Helicon Plasma Source Research Articles

ADAMANT: A surface and volume integral-equation solver for the analysis and design of helicon plasma sources

We present a full-wave numerical tool, dubbed ADAMANT (Advanced coDe for Anisotropic Media and ANTennas), devised for the analysis and design of radiofrequency antennas which drive the discharge in helicon plasma sources. ADAMANT relies on a set of coupled surface and volume integral equations in which the unknowns are the surface electric current density on the antenna conductors and the volume polarization current within the plasma. The latter can be inhomogeneous and anisotropic whereas the antenna can have arbitrary shape. The set of integral equations is solved numerically through the Method of Moments with sub-sectional surface and volume vector basis functions. This approach allows the accurate evaluation of the current distribution on the antenna and in the plasma as well as the antenna input impedance, a parameter crucial for the design of the feeding and matching network. We report several numerical examples which serve to validate ADAMANT against other well-established numerical approaches as well as experimental data. The numerical accuracy of the computed solution versus the number of basis functions in the plasma is also assessed. Finally, we employ ADAMANT to characterize the antenna of a real-life helicon plasma source.

Double layer-like structures in the core of an argon helicon plasma source with uniform magnetic fields

A hot (Te ≈ 10 eV) electron population is observed in the core of a 3 mTorr argon helicon plasma source at 500 W RF power and 900 G uniform axial magnetic field strength, 12 cm from the edge of the helicon antenna. A double layer-like structure consisting of a localized axial electric field of approximately 8 V/cm over 1–2 cm is observed adjacent to the hot electron population. The potential step generated by the electric field is shown to be large enough to trap the hot electrons. To our knowledge this is the first observation of these structures in the core of a helicon discharge.

Preliminary Investigation of Electromagnetic Thrust Characteristics in Electrodeless Compact Helicon Plasma Thruster

In order to realize long-lived electric propulsion systems, we have been investigating an electrodeless plasma thruster. In our concept, high-density plasma from a compact helicon plasma source is accelerated by a magnetic nozzle for the thrust production. In order to optimize the thruster design, investigation of the thrust characteristics is needed. In this study, development of the electromagnetic thrust measurement system for compact helicon plasma thrusters and preliminary experiment of thrust measurement have been conducted. We have successfully developed a torsion-pendulum type thrust stand with measurement resolution of 10 N. From the thrust measurement, the electromagnetic thrust increases with the Ar gas mass flow rate and plasma production power. The thrust force and the electron density jump are observed due to the discharge mode transition from the inductively coupled plasma to the helicon wave excited plasma. The electromagnetic thrust force is up to 340 N (Ar gas mass flow rate of 1.0 mg/s, plasma production power of 400 W).

Ion production cost of a gridded helicon ion thruster

Helicon plasma sources are capable of efficiently ionizing propellants and have been considered for application in electric propulsion. However, studies that estimate the ion production cost of the helicon plasma source are limited and rely on estimates of the extracted ion current. The ion production cost of a helicon plasma source is determined using a gridded ion thruster configuration that allows accurate measurement of the ion beam current. These measurements are used in conjunction with previous characterization of the helicon plasma to create a model of the discharge plasma within the gridded thruster. The device is tested across a range of operating conditions: 343–600 W radio frequency power at 13.56 MHz, 50–250 G and 1.5 mg s−1 of argon at a pressure of 1.6 × 10−5 Torr-Ar. The ion production cost is 132–212 ± 28–46 eV/ion, driven primarily by ion loss to the walls and anode, as well as energy loss in the anode and grid sheaths.

Plasma-Wall Interaction Facilities in Korea

Although the research of plasma-material interaction (PMI) is rather immature comparing the recent success of Korean fusion program, there are several facilities and programs of PMI research in Korea. DiPS (Divertor Plasma Simulator)-2 is a linear device with a four-inch-LaB6 cathode at the Center for Edge Plasma Science (cEps), concentrating on the development of various diagnostics for divertor and scrape-off plasmas, and for PMI research such as tungsten and graphite related phenomena. This is modified from DiPS-1, which were for the simulations of divertor, space and processing plasmas using LaB6 and helicon plasma sources. MP2 (Multi-Purpose Plasma) is a linear device with an eight-inch-LaB6 cathode for PMI in National Fusion Research Institute (NFRI), and will be merged with molten salt (FLiNaK) experiment by using an Electron Cyclotron Resonance (ECR) plasma source. High power plasma torch facilities have been developed at the High-Enthalpy Plasma Research Center in ChonBuk National University, aiming for the development of new materials of the aerospace-, nano-, and automobile-industries, yet recently they have interest in fusion materials. Plasma Immersion Ion Implantation & Deposition (PIIID) facility has been utilized for the research of processing materials in Korean Institute of Science and Technology (KIST), and is to be used for fusion material researches with high energy ions (~70 keV). Electron beam irradiation has been tried for the research of graphite and tungsten at DanKook university. These facilities are to be utilized for the application to KSTAR, ITER and/or Korean DEMO fusion devices. Dust as the by-product of PMI in fusion device is to be characterized and removed in TReD (Transport & Removal experiment of Dust) device in Hanyang University. Plasma sources, diagnostics, and surface analyses of these programs will be explained with design philosophies and basic parameters of plasmas.

High Efficiency Helicon Plasma Source for PMI Studies

The development of fusion energy will require materials resilient to harsh bombardment by energetic plasma. Linear plasma devices are widely used to examine such materials by simulating power loads and particle fluxes, which are envisaged for the future reactor grade plasma devices. To correctly simulate these conditions, the plasma in the simulator should be running continuously and have high enough density, 1019m-3 or higher. A good candidate to produce such a plasma is a helicon plasma source. This paper reviews a helicon plasma source, which is under development in collaboration between the Budker Institute and Forshungzentrum Juelich, in perspective of its application at the JULE-PSI device.

High-Density Helicon Plasma Sources: Basics and Application to Electrodeless Electric Propulsion

The development of unique, high-density helicon plasma sources is described. Characterization of the largest and the smallest source sizes is made along with a discussion of particle production efficiency using Ar gas. Next, we describe an application of helicon sources to plasma propulsion using a new advanced concept without any eroding electrodes, as a review of our Helicon Electrodeless Advanced Thruster (HEAT) project.

Neutral Gas Expansion in a Cylindrical Helicon Discharge Chamber

The expansion of neutral gas in an open-ended cylindrical chamber into a large vacuum facility is investigated to understand neutral particle density distribution. The cylindrical chamber geometry with a single on-axis gas injection port is representative of a helicon plasma source discharge chamber. The neutral gas pressure profile along the cylinder wall is experimentally measured for argon and xenon mass flow rates of 0–200 sccm, at facility pressures up to and and torr Xe, respectively. The average relative error between the experimentally measured pressure profiles and the numerical results obtained with a direct simulation Monte Carlo code is less than 10%, with the maximum relative error below 30%. The single on-axis injection port into the cylindrical discharge chamber results in a nonuniform neutral particle density distribution in the source due to the recirculation zone generated in the rear of the discharge chamber. This highlights the potential impact of the propellant injection system design on the neutral particle depletion phenomenon in helicon plasma sources.

Thrust Measurements of a Radio Frequency Plasma Source

There is interest in the use of a helicon plasma source in propulsive applications as both an ion source and a thruster. Development of a helicon thruster requires a performance baseline as a basis for future optimization and modification. For the first time, the thrust of a helicon plasma source is measured using a null-type inverted pendulum thrust stand at an operating pressure of through the operating range of 215–840 W RF power, 11.9 and 13.56 MHz RF frequency, 150–450 G magnetic field strength, and propellant flow rate for argon. Maximum thrust is found to be 6.3 mN at a specific impulse of 140 s and a maximum specific impulse of 380 s at 5.6 mN. Thrust efficiency is less than 1.4% and demonstrates very-low-power coupling to ion acceleration.

Ion heating and short wavelength fluctuations in a helicon plasma source

For typical helicon source parameters, the driving antenna can couple to two plasma modes; the weakly damped “helicon” wave, and the strongly damped, short wavelength, slow wave. Here, we present direct measurements, obtained with two different techniques, of few hundred kHz, short wavelength fluctuations that are parametrically driven by the primary antenna and localized to the edge of the plasma. The short wavelength fluctuations appear for plasma source parameters such that the driving frequency is approximately equal to the lower hybrid frequency. Measurements of the steady-state ion temperature and fluctuation amplitude radial profiles suggest that the anomalously high ion temperatures observed at the edge of helicon sources result from damping of the short wavelength fluctuations. Additional measurements of the time evolution of the ion temperature and fluctuation profiles in pulsed helicon source plasmas support the same conclusion.

Fusion modeling approach for novel plasma sources

The physics involved in the coupling, propagation and absorption of RF helicon waves (electronic whistler) in low temperature Helicon plasma sources is investigated by solving the 3D Maxwell-Vlasov model equations using a WKB asymptotic expansion. The reduced set of equations is formally Hamiltonian and allows for the reconstruction of the wave front of the propagating wave, monitoring along the calculation that the WKB expansion remains satisfied. This method can be fruitfully employed in a new investigation of the power deposition mechanisms involved in common Helicon low temperature plasma sources when a general confinement magnetic field configuration is allowed, unveiling new physical insight in the wave propagation and absorption phenomena and stimulating further research for the design of innovative and more efficient low temperature plasma sources. A brief overview of this methodology and its capabilities has been presented in this paper.

Flux Tailoring of a Permanent-Magnet Field With a High-Temperature Superconducting Tube for a Helicon Plasma Source

In this paper, we explore modifying the field of a permanent magnet through the use of the Meissner effect, motivated by application to a helicon plasma source. The use of a high-temperature superconducting material is more practical for maintaining a system-wide critical temperature, and it is therefore the focus of our investigation. The field supplied by a permanent magnet is confined to the inside of the tube to obtain a uniform field solution analogous to that of an inviscid incompressible fluid. Discussed are both the theoretical results, which neglect flux pinning and experimental results that show its effect. The experiment to assess the approach uses a tube of Bi-2223 cooled with liquid nitrogen inside of a vacuum chamber. Further analysis shows that the experimental and theoretical results agree and that shaping of the magnetic fields is in fact possible.

Radiofrequency antenna for suppression of parasitic discharges in a helicon plasma thruster experiment

A radiofrequency (rf) antenna for helicon plasma thruster experiments is developed and tested using a permanent magnets helicon plasma source immersed in a vacuum chamber. A magnetic nozzle is provided by permanent magnets arrays and an argon plasma is produced by a 13.56 MHz radiofrequency helicon-wave or inductively-coupled discharge. A parasitic discharge outside the source tube is successfully suppressed by covering the rf antenna with a ceramic ring and a grounded shield; a decrease in the ion saturation current of a Langmuir probe located outside the source tube is observed and the ion saturation current on axis increases simultaneously, compared with the case of a standard uncovered rf antenna. It is also demonstrated that the covered antenna can yield stable operation of the source.

Characteristics of low-aspect ratio, large-diameter, high-density helicon plasmas with variable axial boundary conditions

A low-aspect ratio, high-density helicon plasma source with a large-diameter of ∼74 cm that utilizes an end-launch flat-spiral antenna has been characterized under three different axial boundary conditions. Whereas one end of the device is a quartz-glass window through which an excitation rf wave is injected, the other end is a movable plasma terminating plate of three different kinds: (1) metal with small holes, (2) solid metal, and (3) solid insulator. Using this movable plate, the device aspect ratio A (device axial length/device diameter) can be reduced to ∼0.075 corresponding to the device axial length of 5.5 cm. The plasma production efficiency (PPE, defined as the ratio of the total number of electrons in the plasma to the input rf power) and helicon wave structures are examined for plasmas with various aspect ratios and boundary conditions to characterize our helicon device. Even for the lowest aspect ratio case (A ∼0.075), a plasma with the electron density of 7.5 × 1011 cm−3 can be produced. The PPE of our device is higher than that of other helicon devices that utilize winding-type antennas. Discrete axial wave modes, which can be explained by a simple model, have been identified for helicon waves excited in our low-aspect ratio helicon plasmas. A comparison between the experimental results and helicon wave theory suggests that second order radial modes must have been excited when the electron density is sufficiently high.

Study on Helicon Plasma Lissajous Acceleration for Electrodeless Electric Propulsion

(Received June 27th, 2011) In order to realize a long lifetime of an electric propulsion system, we have been investigating various electrodeless electric propulsion concepts utilizing a helicon plasma source. In one of our concepts, helicon plasma is electromagnetically accelerated using a rotating electric field in the presence of a diverging static magnetic field. This acceleration concept is called the Lissajous acceleration. Plasma acceleration experiments have been conducted and plasma acceleration was evaluated using a Mach probe. Although the experiments showed some features of the electromagnetic acceleration, most increment of the plasma velocity is caused by the increment of the electron temperature. The thrust (4.95 μN) did not reach feasible values for real applications, and therefore, it is important to find a better operational condition with the aid of a theoretical thrust model. We have developed a theoretical thrust model which consists of a trajectory analysis and an electric field penetration model in the electrostatic approximation. The model shows that experimental parameters are off from an optimum operational condition which provides the maximum thrust.

Ducted kinetic Alfvén waves in plasma with steep density gradients

Given their high plasma density (n ∼ 1013 cm−3), it is theoretically possible to excite Alfvén waves in a conventional, moderate length (L ∼ 2 m) helicon plasma source. However, helicon plasmas are decidedly inhomogeneous, having a steep radial density gradient, and typically have a significant background neutral pressure. The inhomogeneity introduces regions of kinetic and inertial Alfvén wave propagation. Ion-neutral and electron-neutral collisions alter the Alfvén wave dispersion characteristics. Here, we present the measurements of propagating kinetic Alfvén waves in helium helicon plasma. The measured wave dispersion is well fit with a kinetic model that includes the effects of ion-neutral damping and that assumes the high density plasma core defines the radial extent of the wave propagation region. The measured wave amplitude versus plasma radius is consistent with the pile up of wave magnetic energy at the boundary between the kinetic and inertial regime regions.

Development of a compact permanent magnet helicon plasma source for ion beam bioengineering

A compact helicon plasma source was developed as a millimeter-sized ion source for ion beam bioengineering. By employing a stacked arrangement of annular-shaped permanent magnets, a uniform axial magnetic flux density up to 2.8 kG was obtained. A cost effective 118 MHz RF generator was built for adjusting forward output power from 0 to 40 W. The load impedance and matching network were then analyzed. A single loop antenna and circuit matching elements were placed on a compact printed circuit board for 50 Ω impedance matching. A plasma density up to 1.1 × 10(12) cm(-3) in the 10 mm diameter tube under the magnetic flux density was achieved with 35 W applied RF power.

VX-200 Magnetoplasma Thruster Performance Results Exceeding Fifty-Percent Thruster Efficiency

H IGH-POWER electric propulsion thrusters can reduce propellant mass for heavy-payload orbit-raising missions and cargo missions to the moon and near-Earth asteroids, and they can reduce the trip time of robotic and piloted planetary missions [1–4]. The Variable Specific Impulse Magnetoplasma Rocket (VASIMR®) VX-200 engine is an electric propulsion system capable of processing power densities on the order of 6 MW=m with a high specific impulse (4000 to 6000 s) and an inherent capability to vary the thrust and specific impulse at a constant power. The potential for a long lifetime is due primarily to the radial magnetic confinement of both ions and electrons in a quasi-neutral flowing plasma stream, which acts to significantly reduce the plasma impingement on the walls of the rocket core. High-temperature ceramic plasma-facing surfaces handle the thermal radiation: the principal heat transfer mechanism from the discharge. The rocket uses a helicon plasma source [5,6] for efficient plasma production in the first stage. This plasma is energized further by an ion cyclotron heating (ICH) RF stage that uses left-hand polarized slow-mode waves launched from the high field side of the ion cyclotron resonance. Useful thrust is produced as the plasma accelerates in an expanding magnetic field: a process described by conservation of the first adiabatic invariant as the magnetic field strength decreases in the exhaust region of the VASIMR [7–9]. End-to-end testing of the VX-200 engine has been undertaken with an optimum magnetic field and in a vacuum facility with sufficient volume and pumping to permit exhaust plumemeasurements at low background pressures. Experimental results are presented with the VX-200 engine installed in a 150 m vacuum chamber with an operating pressure below 1 10 2 Pa (1 10 4 torr), and with an exhaust plume diagnostic measurement range of 5 m in the axial direction and 1 m in the radial directions. Measurements of plasma flux, RF power, and neutral argon gas flow rate, combined with knowledge of the kinetic energy of the ions leaving the VX-200 engine, are used to determine the ionization cost of the argon plasma. A plasmamomentum flux sensor (PMFS)measures the force density as a function of radial and axial positions in the exhaust plume. New experimental data on ionization cost, exhaust plume expansion angle, thruster efficiency, and total force are presented that characterize the VX-200 engine performance above 100 kW. A semiempirical model of the thruster efficiency as a function of specific impulse has been developed to fit the experimental data, and an extrapolation to 200 kWdc input power yields a thruster efficiency of 61% at a specific impulse of 4800 s.

Characterization of Silicon Oxide Films Deposited from a Permanent‐Magnet Helicon Plasma Source

AbstractSilicon oxide films were deposited by an individual PM helicon source using HMDSO as a precursor and N2O as the reactive gas at low temperature. The chemical structure and elemental composition of the films are characterized by Fourier transform infrared spectroscopy and X‐ray photoelectron spectroscopy, and the optical and mechanical properties of the films are evaluated by a thin‐film metrology system using reflectance spectra and a nanoindenter. The experimental results show that the N2O ratio in the plasma has a strong impact on the optical and structural characteristics of the deposited films: with increasing N2O flow ratios, the hardness of the deposited films increases while the deposition rate and carbon content decreases. At the same time, the characteristics of the oxide films gradually shift away from those of polymers towards those of SiO2. The technique presented here may find applications in making barrier films for polymer substrates. magnified image

Characterization of a helicon plasma source in low diverging magnetic fields

A detailed experimental characterization has been performed of a helicon mode in low nonuniform magnetic fields (B0 < 5 mT) for pressures between 0.04 Pa < p0 < 0.4 Pa and rf powers between 50 W < P0 < 400 W, using a number of electrostatic probes as well as circuit measurements of the antenna and matching network system. The helicon mode is produced over a narrow range of magnetic field values, where a distinct density peak is formed that becomes broader (and higher) as the power or pressure is increased. The density peak is found to shift to larger magnetic fields for increasing powers or pressures, giving an almost linear relationship between the maximum density and the magnetic field at this maximum density, in agreement with helicon dispersion theory. The density within the peak (>1017 m−3) is more than an order of magnitude larger than that before or after, and is associated with a corresponding peak in the measured antenna resistance, thus showing a larger percentage of the input power is deposited within the plasma.