Multicusp Magnetic Field Research Articles

Taylor-Couette flow of unmagnetized plasma

Differentially rotating flows of unmagnetized, highly conducting plasmas have been created in the Plasma Couette Experiment. Previously, hot-cathodes have been used to control plasma rotation by a stirring technique [C. Collins et al., Phys. Rev. Lett. 108, 115001 (2012)] on the outer cylindrical boundary—these plasmas were nearly rigid rotors, modified only by the presence of a neutral particle drag. Experiments have now been extended to include stirring from an inner boundary, allowing for generalized circular Couette flow and opening a path for both hydrodynamic and magnetohydrodynamic experiments, as well as fundamental studies of plasma viscosity. Plasma is confined in a cylindrical, axisymmetric, multicusp magnetic field, with Te < 10 eV, Ti < 1 eV, and ne<1011 cm−3. Azimuthal flows (up to 12 km/s, M = V∕cs ∼ 0.7) are driven by edge J × B torques in helium, neon, argon, and xenon plasmas, and the experiment has already achieved Rm ∼ 65 and Pm∼0.2−12. We present measurements of a self-consistent, rotation-induced, species-dependent radial electric field, which acts together with pressure gradient to provide the centripetal acceleration for the ions. The maximum flow speeds scale with the Alfvén critical ionization velocity, which occurs in partially ionized plasma. A hydrodynamic stability analysis in the context of the experimental geometry and achievable parameters is also explored.

Characteristics of the compact plasma device AIT‐PID with multicusp magnetic confinement

AbstractA unique plasma‐generation device called the “Aichi Institute of Technology‐Plasma Irradiation Device” has been developed for plasma–wall interaction (PWI) studies toward realization of a fusion reactor. It has power‐saving characteristics as well as compactness due to the employment of a multicusp magnetic field configuration instead of a strong axial magnetic field. Helium as well as argon plasmas are generated, showing an outstanding property of production of the hot‐electron component which can be controlled by changing the working gas pressure. Utilizing such outstanding properties, some typical studies on PWI are introduced. An azimuthally asymmetric confinement depending on the direction of magnetic field lines, particularly for energetic electrons, has been found. Some discussions on the physical mechanisms are given. IEEJ Trans 2011 DOI: 10.1002/tee.21801

Transition from interpulse to afterglow plasmas driven by repetitive short-pulse microwaves in a multicusp magnetic field

In the power-off phase, plasmas generated by repetitive short-pulse microwaves in a multicusp magnetic field show a transitive nature from interpulse to afterglow as a function of pulse duration tw = 20–200 μs. The ionized medium can be driven from a highly non equilibrium to an equilibrium state inside the pulses, thereby dictating the behavior of the plasma in the power-off phase. Compared to afterglows, interpulse plasmas observed for tw < 50 μs are characterized by a quasi-steady-state in electron density that persists for ∼ 20–40 μs even after the end of the pulse and has a relatively slower decay rate (∼ 4.3 × 104 s−1) of the electron temperature, as corroborated by optical measurements. The associated electron energy probability function indicates depletion in low energy electrons which appear at higher energies just after the end of the pulse. The transition occurs at tw ∼ 50 μs as confirmed by time evolution of integrated electron numbers densities obtained from the distribution function.

Stirring Unmagnetized Plasma

A new concept for spinning unmagnetized plasma is demonstrated experimentally. Plasma is confined by an axisymmetric multicusp magnetic field and biased cathodes are used to drive currents and impart a torque in the magnetized edge. Measurements show that flow viscously couples momentum from the magnetized edge (where the plasma viscosity is small) into the unmagnetized core (where the viscosity is large) and that the core rotates as a solid body. To be effective, collisional viscosity must overcome the ion-neutral drag due to charge-exchange collisions.

Development of low-energy and high-current-density ion beam system

A low-energy ion beam system operating at a dc voltage of less than 300 V was developed using an ion source with a multicusp magnetic field. A high-current-density ion beam of 6.9 mA∕cm(2) was successfully extracted at the electrode. The beam extraction characteristics for flat and concave electrodes were compared. In the case of a concave electrode with a designed focal length of 350 mm, it was observed that the beam profile was sharper than that obtained using a flat electrode.

3D simulation of the electron energy distribution in negative hydrogen ion source

In this paper, we calculate permanent magnet theoretically for a multicusp ion source Coulomb collision between electrons is treated with the "binary collision" model and collisions between the electrons and hydrogen species are treated with the "null-collision" method. A 3D PIC-MCC simulation algorithm is developed, and based on this algorithm the electron deposition process in multicusp ion source is simulated, and the multicusp magnetic field effects on the electron energy distribution and spatial distribution are analyzed. The results show that the spatial non-uniformity of electron distribution comes from high energy electron B× ▽B drift in the filter field.

Trapping of Free Electrons in Multipole System

In this paper, the effective Parameters in the confinement and trapping of fast electrons in plasma source Such as; plasma pressure, wall material of plasma chamber and magnetic mirror rate have been investigated with using Comsol & Geant4 code. The calculations are shown that the Multicusp magnetic field was effective the pressure less than 5 mTor, and the confinement effect becomes stronger with decreasing pressure. It is equivalent to a higher yield of output ions of plasma source. The number of fast electrons trapped in the magnetic field increases with increasing magnetic field intensity and using aluminum for wall material. Optimum conditions of confinement plasma, leading to increased the hot electron density, and ionization efficiency is increased. The results of investigations have demonstrated good correspondence with theoretical calculations, therefore there is the adequacy of the developed approach and the possibility to build more effective source ion on this basis.

Anisotropy induced wave birefringence in bounded supercritical plasma confined in a multicusp magnetic field

Laboratory observation of rotation of the polarization axis (θc∼20°–40° with respect to vacuum) of a penetrating electromagnetic wave through a bounded supercritical plasma (plasma frequency ωp>wave frequency ω), confined in a multicusp magnetic field is reported. Birefringence of the radial and polar wave electric field components (Er and Eθ) has been identified as the cause for the rotation, similar to a magneto-optic medium, however, with distinct differences owing to the presence of wave induced resonances. Numerical simulation results obtained by solving the Maxwell’s equations by incorporating the plasma and magnetostatic field inhomogeneities within a conducting boundary shows a reasonable agreement with the experimental results.

Penetration and screening of perpendicularly launched electromagnetic waves through bounded supercritical plasma confined in multicusp magnetic field

The question of electromagnetic wave penetration and screening by a bounded supercritical (ωp>ω with ωp and ω being the electron-plasma and wave frequencies, respectively) plasma confined in a minimum B multicusp field, for waves launched in the k⊥Bo mode, is addressed through experiments and numerical simulations. The scale length of radial plasma nonuniformity (|ne/(∂ne/∂r)|) and magnetostatic field (Bo) inhomogeneity (|Bo/(∂Bo/∂r)|) are much smaller than the free space (λo) and guided wavelengths (λg). Contrary to predictions of plane wave dispersion theory and the Clemow–Mullaly–Allis (CMA) diagram, for a bounded plasma a finite propagation occurs through the central plasma regions where αp2=ωp2/ω2≥1 and βc2=ωce2/ω2⪡1(∼10−4), with ωce being the electron cyclotron frequency. Wave screening, as predicted by the plane wave model, does not remain valid due to phase mixing and superposition of reflected waves from the conducting boundary, leading to the formation of electromagnetic standing wave modes. The waves are found to satisfy a modified upper hybrid resonance (UHR) relation in the minimum B field and are damped at the local electron cyclotron resonance (ECR) location.

Characterization of compact ICP ion source for focused ion beam applications

A compact, high brightness 13.56MHz inductively coupled plasma ion source without any axial or radial multicusp magnetic fields is designed for the production of a focused ion beam. Argon ion current of density more than 30mA/cm2 at 4kV potential is extracted from this ion source and is characterized by measuring the ion energy spread and brightness. Ion energy spread is measured by a variable-focusing retarding field energy analyzer that minimizes the errors due to divergence of ion beam inside the analyzer. Brightness of the ion beam is determined from the emittance measured by a fully automated and locally developed electrostatic sweep scanner. By optimizing various ion source parameters such as RF power, gas pressure and Faraday shield, ion beams with energy spread of less than 5eV and brightness of 7100Am−2sr−1eV−1 have been produced. Here, we briefly report the details of the ion source, measurement and optimization of energy spread and brightness of the ion beam.

Negative Ion Confinement in the Multicusp Ion Source

To optimize the negative ion source and generate intense beams of negative ions, understanding of transport properties of both electrons and negative ions is indispensable. Transport process of negative hydrogen ions (H−) in a multicusp H− source, has been simulated by three-dimensional Femlab simulation software. Multipolar plasma confinement is known to result in enhanced plasma density, homogeneous plasma of a large volume, and quiescent plasmas. The effect of plasma confinement by applying multi-polar magnetic field was investigated. Results are obtained for ten different configurations of permanent magnet and discussed. Full line cusps are found to give optimum plasma density. Negative ions created on the sidewall hardly can reach the center of the source due to trapping by the multicusp magnetic field. As a result, H− ions created on the sidewall do not have a significant effect on the H− current.

Subcutoff microwave driven plasma ion sources for multielemental focused ion beam systems

A compact microwave driven plasma ion source for focused ion beam applications has been developed. Several gas species have been experimented including argon, krypton, and hydrogen. The plasma, confined by a minimum B multicusp magnetic field, has good radial and axial uniformity. The octupole multicusp configuration shows a superior performance in terms of plasma density (~1.3 x 10(11) cm(-3)) and electron temperature (7-15 eV) at a power density of 5-10 Wcm(2). Ion current densities ranging from a few hundreds to over 1000 mA/cm(2) have been obtained with different plasma electrode apertures. The ion source will be combined with electrostatic Einzel lenses and should be capable of producing multielemental focused ion beams for nanostructuring and implantations. The initial simulation results for the focused beams have been presented.

Development of an all-permanent-magnet microwave ion source equipped with multicusp magnetic fields for high current proton beam production

An all-permanent-magnet (APM) microwave hydrogen ion source was developed to reduce the size and to simplify structure of a conventional solenoid coil microwave ion source developed for reliability improvement of high current proton linac application systems. The difficulty in developing the APM source was sensitive dependence of the source performance on axial magnetic field in the microwave discharge chamber. It was difficult to produce high current proton beam stably without precise tuning of the magnetic field using solenoid coils. We lowered the sensitivity using multicusp magnetic fields for plasma confinement at the discharge chamber sidewall of the source. This enabled stable high current proton beam production with the APM microwave ion source with no tuning coil. The water cooling and the power supply for the coils are not necessary for the APM source, which leads to better reliability and system simplification. The outer diameter of the APM source was around 300 mm, which was 20% lower than the coil source. The APM source produced a maximum hydrogen ion beam current of 65 mA (high current density of 330 mA/cm(2), proton ratio of 87%, and beam energy of 30 keV) with a 5 mm diameter extraction aperture, pulse width of 400 micros, and 20 Hz repetition rate at 1.3 kW microwave power. This performance is almost the same as the best performances of the conventional coil sources. The extracted ion beams were focused with electrostatic five-grid lens to match beam to acceptance of radio-frequency quadrupole linacs. The maximum focused beam current through the orifice (5 mm radius) and the lens was 36 mA and the 90% focused beam half-width was 1-2 mm.

A gas-ion source with a grid-stabilized plasma cathode

A two-stage source of a broad beam of gas ions is described. The source contains a grid-stabilized plasma cathode and an anode stage with a multicusp magnetic field. The emission current of the plasma cathode (which is based on a glow discharge with a hollow cathode) is controlled between 0.1 and 1 A. The voltage that is applied to a bipolar diode between its cathode grid and anode plasma and determines the energy of fast electrons ranges from 50 to 200 V. The operating pressure of the argon in the anode stage is 4 × 10−3−1 × 10−1 Pa. A beam of argon ions having an energy of up to 5 keV and a current of >100 mA is formed by a two-electrode ion-optical system with a working area of ∼50 cm2.

Reduction of the electrostatic coupling in a large-area internal inductively coupled plasma source using a multicusp magnetic field

A large area (1020mm×830mm) inductively coupled plasma (ICP) source has been developed using an internal-type linear antenna with permanent magnets forming a multicusp magnetic field. The large rf antenna voltages, which cause the electrostatic coupling between the antenna and the plasma in a large area internal-type linear-antenna ICP source, were decreased significantly by applying the magnetic field near and parallel to the antenna. Through the application of the magnetic field, an approximately 20% higher plasma density, with a value of close to 1.0×1011cm−3 at a rf power of 2000W, and about three times higher photoresist etch rates were observed, while maintaining the plasma nonuniformity at less than 9%.

Glow-discharge-driven bucket ion source

A two-stage source of broad beams of gaseous ions employing a glow-discharge plasma cathode and an anode chamber with a peripheral multicusp magnetic field has been studied. A grid, whose potential approached the potential of the screen grid of the ion source, separated the cathode and anode stages. The effect of the energy (0–300 eV), the current of injected electrons (0.2–0.8 A), and the gas pressure (argon) on the plasma parameters and the efficiency of ions extraction from the plasma have been analyzed. The current density of the ions extracted from the magnetic-field-free plasma had a nearly homogeneous distribution (∼10%). The beam of 3 keV argon ions was generated using a two-electrode ion optical system with the area of ∼50 cm2. The beam current was 60 mA at the electron current of the plasma cathode equal to 0.2 A.

A multiaperture electron filter for volume-type H− ion source

Negative light ion beams have been used widely in many applications over the past decades. One of the most popular methods of producing H− and D− beams is the so-called volume production, in which a plasma is contained in a multicusp magnetic field. The beam is extracted using an electric field applied in the extraction gap. Accompanying the negative ions is up to 100 times greater number of electrons, which have to be removed from the beam. This is usually done by placing a transverse magnetic field in the extraction region. With this method the electrons can be quite effectively removed from the beam. The problem is that we dump a high intensity, high energy electron beam in the extraction electrode and dissipate a lot of power in the process. The negative ion beam is also deflected in this strong magnetic field which has to be corrected either by tilting the source compared to the extraction or by adding an opposite transverse magnetic field. In the latter method the beam is laterally displaced from the original axis. In the Plasma and Ion Source Technology Group at the Lawrence Berkeley National Laboratory, we are developing a multi-aperture extraction in which the electrons are removed from the beam before they are accelerated to high energy with the use of a honeycomb-structured electron filter and a weak magnetic field. This way the power dissipated by the electrons and the ion beam deflection are minimized. The initial results of these experiments will be described in this presentation.

Monte Carlo simulation of negative ion transport in the negative ion source (Camembert III)

Transport process of negative hydrogen ions (H−) in a large hybrid multicusp H− source, “Camembert III,” has been analyzed by a three-dimensional Monte Carlo simulation code. The realistic geometry and multicusp magnetic-field configuration are taken into account. Various important destruction processes of H− and Coulomb collision with background plasma are also included in the model. Both the volume- and surface-produced H− ion trajectories are followed. For volume-produced H− ions, most of the H− ions can reach the wall in the low-pressure case (1 mTorr), while in the high-pressure case (3 mTorr) most of the H− ions are destructed by volume loss reactions before reaching the wall. This shows that the wall loss is significant at low pressure as in the experiments. For surface-produced H− ions, the influence of its birthplace on the H− current is studied. Negative ions created on the sidewall hardly can reach the center of the source due to trapping by the multicusp magnetic field. As a result, H− ions created on the sidewall do not have a significant effect on the H− current.

Production of pulsed microwave plasma in a tube with a radius below the cut-off value

A plasma is produced by pulsed microwaves in a circular tube with a conducting wall and a radius below the cut-off value for the fundamental circular waveguide mode (TE11). The discharge tube is surrounded with a multicusp magnetic field constructed with permanent magnets which provide a minimum-B configuration. Short pulse (0.05-1.0 µs) microwaves of 3 GHz and 60-100 kW peak power with a repetition frequency of 10-500 Hz are launched into the narrow tube with the propagation vector perpendicular to the cusped magnetic field in a pressure range of 10-2-10 Torr. It has been found that, in addition to occurring at the entrance, a plasma is produced at a location of about one-third length from the tube exit. Plasmas generated at the two locations diffuse mostly along the tube axis with the radial diffusion strongly suppressed by the multicusp field. The density uniformity is better at lower pressures. The spatio-temporal evolution of electrons and ions shows a density growth after the end of a microwave pulse for a few to tens of microseconds depending on the axial position, followed by decay. The profiles have been explained by numerical simulation based upon a model that charged particles are driven by electrostatic and ponderomotive forces and diffuse along the longitudinal direction. The interpulse regime has an electron temperature of about 10 eV. This indicates that the plasma is still active between the pulses, in distinct contrast with the usual afterglows.

Global model of a radiofrequency H2 plasma in DENISE

A global model of a radiofrequency (rf) inductively coupled H2 plasma discharge in the Deuterium Negative Ion Source Experiment (DENISE) has been developed using the numerical code `Global Model Solver' (GMS). The volume-averaged energy and particle balance equations, along with the quasi-neutrality condition, are numerically solved to determine the averaged densities of all the species included in the model and the electron temperature. The effects of the multicusp magnetic field and of the asymmetry of the source chamber are considered in the model. The values of the volume-averaged electron density and the average electron temperature obtained are compared to experimental measurements in the pressure and input power ranges of interest and a reasonably good agreement is found.