Helicon Plasma Source Research Articles

High power light gas helicon plasma source for VASIMR

In the Advanced Space Propulsion Laboratory (ASPL) helicon experiment (VX-10) we have measured a plasma flux to input gas rate ratio near 100% for both helium and deuterium at power levels up to 10 kW. Recent results at Oak Ridge National Laboratory (ORNL) show enhanced efficiency operation with a high power density, over 5 kW in a 5 cm diameter tube. Our helicon is presently 9 cm in diameter and operates up to 10 kW of input power. The data here uses a Boswell double-saddle antenna design with a magnetic cusp just upstream of the antenna. Similar to ORNL, for deuterium at near 10 kW, we find an enhanced performance of operation at magnetic fields above the lower hybrid matching condition.

Modeling electromagnetic field excitation and rf power absorption in a large helicon plasma

Wave propagation and rf power absorption are considered in a large helicon plasma source excited by a flat antenna along the magnetic field. Various single-, four- and six-turn antennas of different dimensions are examined comparatively. The simple physical model and computation code accounting for the most important features of the system are introduced. Profiles of the rf magnetic field and absorbed power and the antenna loading resistance are computed over a broad range of parameters. Computation results are interpreted considering propagation characteristics of the waves, such as the group velocity and characteristic angle, and are compared with measured data.

Evolution of the parallel and perpendicular ion velocity distribution functions in pulsed helicon plasma sources obtained by time resolved laser induced fluorescence

The temporal evolution of parallel and perpendicular ion velocity distribution functions (ivdf) in a pulsed, helicon-generated, expanding, argon plasma is presented. The ivdf's temporal evolution during the pulse was determined with time resolved (1 ms resolution), laser induced fluorescence. The parallel ivdf measurements indicate that, in the expansion region of the plasma and for certain operational parameters, two ion populations exist: a population moving at supersonic speeds (1.1 Mach) resulting from acceleration in an electric double layer (EDL) and a slow moving population (0.7 Mach) generated by local ionization. After 100 ms, although present, the EDL is not fully developed and has not reached a steady-state. Measurements of the perpendicular ivdf indicate constant radial expansion, with ion speeds of ≈400 m s−1, in the expansion region.

Small helicon plasma source for electric propulsion

An investigation was conducted for a very small helicon plasma source of a 2.5 cm i.d. glass tube. Since the operational features of electric propulsion are determined by its plasma density and degree of ionization, the helicon plasma including inductively coupled plasma (ICP) has a niche requirement as high as 10 13 cm − 3 density at the optimized condition of both RF frequency and applied magnetic field. In this paper, by using Ar, the RF frequency was scanned from 27.12 up to 67.80 MHz and the applied magnetic field was moderately varied from 0 to about 0.01 T. The maximum input power was 250 W at this time and 8 × 10 12 cm − 3 was achieved.

Characteristics of a large volume, helicon plasma source

A high-density helicon plasma source with very large volume, 75cm in diameter and 486cm in axial length, has been developed, and its characteristics have been investigated. Furthermore, the radial density profile control has been successfully demonstrated by utilizing two techniques: (1) by changing the magnetic field configurations near the antenna and (2) by changing the antenna radiation-field patterns. Using two types of large-diameter spiral antennae, plasma with density exceeding 1012cm−3 has been produced with several hundreds of watts of radio frequency power. By changing the magnetic field configuration near the antenna, the threshold power and the degree of the density change in a density jump can be varied. The electron density reaches the maximum away from the antenna; then decays weakly along the axial direction.

Control of nitrogen species in helicon plasmas

There are several possible reactions for both growth and decomposition of GaN layers in stable plasma conditions. A greater understanding of the plasma processes involved in the creation of the different N2 species is required before many growth related problems can be resolved. A plasma source capable of selecting different active nitrogen species could be used to study the different reactions related to the growth process. In this paper, we present a spectroscopic study of a helicon plasma source. Five different nitrogen species in the plasma are monitored, namely: N II (atomic ion), N I (atomic neutral), (molecular ion) and two different excited states of the N2 molecule as a function of the filling pressure. We show that by simply changing one plasma parameter (operating pressure), the plasma undergoes a transformation from an essentially atomic nature at low pressure to a dominantly molecular nature at higher pressure. The line intensity variations as a function of electron temperature are consistent with expectations based on the published formation rates for each species.

Three-dimensional laser-induced fluorescence measurements in a helicon plasma

We describe a three-dimensional (3D) laser-induced fluorescence (LIF) diagnostic for argon ions in a helicon plasma source. With three different laser injection orientations at a single spatial location, LIF measurements are performed to determine the 3D ion temperature and the 3D ion flow vector. The measurement process is then repeated at multiple locations in a cross section of the plasma column to create a two-dimensional (2D) map of the 3D ion flow, the ion temperature, and metastable ion density. Scanning in the 2D plane is accomplished by mounting the injection and collection optics on stepping motor driven stages.

Scanning internal probe for plasma particle, fluctuation, and LIF tomographic measurements

An internal scanning probe capable of spatially resolved measurements throughout a horizontal (r,z) cross section of the expansion region between a helicon plasma source and an expansion chamber is described. For complete diagnosis of the expanding magnetoplasma, the probe is designed to simultaneously measure the electron temperature, the electron density, the plasma potential, the magnetic fluctuation spectrum in three dimensions, and the two-dimensional ion velocity distribution function (through a tomographic inversion method). The probe design and operational characteristics as well as representative measurements are presented.

Low-power inductive RF plasma sources for technological applications

The objective of the present paper is to develop an analytic theory of cylindrical low-power RF plasma sources operating at an industrial frequency (f=13.56 MHz, ω=8.52×107 s−1). Inductive surface exciters of electromagnetic fields (exciting antennas) are considered that are positioned either at the side surface of the cylinder or at one of its end surfaces. In the latter case, the plasma flows out of the source through the opposite end surface of the cylinder. A study is made of elongated systems in which the length L of the cylinder exceeds its diameter 2R and of planar disk-shaped systems with L<2R. Simple analytic expressions are derived for electromagnetic fields excited by the antenna in the source plasma. The equivalent plasma resistance and the equivalent RF power deposited in the plasma are calculated for systems with prescribed parameters, i.e., in a non-self-consistent model. Up to now, such sources have been investigated mainly through the numerical solution of the complicated general electrodynamic equations. In the Introduction, the problem is formulated in general terms and the geometry of the sources, as well as the characteristic parameters of the source plasma, is discussed. In Section 2, plasma sources operating without an external magnetic field are investigated. In Section 3, helicon plasma sources in a sufficiently strong external magnetic field are considered. Analytic predictions are compared with the results from solving the problem numerically without using the helicon approximation. Section 4 gives a brief discussion of an electron cyclotron resonance-based RF plasma source. In the Conclusion, the main results of the paper are summarized and the technological efficiency of the sources under consideration is estimated at a qualitative level.

Plasma and electrical characteristics of inductive discharge in a magnetic field

The electron energy distribution function and the azimuthal rf electric field (magnitude and phase) have been measured in a cylindrical inductively coupled plasma (ICP) immersed in a weak magnetic field and driven at 29 MHz. The measurements were performed under conditions of controlled rf power delivered to the plasma electrons taking into account power loss in the induction coil. It has been found that considerable modification of plasma parameters at electron cyclotron resonance (ECR) conditions occurs only at relatively small discharge power and plasma density, when electron–electron collisions are negligible. It is shown that enhancement in the plasma density with application of magnetic field found in earlier experiments is mainly due to a rise in the power transfer efficiency caused by reduction in the ICP sustaining rf electric field. The rf field reduction is due to strong cyclotron damping at ECR conditions and due to the onset of propagation and absorption of the wave for which the phase velocities were found to be close to the electron thermal velocity. This wave regime occurs at magnetic fields higher than that at ECR but much smaller than that in typical helicon plasma sources.

Low-Power RF plasma sources for technological applications: III. helicon plasma sources

The third part of the paper is devoted to an investigation of the so-called helicon plasma sources. These are RF devices operating with a relatively weak external magnetic field, which is, at the same time, strong enough for the resonant electron gyrofrequency to be much higher than the industrial frequency (ω=8.52×107 s−1). As in [1, 2], a study is made of elongated cylindrical plasma sources in a longitudinal (directed along the cylinder axis) magnetic field and planar disk-shaped plasma sources in a transverse (perpendicular to the plane of the disk) magnetic field. A comparison of the present results with the results that were obtained in [3] without using the helicon approximation leads to the conclusion that elongated helicon sources hold great promise for plasma technologies.

Development of very large helicon plasma source

We have developed a very large volume, high-density helicon plasma source, 75 cm in diameter and 486 cm in axial length; full width at half maximum of the plasma density is up to ∼42 cm with good plasma uniformity along the z axis. By the use of a spiral antenna located just outside the end of the vacuum chamber through a quartz-glass window, plasma can be initiated with a very low value of radio frequency (rf) power (&amp;lt;1 W), and an electron density of more than 1012 cm−3 is successfully produced with less than several hundred Watt; achieving excellent discharge efficiency. It is possible to control the radial density profile in this device by changing the magnetic field configurations near the antenna and/or the antenna radiation-field patterns.

Parallel velocity and temperature of argon ions in an expanding, helicon source driven plasma

The parallel ion flow in a high-density helicon source plasma expanding into a region of weaker magnetic field is measured as a function of neutral pressure, magnetic field strength, rf power and rf driving frequency. The dependence of the parallel ion flow and parallel ion temperature, measured by laser induced fluorescence, on the plasma density, electron temperature and floating potential, measured with an rf-compensated Langmuir probe, is also examined. At the end of the helicon plasma source, the ion velocity space distribution changes from a single subsonically drifting Maxwellian population to a supersonic ion beam (≈15 eV) plus a cold, subsonically drifting background ion population. At 38 cm into the expansion region beyond the end of the plasma source, the supersonic ion beam is not observed.

Development of a compact helicon ion source for neutron generators

A compact helicon ion source has been designed for a neutron generator. Energetic deuterium beams of 120 keV from this ion source will be directed onto a Ti-coated copper target where D–D nuclear fusion reactions take place and generate 108 n/s of neutrons. High-density radio frequency (rf) plasma sources such as a helicon plasma source, known for its highest efficiency of generating high-density plasmas, are chosen for the development of a high-current density compact ion source. Highest plasma densities for hydrogen plasmas are obtained at relatively low magnetic fields of 100–300 G although plasma densities of helicon plasmas are well known to be almost linearly dependent on magnetic field strength. With Nagoya type III antenna, plasma densities of up to 2×1011 cm−3 are obtained with 1.1 kW rf power at the frequency of 13.56 MHz. With the prototype helicon ion source, hydrogen beam currents of up to 44 mA at 23 kV have been extracted in continuous wave operations with the beam current density of 28.5 mA/cm2. With helical antenna instead of Nagoya type III antenna, higher plasma densities of up to 1012 cm−3 are obtained and open up a new possibility of much higher beam current densities.

Theoretical components of the VASIMR plasma propulsion concept

The ongoing development of the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) involves basic physics analysis of its three major components: helicon plasma source, ion cyclotron-resonance heating module, and magnetic nozzle. This paper presents an overview of recent theoretical efforts associated with the project. It includes (1) a first-principle model for helicon plasma source, (2) a nonlinear theory for the deposition of rf-power at the ion cyclotron frequency into plasma flow, and (3) a discussion of the plasma detachment mechanism relevant to VASIMR.

Near infrared spectra of carbon deposited layers from tore supra under plasma particle bombardment

During the 1999 experimental Tore-Supra campaign, an unexpected high near infrared (NIR) emission was observed when looking at carbonaceous surfaces under the impact of plasma and high energy ions: a radiation different from blackbody. Such an extra luminance phenomenon has now been produced in controlled way in laboratory experiments. Here we report on measurements using a helicon plasma source at LPIIM, Marseille. As with previous experiments on Tore Supra, the laboratory observed NIR spectra are different from simple blackbody radiation curves under certain conditions. Luminescence phenomenon is discussed to explain extra luminance from carbon surfaces under plasma particles impact.

Modeling of a helicon plasma source

A two-dimensional axisymmetric helicon plasma source with three-dimensional wave components is modeled. Plasma fluid equations are solved self-consistently with helicon wave equations. Electron inertia is taken into account to simulate the propagation of electrostatic wave components. The purpose of this paper is to describe the model in detail and present the results to verify wave propagation characteristics, power deposition mechanisms, especially bulk versus peripheral power absorption. We also study the effect of wave absorption characteristics on discharge plasma density and temperature profiles.

Strong ion-sound parametric turbulence and anomalous anisotropic plasma heating in helicon plasma sources

Experimentally observed anisotropic ion heating in helicon plasma sources may result from the interaction of ions with ion-sound turbulence. The ion-sound turbulence develops due to excitation of the short scale, kinetic, parametric ion-sound instability. From the quasilinear equation for the ion distribution function, which includes the induced scattering of ion-sound waves by ions, the turbulent heating rate is determined. This anomalous ion heating is predominantly across the magnetic field and can lead to strong turbulence effects, such as the collective statistical effect of ion-sound turbulence on individual ion orbits (resonance broadening). Resonance broadening is a strong turbulence saturation mechanism for ion-sound turbulence. The energy density of the ion-sound turbulence in the saturated state, the ion and electron heating rates, as well as the effective collision frequency arising from the anomalous absorption of the helicon wave, are estimated and compared to experimental data.

Development of an RF Plasma Source with Mirror Field for a Compact Neutron Generator

An RF plasma source has been designed and constructed for a compact neutron generator. The generation of neutrons is based on the D-D/D-T fusion reactions, producing 2.5/14.1MeV neutrons, respectively. The neutron yield of this device depends on the density of D or T monoatomic beams, which can be extracted from low-pressure high-density plasma sources, especially RF-driven plasma sources, such as an ICP (inductively coupled plasma) or a Helicon plasma source. In this presentation, the design of an ICP source with mirror field has been performed arranging targets in coaxial geometry. Plasma targets without any solid target as well as solid targets are arranged in this design for the comparison study and the effects of the mirror field on the plasma properties will be evaluated.

Progress in Experimental Research of the Vasimr Engine

The Advanced Space Propulsion Laboratory (ASPL) of NASA's Johnson Space Center is performing research on a Variable Specific Impulse MagnetoPlasma Rocket (VASIMR). The VASIMR is a high power, radio frequency (RF) driven magnetoplasma rocket, capable of very high exhaust velocities, > 100 km/s. A NASA-led research team involving industry, academia and government facilities is pursuing the development of this concept in the United States. The ASPL's experimental research focuses on three major areas: helicon plasma production, ion cyclotron resonant frequency (ICRF) acceleration and plasma expansion in a magnetic nozzle. The VASIMR experiment (VX-10) performs experimental research that demonstrates the thruster concept at a total RF power on the order of 10 kW. A flexible four-magnet system, with a 1.3 Tesla maximum magnetic field strength, allows axial magnetic field profile shape effects to be studied. Power generated at 10 – 50 MHz with about 3 kW is used to perform helicon plasma source development. A 3 MHz RF transmitter capable of 100 kW is available for ICRF experiments. The primary diagnostics are: gas mass flow controllers, RF input power, Langmuir probes, Mach probe, retarding potential analyzers (RPA), microwave interferometer, neutral pressure measurements and plasma light emission. In addition, many thermocouples are attached inside the vacuum chamber to measure heat loads around the plasma discharge.Helicon research has been done with hydrogen, deuterium, helium, nitrogen, argon, xenon and mixtures of these gases. Optimization studies have been performed with the magnetic field axial profile shape, antenna geometry, gas flow rate, gas tube geometry and RF frequency. ICRF experiments have begun, primarily using a high density (> 1018/m3) helium helicon discharge as a target. Over 6 kW of power has been applied using a simple antenna array. The latest results of helicon and ICRF experiments will be presented.