Coaxial Plasma Source Research Articles

A compact coaxial electron cyclotron resonance plasma source

A compact coaxial electron cyclotron resonance (ECR) plasma source is built for plasma deposition experiments. The ECR plasma is produced in a coaxial line configuration and hence the source is compact. The plasma parameters (plasma density and electron temperature) are measured using a Langmuir probe. The plasma parameters are mainly dependent on the center conductor (stub) dimensions of the coaxial line. The characterization of plasma for both conical and cylindrical stubs is carried out and it is found that the conical stub produces relatively denser and more stable plasma than the cylindrical stub. The typical plasma density and electron temperature are 3×1010 cm−3 and 5 eV, respectively, for argon plasma.

Measurements of the impressed electric field inside a coaxial electron cyclotron resonance plasma source

The magnitude and spatial variation of the impressed electric-field patterns inside a compact electron cyclotron resonance ion/plasma source are experimentally measured for argon and nitrogen feed gases. This ECR plasma source consists of several components: a resonant coaxial coupling section, an evanescent circular waveguide section, coupling loop, and the ECR discharge load itself. The electric fields inside the coaxial and circular waveguide sections are measured as the operating pressure and input power and are varied from 0.2 to 2.0 mTorr and 100 to 170 W, respectively. The measured fields verify that a standing wave with a maximum of 20–40 kV/m exists inside the coaxial section of length l. For matched conditions the length of this section varies only slightly between 0.6 and 0.7λ as pressure, power, and gas type vary. However, the evanescent impedance-matching circular waveguide section of length d changes from 2.5 for argon to 3.2 cm for nitrogen, indicating that the gas type influences the plasma impedance. Field pattern measurements in the ECR section of the source demonstrate the presence of nonevanescent fields in the discharge region. Measured plasma loaded and unloaded quality factors varied from 220 to 1800, respectively, indicating that 87% of the net input power is coupled into the discharge load. Additional calculations of conductive wall losses show that about 6% of the input power is lost in the cavity walls, and the remaining 7% is lost in the coupling loop.

A magnetized coaxial source facility for the generation of energetic plasma flows

The design features and characterization of a new magnetized coaxial plasma source facility for the generation of energetic plasma stream flows are presented. Careful, yet simple design of a low-inductance (<or approximately=200 nH) external power delivery system provides high-current (>or approximately=130 kA) pulses to the plasma source with fast rise ( approximately 26 mu s). This allows for the production and acceleration of dense ( approximately 2*1015 cm-3), high-quality (10-40 eV), self-field plasmas to approximately 105 m s-1. We are then afforded the laboratory study of plasma streams with a wide variety of applications including, but not limited to, advanced thrusters for electric propulsion, astrophysical jets, critical ionization velocity, magnetic fusion, large-scale plasma etching and deposition, etc. Comparison to an ideal MHD model is made indicating reasonably good agreement in these self-field discharges, while elucidating the advantages of strong applied magnetization to provide nozzling in a magnetically constricted flow.

Initial Results on High Enthalpy Plasma Generation in a Magnetized Coaxial Source

Initial investigation on high enthalpy plasma stream generation in the North Carolina State University Coaxial Plasma Source[1] (CPS) facility is presented. Tenuous, yet high enthalpy, flows are produced from this Magnetized Coaxial plasma Gun (MCG) which allow laboratory study of plasma streams with a wide variety of applications. The applicability includes, but is not limited to, advanced thrusters for electric space propulsion, astrophysical jets and critical ionization phenomena, magnetic fusion compact toroid devices and tokamak fueling, large scale plasma etching and deposition, etc.

VHF Coaxial Helix Plasma Source for a-Si:H

ABSTRACTCoaxial helical resonators are used in the field of high frequency engineering for narrow band selection of signals from about 10 MHz to 500 MHz (VHF and low UHF range) [1, 2]. Their favorable geometric construction suggests their utilization as plasma source for etching and deposition of a-Si:H at frequencies between 30 and 200 MHz - an additional, but still unusual application.The resonator consists of a cylindric outer conductor, which is closed on one side by a bottom plate, and a rotationally symmetrical inner conductor of spiral form (helix) Measuring γ/4. One end of the helix is grounded to the bottom plate (short circuit), the other end is open (open circuit). Power supply is feasible either galvanically by connecting to the helix or inductively by means of a coupling coil. Applying a suitable coupling factor allows operation at resonance without matchbox. The plasma is generated within the resonator and above its open end, which can be adjusted to special conditions of application with the aid of several grids. In contrast to the source commonly used, the coaxial helix plasma source has proved advantageous with regard to simple design, easy mounting, Minimal outer electrical equipment, no dependence of generated plasma on substrate measures, good breakdown and discharge behavior, low supply voltage (some tens of volts), high power density, and, due to a discharge occurring without filament, also to life durability and no thermally generated particle emission. The application of very high frequencies leads to a low ion bombardment intensity of a-Si:H, and thus additional magnetic fields are not necessary.

Comparison of the operational performance of a compact electron cyclotron resonance plasma source at excitation frequencies of 2.45 GHz or 915 MHz

This article presents the results of a study in which the performance of a compact coaxial electron cyclotron resonance plasma source for molecular-beam epitaxy has been experimentally evaluated and compared at the two microwave excitation frequencies of 2.45 GHz and 915 MHz. Diagnostic measurements, using Langmuir probes and a multigrid energy analyzer, are made at absorbed power levels varying from 80 to 250 W, at flow rates ranging from 2 to 30 sccm and the corresponding pressures of ∼10−5–10−4 Torr. It is seen that the plasma source is capable of producing high density discharges, about high 1010 to low 1011 cm−3, with excitation at either 915 MHz or 2.45 GHz. Also, at either frequency, ion energy distribution functions are relatively narrow, with full width at half-maxima of less than 20 eV for a typical operating condition. However, the average ion energy for a 915 MHz discharge (30–50 eV) is in general higher than the average ion energy of a 2.45 GHz discharge (15–25 eV), when operated at the same power and pressure conditions. As measured by the double Langmuir probe, the electron temperatures of the high energy tail are higher for 915 MHz excitation than similar electron temperatures of a 2.45 GHz excited discharge.

High-flux source of low-energy neutral beams using reflection of ions from metals

Reflection of low-energy (&amp;lt;100 eV) ions from surfaces can be applied as a method of producing high-flux beams of low-energy neutral particles, and is an important effect in several areas of plasma technology, such as in the edge region of fusion devices. We have developed a beam source based on acceleration and reflection of ions from a magnetically confined coaxial rf plasma source. The beam provides a large enough flux (over 4 A ion current, or 5×1016 atoms/cm2 s at 10-cm range) to allow the energy distribution of the reflected neutrals to be measured despite the inefficiency of detection, by means of an electrostatic cylindrical mirror analyzer coupled with a quadrupole mass spectrometer. Energy distributions have been measured for oxygen, nitrogen, and inert gas ions incident with from 15 to 70 eV reflected from amorphous metal surfaces of several compositions. For ions of lighter atomic mass than the reflecting metal, reflected beams have peaked energy distributions; beams with the peak at 4–32 eV have been measured. The energy and mass dependences of the energy distributions as well as measurements of absolute flux, and angular distribution and divergence are reported. Applications of the neutral beams produced are described.

Development of coaxial ECR plasma source for tube inner coating

We developed an electron cyclotron resonance (ECR) plasma source with a coaxial microwave mode which has no cut-off length for coating inside a metallic tube of small diameter and shorter than the cut-off length for the microwaves used. In a discharge with a narrow gap less than the wavelength of the microwaves, both the magnetic field and the electron loss to the boundary play an important role in the condition for breakdown. We theoretically and experimentally investigated the gaseous breakdown conditions for microwave power in the presence of an external crossed magnetic field. Using a new calculation method to treat the effects of electron collisions in a wide range of gas pressures the result indicates that the ECR condition ( ω = ω ce where ω is the microwave frequency and ω ce is the electron cyclotron frequency) has significant advantages for such a narrow-gap microwave discharge. The plasma is found to be produced at B of around 875 Gauss for the pressure range from 1 × 10 −4 to 5 × 10 −3 Torr. The discharge area broadened with increasing gas pressure. Good agreement is found between the calculated and measured discharge area. The typical electron density n e and electron temperature T e were also measured to be 8 × 10 10 cm −3 and 10 eV, respectively, which parameters are suitable for reactive sputtering or plasma chemical vapour deposition.

The impedance and energy efficiency of a coaxial magnetized plasma source used for spheromak formation and sustainment

Electrostatic (dc) helicity injection has previously been shown to successfully sustain the magnetic fields of spheromaks and tokamaks. The magnitude of the injected magnetic helicity balances (within experimental error) the flux lost by resistive decay of the toroidal equilibrium. Hence the problem of optimizing this current drive scheme involves maximizing the injected helicity (the voltage-connecting-flux product) while minimizing the current (which multiplied by the voltage represents the energy input and also possible damage to the electrodes). The impedance (voltage-to-current ratio) and energy efficiency of a dc helicity injection experiment are studied on the CTX spheromak [Phys. Fluids 29, 3415 (1986)]. Over several years changes were made in the physical geometry of the coaxial magnetized plasma source as well as changes in the external electrical circuit. The source could be operated over a wide range of external charging voltage (and hence current), applied axial flux, and source gas flow rate. A database of resulting voltage, helicity injection, efficiency, electron density, and rotation has been created. These experimental results are compared to an ideal magnetohydrodynamic theory of magnetic flux flow. The theory is parametrized by the dimensionless Hall parameter, the ratio of electric to mass current. For a constant Hall parameter the theory explains why the voltage depends quadratically on the current at constant flux. The theory also explains the approximately linear dependence of the impedance-to-current ratio on the current-to-flux ratio of the source. The current-to-flux ratio itself (the energy-per-unit helicity of the source) is bounded below by considerations of force balance. While the rotation of the flow is not understood, the density of the sustained spheromak is shown to be related to the mass flow in the source, supporting the constant Hall parameter assumption. The overall efficiency of sustainment through dc helicity injection is limited by the usual Ohmic resistive decay, by the force-balance limits on the current-to-flux ratio, by the losses of the external electrical circuit, and by the fundamental limitations on the achievable impedance of flux flow in a magnetized plasma. Even so, ratios of spheromak magnetic energy to capacitor bank energy of over 17% have been achieved on CTX. Ignoring external circuit losses the efficiency of electrostatic helicity injection for converting energy received by the coaxial source to the energy of the spheromak magnetic field has exceeded 70%.

ATOMIC OXYGEN BEAM SOURCE FOR ORBITAL ENVIRONMENT EXPERIMENTS

ABSTRACT We describe a new system for the production of high flux neutral beams of low energy (5-10 eV) atoms, for use in beam-surface interaction and surface modification studies. This source can recreate the flux of superthermal atomic oxygen and other neutral species to which spacecraft surfaces are exposed in low Earth orbit. The system is based upon a magnetically confined (3-4 kG) co-axial plasma source driven by 1 kW RF at 2.45 GHz. The beam is. produced by the acceleration of plasma ions onto a negatively biased plate of high-Z metal (Mo or Pt); the ions are neutralized and reflected by the surface, retaining a large fraction of their incident kinetic energy, forming a beam of superthermal atoms. The source can provide neutral flux > 5 × 10 16 /cm2 s and fluence > 1020 cm2 in a five hour exposure. Such a high fluence source can provide accelerated simulation testing of materials and coatings for space applications. To date, samples of carbon film, carbon-based paint (Z-302), Kapton, mylar, teflon,...

Experimental determination of the conservation of magnetic helicity from the balance between source and spheromak

The conjecture that magnetic helicity (linked flux) is conserved in magnetized plasmas for time scales that are short compared to the resistive diffusion time is experimentally tested in the CTX spheromak [Phys. Rev. Lett. 45, 1264 (1980); 51, 39 (1983); Nucl. Fusion 24, 267 (1984)]. Helicity is created electrostatically by current drawn from electrodes. The magnetized plasma then flows into a conducting flux conserver where the energy per helicity of the plasma is minimized and a spheromak is formed on a relaxation time scale of many Alfvén times. The magnetic field strength of the equilibrium is subsequently increased and sustained. The amount of helicity created by the magnetized coaxial plasma source, the helicity content of the spheromak equilibrium, and the resistive loss of the helicity are measured to determine the balance of helicity between source and spheromak with a ±16% uncertainty. In CTX the amount of energy that must be rapidly dissipated within the conducting boundary while conserving helicity in the process of sustaining the spheromak is experimentally controllable, and has varied from 1.8 times the spheromak magnetic energy to greater than 10 times. The relaxation, or minimization of the energy-to-helicity ratio, determines the gross structure (the normalized spatial profile) of the spheromak, while the conservation of helicity itself determines the magnitude and time dependence of the magnetic fields of the spheromak equilibrium. Helicity balance tests are done by individually varying the sign and magnitude of the source voltage and flux, and by observing sustainment of spheromaks with fields opposing those of the source. A threshold for helicity injection from the source is measured and related to the source and entrance region size. During times short compared to resistive diffusion time scales the helicity is shown to be conserved with a ±12% uncertainty using no free parameters. For longer times the resistive dissipation Its value is independently measured and appears to be related to the expected classical resistive decay. Absolutely calibrated bolometer measurements are consistent with excess source energy heating the spheromak plasma during the sustainment by electrostatic helicity injection.

The Ohmic heating of a spheromak to 100 eV

The first spheromaks with Thomson-scattering-measured electron temperatures of over 100 eV are described. The spheromak is generated by a magnetized coaxial plasma source in a background gas of 30 mTorr of H2, and it is stably confined in an oblate 80 cm diam copper mesh flux conserver. The open mesh design allows rapid impurity transport out of the spheromak. The peak temperature, measured using multipoint Thomson scattering, is observed to rise from approximately 25 eV to over 100 eV in about 0.2 msec due to Ohmic heating from the decaying magnetic fields. Density (∼5×1013 cm−3) and magnetic fields (approximately 2 kG) are measured using interferometry and magnetic probes.

516. Energy characteristics of a coaxial plasma source: A G Belikov et al, Zh Tekh Fiz, 41 (9), Sept 1971, 1881–1886 ( in Russian)

516. Energy characteristics of a coaxial plasma source: A G Belikov et al, Zh Tekh Fiz, 41 (9), Sept 1971, 1881–1886 ( in Russian)