Helicon Plasma Source Research Articles

High-density plasma silicon oxide thin films grown at room-temperature

The fabrication at room-temperature of thin (<8nm) silicon oxide films has been achieved, in a high-density helicon plasma source reactor using Ar/O2 mixture, exhibiting relatively low concentration of fixed oxide and interface charges, after annealing. The developed plasma oxidation process was employed in order to fabricate SOI-MOSFETs at low thermal budget with competitive operating characteristics.

Development of a Helicon Plasma Source for the Measurement of He* Component in a He0 Beam

A high density plasma source using helicon discharge was designed for the measurement of the population ratio of the metastable state and ground state in a helium neutral beam. The required density of the plasma source was investigated in terms of the rate coefficient relevant to the beam-plasma interaction. The charge-exchange for the metastable state is a dominant process in a beam-energy range of 30-300 keV for the beam attenuation, promising the measurement of the metastable fraction in the helium neutral beam using this plasma source. The first plasma with the electron density of 3.4 × 1012cm−3 and the electron temperature of 10 eV was successfully produced in argon gas.

High-beta plasma effects in a low-pressure helicon plasma

In this work, high-beta plasma effects are investigated in a low-pressure helicon plasma source attached to a large volume diffusion chamber. When operating above an input power of 900W and a magnetic field of 30G a narrow column of bright blue light (due to Ar II radiation) is observed along the axis of the diffusion chamber. With this blue mode, the plasma density is axially very uniform in the diffusion chamber; however, the radial profiles are not, suggesting that a large diamagnetic current might be induced. The diamagnetic behavior of the plasma has been investigated by measuring the temporal evolution of the magnetic field (Bz) and the plasma kinetic pressure when operating in a pulsed discharge mode. It is found that although the electron pressure can exceed the magnetic field pressure by a factor of 2, a complete expulsion of the magnetic field from the plasma interior is not observed. In fact, under our operating conditions with magnetized ions, the maximum diamagnetism observed is ∼2%. It is observed that the magnetic field displays the strongest change at the plasma centre, which corresponds to the maximum in the plasma kinetic pressure. These results suggest that the magnetic field diffuses into the plasma sufficiently quickly that on a long time scale only a slight perturbation of the magnetic field is ever observed.

Development of a High-Current Helicon Ion Source With High Monatomic Fraction for the Application of Neutron Generators

A high-current helicon ion source has been developed for neutron generator application. High-density plasmas with high electron temperatures are generated by launching helicon waves with a half-helical antenna in various magnetic field configurations. Helicon plasmas with high power efficiency are favorable in building ion sources with high current density and high monatomic ion beam fraction that are required for compact neutron generators. A maximum hydrogen beam current of 50 mA is extracted at 34 kV from the helicon plasma source with 1.9-kW RF power at 13.56 MHz. High current densities of 212 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and high atomic fractions of up to 94% are achieved with this source. These measured beam parameters are mostly consistent with plasma parameters, except for unusually high monatomic beam fraction, which may be due to the existence of energetic electrons in helicon plasmas.

Honeycomblike large area LaB6 plasma source for Multi-Purpose Plasma facility

A Multi-Purpose Plasma (MP(2)) facility has been renovated from Hanbit mirror device [Kwon et al., Nucl. Fusion 43, 686 (2003)] by adopting the same philosophy of diversified plasma simulator (DiPS) [Chung et al., Contrib. Plasma Phys. 46, 354 (2006)] by installing two plasma sources: LaB(6) (dc) and helicon (rf) plasma sources; and making three distinct simulators: divertor plasma simulator, space propulsion simulator, and astrophysics simulator. During the first renovation stage, a honeycomblike large area LaB(6) (HLA-LaB(6)) cathode was developed for the divertor plasma simulator to improve the resistance against the thermal shock fragility for large and high density plasma generation. A HLA-LaB(6) cathode is composed of the one inner cathode with 4 in. diameter and the six outer cathodes with 2 in. diameter along with separate graphite heaters. The first plasma is generated with Ar gas and its properties are measured by the electric probes with various discharge currents and magnetic field configurations. Plasma density at the middle of central cell reaches up to 2.6 x 10(12) cm(-3), while the electron temperature remains around 3-3.5 eV at the low discharge current of less than 45 A, and the magnetic field intensity of 870 G. Unique features of electric property of heaters, plasma density profiles, is explained comparing with those of single LaB(6) cathode with 4 in. diameter in DiPS.

Experimental Study on the Density Peaking of an RF Plasma at a Low Magnetic Field

Density peaking of a plasma at a low magnetic field is investigated. In this experiment, a helicon plasma source with a 15-cm plasma column diameter is used. An rf power of 200 ∼ 800 W with a frequency of 6 ∼ 14 MHz is fed to a half-turn helical antenna located outside a Pyrex tube plasma enclosure. Experimental results show a loading resistance and an ion density peaking at several tens of gauss, and the peaking magnetic fields of these two parameters are reasonably the same. In addition, the dependency of the peaking magnetic field on the ion density seems to be matched with the dependence of the mode coupling magnetic field on the ion density. Mode coupling of a short-wavelength electrostatic wave called the TG mode and the helicon mode is formed at the corresponding magnetic field when the effect of electron inertia is included in the rf plasma with an axial magnetic field. The experimental result supports that density peaking at a low magnetic field is possibly caused by eigenmode formation around a mode coupling magnetic field. The loading resistance is usually found to be high for the eigenmodes, and a higher loading resistance yields an efficient rf power coupling to the plasma, thus, a higher plasma density. A plasma with a low magnetic field may have advantages, such as a relatively uniform plasma profile and a simple system for magnets and the power supply. This kind of density enhancement at a low magnetic field is, therefore, useful for applications requiring an efficient, uniform plasma.

Experimental characterization of a density peak at low magnetic fields in a helicon plasma source

We investigated characteristics of a density peak observed in a magnetic field B0 lower than 100 G in the case of using helicon plasma sources with particular wavelength. For B0 > 30 G, the antenna launches an electromagnetic wave as a slow wave, the phase velocity of which becomes close to the electron thermal velocity under the density-peak condition for various gas pressures. The Landau damping frequency is higher than the electron–neutral and the electron–ion collision frequencies, which indicates that the wave produces the plasma via Landau damping at low B0. The wavelengths estimated from the density ne and B0 for the density peaks agree with those of the electromagnetic field generated by helicon antennas of various lengths. The measured density is found to vary under the condition of the agreement between wavelengths of the propagating wave and the antenna-excited field during the density increase. One of the causes of the density peak appearing as a function of B0 is considered to be the wavelength variation and the corresponding change of phase velocity of the slow wave which is enabled to propagate in the plasma by the introduction of B0.

Nitrogen dissociation degree in the diffusion region of a helicon plasma source obtained by atomic lines to molecular band intensities ratio

Estimates of the dissociation degree in the diffusion region of a nitrogen helicon plasma source based on optical emission spectroscopy and Langmuir probe measurements are presented. The estimation procedure relies on measurements of the ratios of the intensities of the atomic triplet 3pS04→3sP4 (742.36, 744.23, and 746.83nm) to the intensity of the 4-2 band of the first positive system (AΣu+3→BΠg3) at 750.39nm and the measured relative vibrational distribution of the BΠg3 state. The electron energy distribution function, obtained from the second derivative of the Langmuir probe characteristic, and published excitation cross sections are used to calculate the electron-impact excitation rate coefficients—which are then compared to the atomic line and molecular band intensities to calculate the dissociation degree. For two distinct operating regimes, capacitively and inductively coupled, dissociation fractions of 5% and 13% are obtained in the expansion region of a 10mTorr, 500W, 10.74MHz helicon generated nitrogen plasma. The dissociation degree results suggest that the helicon source can provide excited molecular species for plasma assisted molecular beam epitaxy of III-group nitrides.

Helicon plasma source in the ion-acoustic frequency range

A helicon plasma source operating in the ion cyclotron frequency range is studied theoretically. It is shown that, even with a purely inductive antenna exciting a helicon wave in a plasma at ion-acoustic frequencies, the effective resistance characterizing the absorption of high-frequency field energy is determined by the ion-acoustic field generated by the helicon wave. Calculations show that such a plasma source can operate very efficiently.

Line ratio diagnostics in helium and helium seeded argon plasmas

We investigate the potential use of line ratio diagnostics to evaluate electron temperature in either helium or helium seeded argon plasmas. Plasmas are produced in a helicon plasma source. A rf compensated Langmuir probe is used to measure both the electron temperature and plasma density while a spectrometer is used to measure He I line intensities from the plasma. For all plasma densities where the electron temperature remains at 5 ± 1 eV, three He line ratios are measured. Each experimental ratio is compared with the prediction of three different collisional radiative models. One of these models makes uses of recent R-matrix with pseudo-states calculations for collisional rate coefficients. A discussion related to the different observations and model predictions is presented.

Large-area helicon plasma source with permanent magnets

A helicon plasma source has been designed using annular permanent magnets to produce the required DC magnetic field (B-field). With the discharge tube located in the remote field, rather than the internal field of the magnet rings, the plasma can be injected into a processing chamber containing the substrate to be treated. The discharge tube, radiofrequency (RF) antenna and magnet size were optimized by computation and tested by experiment. A distributed source comprising eight individual discharges was constructed and tested. Such sources are capable of producing downstream densities >1012 cm−3 (in argon) over an arbitrarily large area for high-flux applications.

Development of Two Propulsion Systems with Helicon Plasma

Two space propulsion systems, which are called K2H (KBSI-KAIST-Hanyang University) and DiPS (Diversified Plasma Simulator) devices, are being developed in parallel to explore the space propulsion parameters and optimal helicon operation conditions with the concept of VASIMR (Variable Specific Impulse Magnetoplasma Rocket). Both devices utilize the open-ended magnetic configuration. K2H has three regions such as helicon source, ion cyclotron resonance heating (ICRH), magnetic nozzle and expansion regions. DiPS is the space plasma simulator and composed of three major sections: helicon plasma source, extraction region and space simulation region. Helicon plasmas are generated for both devices by 13.56 MHz rf power using M=+1 right-helical antenna at pressure of several mTorr. Initial plasma parameters such as density, temperature, and drift velocity were measured by a laser induced fluorescence (LIF) system and a fast scanning electric probe system with an rf-compensated Langmuir probe and a Mach probe at ICRH and magnetic nozzle region. The results are given as follows: plasma density n = 1011 - 1013 cm-3 (K2H) and 1012 - 1013 cm-3 (DiPS), electron temperature Te = 3 - 9eV (K2H) and 2 - 4 eV (DiPS), ion temperature Ti = 0.144 - 0.164 eV (K2H), and drift velocity vd = 0.8 - 1.55 km/s (K2H). A simple analysis of the results is provided.

Dynamics of hydrogenic retention in molybdenum: First results from DIONISOS

The DIONISOS (Dynamics of ION Implantation and Sputtering of Surfaces) experiment is described. DIONISOS features steady-state plasma exposure of material surfaces, with simultaneous, in situ, non-destructive depth profiling on the same surfaces with ion beam analysis (IBA). A steady-state RF helicon plasma source produces a fusion ‘edge-like’ low temperature (T∼1–10eV), high-density (⩽1017–18m−3), steady-state, cylindrical, plasma (ϕ∼50mm). The exposure stage allows for the control of sample surface temperature (300–800K), incident ion energy (10–500eV), and ion beam and magnetic field incident angle (360° rotation), during bombardment with high plasma flux (∼1020–1022s−1m−2). First results from DIONISOS show a large and evolving concentration of deuterium (∼1000appm) trapped deep (∼5μm) in molybdenum. IBA during plasma exposures shows the surface dynamic deuterium inventory increased by a factor of 2 over the ‘long-term’ trapped inventory at an incident ion flux density of ∼1.5×1021D/m2s.

Resonant power absorption in helicon plasma sources

Helicon discharges produce plasmas with a density gradient across the confining magnetic field. Such plasmas can create a radial potential well for nonaxisymmetric whistlers, allowing radially localized helicon (RLH) waves. This work presents new evidence that RLH waves play a significant role in helicon plasma sources. An experimentally measured plasma density profile in an argon helicon discharge is used to calculate the rf field structure. The calculations are performed using a two–dimensional field solver under the assumption that the density profile is axisymmetric. It is found that RLH waves with an azimuthal wave number m=1 form a standing wave structure in the axial direction and that the frequency of the RLH eigenmode is close to the driving frequency of the rf antenna. The calculated resonant power absorption, associated with the RLH eigenmode, accounts for most of the rf power deposited into the plasma in the experiment.

Plasma ionization by annularly bounded helicon waves

The general solution to the electrostatic and magnetic fields is derived with respect to the boundary conditions of a coaxial helicon plasma source. The electric field contours suggest that a simple antenna design can ionize the gas in a coaxial configuration. In addition, the power deposition as a function of excitation frequency is derived. The solution is validated by comparison with the standard cylindrical helicon plasma source. Further, a parametric study of source length, channel radius, channel width, and antenna excitation frequency are presented. This study suggests that it is possible to create a helicon plasma source with a coaxial configuration.

Electric Probe Systems for Diversified Plasma Simulator (DiPS)

AbstractDiPS (Diversified Plasma Simulator), a new versatile linear machine (length=3400 mm, diameter=200‐600 mm), is developed for the simulations of divertor, space and processing plasmas with various electric probes: fast‐scanning systems with single, triple, and Mach Probes, slow‐scanning water‐cooled single probe, griddedenergy analyzer (GEA), and several fixed single probes. For the verification of current probe theories and the development of new theories with magnetic vector field, collisional effects, various particle sources, and wide range of plasma parameters, two different plasma sources are installed: (1) For a stable high density dc plasma, a LaB6 disk is used as the thermal electron emission source with 5 kW graphite heater. Initial plasma density of the LaB6 source is 5 × 1012 cm−:3, electron temperature is 8‐10 eV with magnetic field around 1 kG. The electron density would be decreased severely, i.e., to 106–108 cm−3 with grid, and expanded into space simulation region without magnetic field; (2) Helicon plasma source is also installed for a processing simulation in DiPS and for the space propulsion study with magnetic expansion, which generates the plasma with density of 2 × 1013 cm−3 and electron temperature 3‐4 eV for the rf power of 2.5 kW at optimum pressure of 7.5 mTorr. As a unique feature for the diversified uses of sources and divertor simulator experiments, space and processing simulation chambers can be detached from and attached to the divertor simulation chamber on the rail. Initial data of various electric probes (single, triple, emissive, mach probes, GEA, etc.) are introduced for the the following conditions of DiPS with B= 1 kG, P= 130 mTorr at source region, P= 2 mTorr at divertor simulation region, ne = 1×1012 – 1×1014 cm−3, Te = 2 ‐ 3 eV, Ti = 0.2 eV with laser induced fluorescence (LIF) (Ar gas), ne = 1 × 1012 – 1 × 1013 cm−3, Te =5 ‐ 6 eV (He gas) at divertor simulation region, Ti = 0.47 ‐ 0.74 eV with GEA at space simulation region and vd = 500 ‐ 600 m/s. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Momentum and Heat Flux Measurements Using an Impact Target in Flowing Plasma

An impact target plate was used to determine the momentum and heat flux quantities from a helicon plasma source. The plasma source is the first stage of the Variable Specific Impulse Magnetoplasma Rocket, which uses radio frequency waves to create and energize a flowing plasma. The momentum and heat flux quantities are determined separately and agree favorably. They also agree with expected flux quantities calculated from the plasma parameters determined by other probes. The momentum and heat flux quantities were obtained at two axial locations in the exhaust or magnetic nozzle region of the device using helium and argon propellant. Only data from the plasma source are used, which shows that the plasma source does produce a flowing plasma with significant momentum. The flux data at the two axial locations verify the increase in momentum as the plasma flows through the diverging magnetic field in the magnetic nozzle region. Although the axial location for maximum momentum (or thrust) was not determined, forces of several millinewtons were measured for a combined neutral particle and plasma flow. The ionization fraction for these data was approximately 22 %. Neutral particles entrained in the flow provided a significant part of the flow momentum thereby contributing to the potential thrust. The technique of measuring the momentum flux and heat flux in a flowing plasma is described.

Development of a strong field helicon plasma source

We developed a high-density helicon plasma source with a very strong field of up to 10kG. Using a double-loop antenna wound around a quartz tube, 9.5cm in inner diameter and 90cm in axial length, initial plasmas with a high density more than 1013cm−3 were successfully produced with a radio frequency power less than a few kilowatts, and with changing magnetic fields, fill pressures, and gas species.

Plasma propulsion for interplanetary flight

The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is a high-power propulsion concept using radio waves to create and accelerate plasma in a magnetic nozzle. Important features are its high and variable exhaust velocity, which greatly enhances performance. A NASA-led, research team is developing this technology in the U.S. Recent advances include demonstration of efficient propellant utilization in its helicon plasma source and experimental verification of high ion acceleration through ion cyclotron wave resonance absorption, as predicted by theory. This paper describes the physics and engineering of VASIMR, reviews recent progress and discusses its application in a piloted Mars mission architecture.

Focusing magnetic field contribution for helicon plasma on Mini-RFTF

The Mini-Radio Frequency Test Facility (Mini-RFTF) has contributed to helicon plasma source optimization for the Variable Specific Impulse Magnetoplasma Rocket (VASIMR). In this paper, we report how the source, using hydrogen and helium, can be optimized for non-uniform geometries that are constrained to be suitable for VASIMR. The behavior of the density transition into high-density helicon operation was found to significantly depend on the magnetic field geometry. Operation indicates that a strongly focused magnetic geometry produces a continuously increasing density with increasing net power. For helium operation, mirror confinement effects possibly explain the observed plasma sustainment on upstream and downstream for several magnetic field geometries.