study, SiC etching was carried out using fluorine-based magnetized inductively coupled plasmas. The SiC etch rates and etch selectivities of SiC to Cu and Ni were investigated for the purpose of obtaining high etch rates in the application of SiC etching to various optical devices and micro-electromechanical systems (MEMS). Among SF6, CF4 and NF3, SF6 showed the highest SiC etch rates and etch selectivities to Cu and Ni, due to its highest F atomic density and to the formation of nonvolatile fluoride on Cu and Ni. Cu generally showed higher etch selectivity than Ni, possibly due to the easier formation of fluoride in this case. The application of a weak axial magnetic field ranging from 0 to 80 G showed maximum SiC etch rates at 40 G, possibly due to the formation of a resonance mode. When a field of 40 G was applied, the SiC etch rate was increased approximately two times and, in this condition, the F atomic density and ion densities in the plasma were also at a maximum. The highest SiC etch rate obtained in our experiment was 2020 nm/min with an inductive power of 1400 W, a bias voltage of -600 V, a pressure of 10 mTorr of SF6, and a magnetic field of 40 G. The etch selectivity to Ni obtained in this condition was about 40.

Interchange Method in Compressible Magnetized Couette Flow: Magnetorotational and Magnetoconvective Instabilities

We obtain the general forms of the axisymmetric stability criteria in a magnetized compressible Couette flow using an energy variational principle, the so-called interchange or Chandrasekhar s met hod, which we applied successfully in the incompressible case. This formulation accounts for the simultaneous presence of gravity, rotation, a toroidal magnetic field, a weak axial magnetic field, entropy gradients, and density gradients in the initial equilibrium state. The power of the method lies in its simplicity which allows us to derive extremely compact and physically clear expressions for the relevant stability criteria despite the inclusion of so many physical effects. In the implementation of the method, all the applicable conservation laws are explicitly taken into account during the variations of a quantity with dimensions of energy which we call the free energy function. As in the incompressible case, the presence of an axial field invalidates the conservation laws of angular momentum and azimuthal magnetic flux and introduces instead isorotation and axial current conservation along field lines. Our results are therefore markedly different depending on whether an axial magnetic field is present, and generalize in two simple expressions all previously known, partial stability criteria for the appearance of magnetorotational instability. Furthermore, the coupling between magnetic tension and buoyancy and its influence to the dynamics of nonhomoentropic magnetized flows becomes quite clear from our results. In the limits of plane-parallel atmospheres and homoentropic flows, our formulation easily recovers the stability criteria for suppression of convective and Parker instabilities, as well as some related special cases studied over 40 years ago by Newcomb and Tserkovnikov via laborious variational techniques.

Effect of time-varying axial magnetic field on photoresist ashing in an inductively coupled plasma

Time-varying axial magnetic field is added to an inductively coupled plasma. Weak axial magnetic field of about 10–15 G can be obtained by a pair of Helmholtz coils attached to the chamber. This scheme has been applied to etch SiO2 and silylated photoresist, where processing pressures are generally below 7 Pa, and ions are major reaction species. In order to extend this concept to other etching conditions, this method is applied to photoresist ashing, where processing pressure is usually higher than 133 Pa, and downstream oxygen radicals are important species. It is found that 60 Hz of the ac current gives the maximum ash rate and that ash rate is increased from 6.1 to 7.0 μm/min, and uniformity is improved from 8% to 4.5% over an 8 in. wafer. When aluminum baffle is placed between plasma source and reaction chambers, the ash uniformity improves further to 2.0% over a 300 mm wafer. The presence of axial magnetic field increases the plasma density and thus ash rate, and better uniformity comes from redistribution of ions and radicals during the off period in a periodic magnetic field.

Gas-filled electron diode based on a glow discharge

Stable ignition and sustention of a pulsed discharge with a current of up to 180 A and duration of 12 µs at a pressure of 10−1–10−2 Pa are achieved in a glow-discharge plasma cathode with the help of an auxiliary initiating discharge. An electron emission current density of up to 100 A/cm2 and accelerating voltageof 15 kV are obtained in a gas-filled diode based on this type of a plasma cathode. An electron beam witha neutralized space charge can be transported almost without losses in a weak axial magnetic field alonga plasma channel formed due to the gas ionization by the accelerated electrons over a distance of up to 30 cm.

GENERATION OF HIGH-CURRENT LOW-ENERGY ELECTRON BEAMS IN SYSTEMS WITH PLASMA EMITTERS

Generation and transport of high-current electron beams are investigated in gas-filled diodes with plasma emitters based on arc and glow discharges. A space-charge neutralized beam with a current up to 1 kA was produced in a diode with a plasma emitter based on an arc discharge for an accelerating voltage of 15 kV. The beam is constricted from 8 cm down to 1 cm in diameter by a self-magnetic field and is transported through a distance of over 20 cm with an efficiency of 70%. A beam with a current of 80 A and a current density up to 100 A/cm2 was produced in a glow-discharge diode. The beam was transported through a distance of 30 cm in a weak axial magnetic field with induction B = 0.015 T.

Improved etching characteristics of silicon-dioxide by enhanced inductively coupled plasma

Among various types of plasma sources, such as electron cyclotron resonance (ECR), planar inductively coupled plasma (ICP) (or TCP), helical resonator, or surface wave plasma, the ICP type plasma source has been well known as a promising source of uniform high density plasma with the ability of expansion and economical merit. We have proposed and developed a novel plasma process technique, named ‘Enhanced-ICP’, which uses periodic control of weak axial magnetic field to provide better uniformity and a higher etch rate with higher plasma density than normal continuous wave mode (cw)-ICP. The E-ICP process, so far, is considered as a damage-free technique with much improved characteristics. It has already shown better spatial uniformity than 1% and much improved etch rate within a diameter of 10 cm for a photo-resist etch process. The adoption of E-ICP operation for SiO 2 etch with CF 4 gas also provides better etch profile and higher etch rate than cw-ICP process. The optimum frequency of axial magnetic field for CF 4 gas is ∼80 Hz, which is approximately two times that of oxygen plasma. The etch rate of E-ICP operation in the vertical direction is approximately 1.5 times larger than that of a cw-ICP, while side etch rate does not vary noticeably, to provide more vertical etch profile than normal.

Plasma diagnostics of magnetic field assisted ionized magnetron sputtering

The effects of an axially applied axial magnetic field on ionized magnetron sputtering with an internal inductively coupled plasma (ICP) antenna were diagnosed with a Langmuir probe, an impedance probe, and optical emission spectroscopy (OES) to find an operation regime for low energy ionized deposition for low defect density films. A very weak axial magnetic field (Bz) of 20 G was found to reduce plasma potentials by 45% while keeping the ion current density at 80% of the value without Bz. Also based on OES measurements, the plasma density was increased by three times with 5 G of Bz but the coil sputtering was markedly reduced by increasing Bz. However, the radial uniformity of ion saturation current with Bz was about 2.5%–10.4% which was less than the 1.3%–5.5% observed without Bz (std. dev./avg.×100%). However, when the substrate was moved close to ICP coil by 1 cm from 5 cm above the ICP antenna, the radial uniformity was improved to 0.9%–5%, which is thought to be due to a balance between E-mode and H-mode discharge.

Extrap mode confinement in Extrap T1-U

An extensive series of experiments in the Extrap mode has been carried out on Extrap T1-U in an attempt to obtain hot, collisionless discharges. Using no external axial magnetic field, performance has been limited to low temperatures and low currents at high loop voltages kV, leading to energy confinement times of only a few s or Alfvén times. The discharge length is typically of the order of greater than 100 Alfvén times. Imposing a weak axial field at peak current) the temperature could be increased to at toroidal currents . Energy confinement times up to 15 Alfvén times were then obtained. The poor confinement is attributed to the weak axial magnetic field, generating strong instability-induced fluctuations and related radial transport in the core plasma. A turbulence core plasma transport model is suggested, based on theoretical evidence of linear instability in the kinetic regime. Self-generation of axial magnetic field is observed when the magnetic X-points, generated by the plasma current and an external octupole field, approach or pass beyond the wall. Edge transport appears to be dominated by line radiation and heat conduction to the wall along open field lines.

Helicon plasma source excited by a flat spiral coil

We have characterized a new helicon plasma source with an end-launch antenna configuration. Power at 13.56 MHz is coupled via a four-turn flat spiral coil into an m=0 helicon mode with the application of a weak (B≳5 G) axial magnetic field. Plasma parameters were measured with Langmuir probes, and the structure and absorption of the helicon wave fields were determined with magnetic induction probes. Plasma densities of 1011–1012 cm−3 were produced in argon for pressures in the 1–100 mTorr range with a 5–60 G magnetic field. Radio frequency power absorption occurs primarily via collisionless Landau damping for pressures below 5–10 mTorr and collisional damping dominates at higher pressures. Single pass absorption occurs for magnetic fields less than 20 G. This source does not require a separate source chamber, and thus combines the compactness of flat coil inductively coupled sources with the advantages of remote plasma generation found in wave supported sources. The source can be easily optimized for a variety of operating conditions, because the helicon parallel wavelength is not constrained.

Millimeter wave amplification in a free electron laser with a focusing wiggler

A mm wave amplifier experiment on a free electron laser was performed using an intense, mildly relativistic electron beam (0.7–0.9 MeV) with a combination of a focusing planar wiggler and a weak additional axial magnetic field in the so-called ‘‘group 1’’ region. A beam transport ratio of over 80%, through the whole wiggler length, was realized. In an amplification experiment, a spatial growth rate of 56 dB/m was obtained at a frequency of 45 GHz, and a power saturation was observed at a level of 6 MW, where the total gain was estimated to be 52 dB. It was shown that the off-axis component of the beam contributes considerably to the rf amplification in the focusing wiggler.

Solitary resonance in the velocity-selective magnetic-optical activity spectrum of the ^87Rb D_2 line

We report the velocity-selective magnetic-optical activity of a (87)Rb vapor when the counterpropagating pump and probe beams are linearly cross polarized. A semiconductor laser tuned to the D(2) line (780 nm) is used to perform the experiment. When a weak axial magnetic field is applied to the vapor, the magnetic-optical activity spectrum reveals a narrow dominant peak, the solitary resonance, corresponding to the cycling transition 5S((1/2)), F = 2 ?5P(3/2), F = 3. The light-shift origin of this spectrum is discussed.

Frequency shifts of a two-mode He–Ne/CH4laser stabilized by frequency resonances, under the action of a magnetic field in the active medium

Experimental and theoretical studies were made of the influence of a weak axial transverse magnetic field on the frequency shifts of a two-mode He–Ne/CH4 laser (λ = 3.39 μm) stabilized using narrow saturated dispersion resonances in the radiation frequency. In some cases a magnetic field applied to the laser active medium eliminated the asymmetry of the frequency resonances observed when the centers of the gain profile (ω0( + )) and the absorption line (ω0( – )) did not coincide. At ω0( + ) ≈ ω0( – ), the magnetic field does not influence the profile of the frequency resonances or the laser frequency shifts.

High-frequency noise on antennas in plasmas

Fluctuations in plasmas at frequencies near the electron plasma frequency (ωp ) have been measured with in situ wire antennas. Such observations are important to the understanding of basic plasma properties (discrete versus collective effects) and the use of antennas in plasmas. The experiments are performed in a large (1 m diameter ×2 m) pulsed dc discharge and afterglow plasma (ne<1012 cm−3, kTe<5 eV) with a weak axial magnetic field (B0=5 G). The fluctuations are detected from wire antennas (length L≫radius a≳Debye length λD) with a low-noise microwave receiver. The observations reveal three different physical processes that determine the noise spectra: (i) single particle shot noise, (ii) collective oscillations by bounded sheath–plasma resonances, and (iii) noise enhancements by longitudinal plasma waves. The first phenomenon (shot noise) gives rise to broadband noise (ωωp) on both electric and magnetic antennas. In the evanescent regime (ω<ωp) , the shot noise is induced by random electron motions through distances of, at most, a collisionless skin layer (c/ωp) around the antenna. Recalling that electron transit-time effects cause absorption of waves in a collisionless skin layer (known as ‘‘anomalous’’ skin absorption) the present observation of a collisionless skin emission effect can also be understood by the equivalence of blackbody absorption and emission coefficients. The second phenomenon (sheath–plasma resonance) is observed as a narrow resonant enhancement in the shot noise below cutoff (ω<ωp) on electric antennas surrounded by sheaths or dielectrics. The series sheath–plasma resonance, usually identified from reflection/absorption measurements with incident waves, is established here, for the first time, as a feature of thermal emission spectra from antennas. The third phenomenon (plasma dielectric ε(ω,k)→0) produces a broad noise enhancement at ω≳ωp on electric antennas. It can only be observed in the open-loop voltage of dipoles measured with a high-impedance transformer between antenna and 50Ω transmission line. The enhanced noise is electrostatic, randomly polarized, but not significantly enhanced by the presence of energetic electron tails. Existing theories can describe the new observations qualitatively but not quantitatively.

Stability analysis of dynamic Z pinches and theta pinches

The stability of diffuse, dynamic Z pinches and theta pinches is analyzed. Spectra of the growth rates of hydromagnetic instabilities of dynamic pinches are calculated. The acceleration of plasma pinches during compression and expansion strongly modifies the growth of bulk instabilities. Also, even a relatively weak axial magnetic field substantially suppresses instabilities in Z pinches, in agreement with recent experiments. The initial axial field Bz 0 needed for a stable implosion of a pinch of initial radius R0 and average current Ī is Bz 0 ≳(10–30 kG) Ī (MA)/R0(cm). The approximate analytic solutions of the linearized equations are shown to agree well with the exact numerical solutions. The stability analysis demonstrated here for dynamic pinches is broadly applicable to the stability of dynamic plasmas in general.

Rotation of elliptical oscillation in a molecular beam maser in a weak axial magnetic field

The oscillation polarization ellipse in a molecular beam disc resonator maser is shown to be strongly rotated when a weak axial magnetic field is applied. The phenomenon is a microwave analogue of a similar behaviour observed in gas lasers, and is interpreted here in terms of lumped resonator anisotropies.