Coaxial Plasma Gun Research Articles

Neutron angular distribution in a plasma focus obtained using nuclear track detectors.

The dense plasma focus (DPF) is a coaxial plasma gun in which a high-density, high-temperature plasma is obtained in a focused column for a few nanoseconds. When the filling gas is deuterium, neutrons can be obtained from fusion reactions. These are partially due to a beam of deuterons which are accelerated against the background hot plasma by large electric fields originating from plasma instabilities. Due to a beam-target effect, the angular distribution of the neutron emission is anisotropic, peaked in the forward direction along the axis of the gun. The purpose of this work is to illustrate the use of CR-39 nuclear track detectors as a diagnostic tool in the determination of the time-integrated neutron angular distribution. For the case studied in this work, neutron emission is found to have a 70% contribution from isotropic radiation and a 30% contribution from anisotropic radiation.

Laminar plasma dynamos

A new kind of dynamo utilizing flowing laboratory plasmas has been identified. The conversion of plasma kinetic energy to magnetic energy is verified numerically by kinematic dynamo simulations for magnetic Reynolds numbers above 210. As opposed to intrinsically-turbulent liquid-sodium dynamos, the proposed plasma dynamo corresponds to laminar flow topology. Modest plasma parameters (1–20 eV temperatures, 1019–1020 m−3 densities in 0.3–1.0 m scale-lengths driven by velocities on the order of the Alfvén critical ionization velocity) self-consistently satisfy the conditions needed for the magnetic field amplication. Growth rates for plasma dynamos are obtained numerically with different geometry and magnetic Reynolds numbers. Magnetic-field-free coaxial plasma guns can be used to sustain the plasma flow and the dynamo.

Spheromaks, solar prominences, and Alfvén instability of current sheets

Three related efforts underway at Caltech are discussed: experimental studies of spheromak formation, experimental simulation of solar prominences, and Alfven wave instability of current sheets. Spheromak formation has been studied by using a coaxial magnetized plasma gun to inject helicity-bearing plasma into a very large vacuum chamber. The spheromak is formed without a flux conserver and internal λ profiles have been measured. Spheromak-based technology has been used to make laboratory plasmas having the topology and dynamics of solar prominences. The physics of these structures is closely related to spheromaks (low β, force-free, relaxed state equilibrium) but the boundary conditions and symmetry are different. Like spheromaks, the equilibrium involves a balance between hoop forces, pinch forces, and magnetic tension. It is shown theoretically that if a current sheet becomes sufficiently thin (of the order of the ion skin depth or smaller), it becomes kinetically unstable with respect to the emission of Alfven waves and it is proposed that this wave emission is an important aspect of the dynamics of collisionless reconnection.

Very broad vacuum arc ion and plasma sources with extended large area cathodes

Two versions of vacuum arc sources for producing accelerated ion beams and plasma flows are described. The sources are distinguished by use of a dc vacuum arc discharge for plasma generation on the basis of a simple linear or coaxial plasma gun with extended large area (70 300 and 1500 cm2) cathodes and microparticle filtering system. The dc vacuum arc discharge combined with the diode extraction system enables us to use the single source for dc mode of plasma flow formation and repetitively pulsed mode of ion beam generation. Experimental and numerical results are presented on the repetitively pulsed accelerated ion beam formation in the vacuum arc sources with large area cathodes.

Design and Operation of a Passively Switched Repetitive Compact Toroid Plasma Accelerator

The design and operation of a spheromak-like compact toroid (SCT) plasma accelerator is described. As an example application, some principles are presented for using the device as a plasma injector to fuel a tokamak plasma. The device forms and accelerates an SCT plasma. The SCT is a self-contained structure of plasma with embedded poloidal and toroidal magnetic fields and their associated currents that provide plasma confinement and structural integrity. The SCT is formed in a magnetized coaxial plasma gun and then accelerated within coaxial electrodes. The typical mass of an SCT for tokamak fueling is from several tens to several hundreds of micrograms and is accelerated up to a velocity of ~2 × 105 m/s. Larger-mass SCTs can be produced, and higher velocities are possible. This is important for other applications such as space propulsion, X-ray generation, fast-opening plasma switches, and low-temperature high-density plasma simulators.The novel features of the device are as follows: (a) it can be operated in a repetitive mode, (b) the high-energy capacitor bank to form the SCT is switched by initiating breakdown with fast gas injection, (c) the required delay between formation and acceleration is achieved passively with saturable inductors that switch the high-energy accelerator capacitor bank, and (d) a drift section has been added within the toroidal field region to study cross-field propagation prior to tokamak penetration.With the device installed on the Davis Diverted Tokamak (DDT), measurements are taken to study tokamak fueling. Typical rep-rated parameters are as follows: the SCT poloidal magnetic field at the outer electrode = 0.4 T, the stored formation bank energy = 1600 J, and the stored accelerator bank energy = 3600 J. The lower bound on SCT kinetic energy leaving the accelerator = 40 J (inferred from electron line density measurements). Typical SCT velocity is 15 to 20 cm/μs. The maximum rep-rate achieved so far with the device is 0.2 Hz and is currently limited by vacuum pumping capacity. Reliable operation has been demonstrated for 1000 consecutive shots. Higher-energy single shots have also been taken to study SCT propagation through an open guide tube and to study penetration of the SCT into the DDT tokamak vessel with both tokamak plasma discharges and vacuum toroidal magnetic field only.

Scaling studies of spheromak formation and equilibrium

Formation and equilibrium studies have been performed on the Swarthmore Spheromak Experiment (SSX). Spheromaks are formed with a magnetized coaxial plasma gun and equilibrium is established in both small (dsmall=0.16 m) and large (dlarge=3dsmall=0.50 m) copper flux conservers. Using magnetic probe arrays it has been verified that spheromak formation is governed solely by gun physics (in particular the ratio of gun current to flux, μ0Igun/Φgun) and is independent of the flux conserver dimensions. It has also been verified that equilibrium is well described by the force free condition ∇×B=λB (λ=constant), particularly early in decay. Departures from the force-free state are due to current profile effects described by a quadratic function λ=λ(ψ). Force-free SSX spheromaks will be merged to study magnetic reconnection in simple magnetofluid structures.

An investigation of the pulsed plasma for deposition of thin film materials

A new technology for generating pulsed plasma was developed for the deposition of thin film materials according to the concept of the coaxial plasma gun. A model has been developed to describe the generation and acceleration of pulsed plasma. Diagnostic measurements were carried out to understand the basic process of generation of pulsed plasma by using the electric probes and mass spectroscopy. It was found that the plasma density and composition strongly depend on the discharging voltage and inner-electrode materials. The electron density for a titanium inner-electrode is higher than that for a graphite inner-electrode. Typical values for the electron density for the pulsed plasma are n e=10 14 for graphite innerelectrode and 10 15 cm −3 for titanium inner electrode. The electron temperature is T e=65 eV. The velocity of pulsed plasma increases proportionally with discharging voltage. The pulsed plasma mainly consists of two kinds of ions: working gas ions and inner-electrode ions. The ratio between two kinds of ions depends on the inner-electrode materials and the discharging voltage. The ion amounts of the inner-electrode, which has low conductivity, are much lower than that of working gas and have no relationship with discharging voltage. However, if the inner-electrode is made of metal materials with good conductivity, the plasma amounts of two kinds of ions can be compared with each other. The pulsed plasma–material interaction is also discussed in detail.

Experiments on 5 MeV proton beam focusing by plasma lens

The plasma lens is investigated for focusing of a proton beam of energy 5 MeV, current 10 mA, and pulse duration 30 μs, obtained on the accelerator “Ural-5” with radio-frequency quadrupole (RFQ) radial-phase focusing. The parameters of the plasma flow produced by a coaxial plasma gun are the following: plasma density 1011–1015 cm−3, temperature 1–3 eV, velocity 107 cm/s, and time duration 500 μs. The magnetic-field value of a short coil is 500–1000 Oe. The focusing coefficient is ten times the compression at the length of 30 cm obtained. The dependence of ion-beam size upon various parameters of the plasma lens is investigated.

Scaling Relations for the Production and Acceleration of a J × B Driven Rotating Plasma

The density and velocities (rotational and axial) of a rotating plasma driven by the J ×B Lorentz force and their parametric dependence on radial current I and axial magnetic field Bz are investigated in ranges of 3–11 kA and 0–3.6 kG, respectively. Results are compared with those from a simplified single-fluid model that describes the motion of an incompressible plasma with uniform density under the existence of ion production by electron-impact ionization and ion loss by convection to electrodes. In the experiment, argon plasmas are produced by the pulsed discharge (28 µs) of a coaxial plasma gun and are accelerated in both the azimuthal and axial directions by J ×Bz and J ×Bθ forces, respectively, where Bθ is induced by I. Typical values of the plasma density n, the rotational frequency ω and the axial velocity vz are ∼1021 m-3, ≃1 ×106 rad/s and ≃2 ×104 m/s, respectively, at I=9 kA and Bz=3.6 kG. Scaling laws derived from the experiment: n ∝I2Bz0, ω∝I1Bz1n-1 and vz ∝I2Bz0n-1, are found to agree well with predictions of the model. As a result, plasma production and acceleration are found to be strongly related to each other.

Modified Thomson spectrometer for the detection of low energy (<1 keV) high power ion beams

By applying an after acceleration electric field, the detection and registration of the ion species and spectral distribution of low energy ion beams (<1 keV) have been achieved by the Thomson spectrometer with CR-39 nuclear track detector as detection plate. With the use of this modified spectrometer, the low energy and high energy density plasma ion beams generated by a coaxial plasma gun have been detected and investigated. The ion beams were mainly N+ ions and were distributed in the energy range from E∼0.12 to 0.4 keV.

Coating on metallic samples produced by a small energy plasma focus

A surface treatment of metallic samples irradiated by a nitrogen ion beam generated in a co-axial plasma gun is presented. By placing a titanium insert at the end of the inner electrode of the device, a TiN coating was produced on the surface of the sample. X-ray diffraction, scanning electron microscopy, electron probe x-ray micro-analysis and x-ray photoelectron speectroscopy were employed to study the surface of the treated sample. It was found that the main role of the ion beam was to produce a strong pre-heating of the superficial layers of the substrate, thus favouring the diffusion of the impinging particles.

Deposition of diamond-like carbon films on an iron substrate by a pulsed high-density plasma beam at room temperature

Diamond-like carbon (DLC) films were produced on the surface of a pure iron substrate at room temperature using a pulsed high-density plasma beam from a coaxial plasma gun. DLC films have a wide mixing interlayer with substrates, which can improve their adhesion to the substrates. The plasma density ranged between 1014 and 1016 cm−3, the plasma electron temperature was 10–50 eV, the mean plasma speed 30–50 km s−1, and the pulse width about 60 μs. Hardness and thickness measurements, a scratch tester, Raman spectroscopy, Auger electron spectroscopy, and X-ray diffraction were used to characterize the DLC films.

High power density pulsed plasma deposition of titanium carbonitride

Titanium carbonitride [Ti(CN)] films were deposited on the surface of No. 45 steel for modification using the high power density pulsed plasma technique. The pulsed plasma is generated from a coaxial plasma gun and has a high electron temperature of 105–106 K and a high electron density of 1014–1016 cm−3; high translation titanium carbonitride [Ti(CN)] films were deposited at 30–50 km/s and were only 60 μs wide. The substrates were maintained at room temperature during the film deposition, but a stronger adhesion of film to substrate existed. A wide mixing interface between the Ti(CN) films and the substrate formed and resulted in a high adherency of the films to the substrate. The surface microhardness of No. 45 steel increased remarkably due to the formation of titanium carbonitride films.

Pulsed plasma growth of cubic and diamond-like boron nitride thin films at low pressure and room temperature

The formation of boron nitride thin films on NaCl substrates at room temperature using a pulsed high energy density plasma (PHEDP) is investigated. A coaxial plasma gun is built to produce the PHEDP with pulse width of several tens of (μ, power density of 104 W/cm2, electron density of 1014 to 1016cm−3, electron temperature of 10 to 100 eV, and velocity of (3 to 5) × 104 m/s. Mixtures of B2H6 + N2 are used as the gas source of PHEDP. The presence of c-BN and β-BN is confirmed by infrared absorption spectrum (IR), transmission electron microscopy (TEM), and electron diffraction. The structure and features of the deposited films vary with experimental conditions. The mechanism of the synthesis of the BN films by PHEDP is also discussed.

Review of Experimental Investigations on Compact Toroids and Compact Tokamaks Using TS-3 Device

The TS-3 device at the University of Tokyo has been used to produce tree boundary spheromaks or spheromak-like compact toroids. Plasma production is accomplished either by Z-θ discharges or by means of magnetized coaxial plasma guns installed at both ends of the device. The plasmas produced have a minor=major radius of about 15 to 20 cm with a natural decay time of about 30 to 50 μs and a toroidal plasma current of about 30 to 60 kA. A unique feature of TS-3 device is the possession of production regions at both ends of the device, and concequently the ability of producing two adjacent compact toroids which can be merged through magnetic reconnection. Another feature of TS-3 device is the possibility of external application of a toroidal field with the aid of an optional center conductor assembly that can carry an axial current ranging from 0 to ±80 kA. This construction enables us to produce compact toroidal plasmas of various types from reversed field pinch(RFP) to tokamak in terms of the difference in q profile. The variation of both poloidal plasma current and external toroidal field current permits the change in magnetic configuration of merging plasmas, enabling the reconnection angle to continuously vary from about 20° (tokamak merging) through 90° (cohelicity spheromak merging) to 180° (counter-helicity spheromak merging to produce field reversed configurations(FRC)). When the coaxial guns are installed at both ends of the device in place of the center conductor, a center plasma current can be injected to form flux-core spheromaks (or bumpy z-pinches). Novel research subjects that have emerged from TS-3 experiments are; (1) the investigation of three dimensional effects of magnetic reconnection in laboratory plasmas. (2) the formation of FRC plasmas by a counter-helicity spheromak merging, (3) non-OH production and merging of tight aspect ratio tokamaks, (4) the stabilization of tilt motions of tight aspect ratio tokamaks, and (5) the formation and compression (flux amplification) of free-boundary tilt stabilized flux-core spheromaks.

Room temperature synthesis of cubic boron nitride films by pulse high energy density plasma

Cubic boron nitride films have been prepared on the Si(100) oriented substrate at room temperature by a pulse high energy density plasma (PHEDP). The pulse plasma is generated in a coaxial plasma gun. The working gas was a mixture of 50%B 2H 6 + 50%N 2. The structure of boron nitride films strongly depends on the voltages between inner and outer electrodes. The boron nitride films were characterized by scanning electron microscopy, electron diffraction and infrared absorption spectroscopy.

Compression of Compact Toroids in Conical-Coaxial Geometry

Experiments to form, compress, and accelerate compact toroids are described. A 1-m-diam, two-stage, puffed gas, magnetic field embedded coaxial plasma gun is used. Emphasis is on conical compression. Discharges were in the several mega-ampere, few microsecond rise time range. Magnetic probe data suggest that l/(r{center_dot}{delta}r) compression of the toroid field is achieved, consistent with theoretical prediction. The magnetic field pulse and electron density pulse due to the compact toroid correlate in space and time. The compact toroid species is the injected gas species and precedes electrode plasma by several microseconds. The poloidal magnetic field precedes the azimuthal magnetic field. The time of arrival of the axial magnetic field compared with the axial position is consistent with the mean current axial position trajectory obtained from inductance growth. 8 refs., 6 figs.

Current Delivery and Radiation Yield in Plasma Flow Switch-Driven Implosions

Vacuum inductive-store, plasma flow switch-driven implosion experiments have been performed using the Shiva Star capacitor bank (1300 {mu}f, 3 nH, 120 kV, 9.4 MJ). A coaxial plasma gun arrangement is employed to store magnetic energy in the vacuum volume upstream of a dynamic discharge during the 3- to 4-{mu}s rise of current from the capacitor bank. Motion of the discharge off the end of the inner conductor of the gun releases this energy to implode a coaxial cylindrical foil. The implosion loads are 5-cm-radius, 2-cm-long, 200 to 400 {mu}g/cm{sup 2} cylinders of aluminum or aluminized Formvar. With 5 MJ stored initially in the capacitor bank, more than 9 MA are delivered to the implosion load with a rise time of nearly 200 ns. The subsequent implosion results in a radiation output of 0.95 MJ at a power exceeding 5 TW (assuming isotropic emission). Experimental results and related two-dimensional magnetohydrodynamic simulations are discussed. 10 refs., 12 figs.

A study of a coaxial plasma gun

The plasma generated by a coaxial plasma puff-gun is studied using a double Langmuir probe and spectrometer. An analytical solution of the plasma sheath motion is also presented. The charge voltage of the energy-storage capacitors of the gun ranges from 1.5 to 4.0 kV, the plasma electron temperature from 10 to 20 eV, the kinetic energy from 45 to 310 eV and the density from 5*1013to 7*1014cm-3. An approximate solution has been obtained under the assumption of the snowplow model, and the comparison of the predictions of the theory with our experimental results indicates general qualitative agreement.

A new technique for deposition of titanium carbonitride films at room temperature by high energy density pulse plasma

Thin films of titanium carbonitride were deposited by high energy density pulse plasma technique onto a No. 45 steel substrate at room temperature. The pulse plasma was generated from a coaxial plasma gun. The plasma has a high electron temperature, high electron density, high translational speed and a short width of pulse. The mechanical properties of No. 45 steel were improved greatly by deposition titanium carbonitride films.