Helicon Device Research Articles

Development of a compact high-density blue core helicon plasma device under 2000 G magnetic field of ring permanent magnets

A helicon wave plasma source in a tube of ring permanent magnets (PMs) has been constructed to study the effect of the configuration of the magnetic field with zero magnetic points on plasma parameters. This device also serves as an exploration platform for a simple, compact helicon wave plasma source adaptable to engineering applications. A small-diameter (26 mm) high-density (∼1018 m−3) blue core plasma is produced in ∼1 Pa argon by helicon RF (radio-frequency) discharge using a Nagoya III antenna under magnetic field (∼2 kG) of compact ring PMs (length ∼204 mm). Operational parameters, i.e. RF power and neutral gas pressure are scanned and plasma density is measured by an RF compensated probe to explore the operating characteristics of the device. Iconic feature of a helicon discharge, such as blue core plasmas and E-H-W mode transitions are well observed in the device, despite the wavelength calculated using the conventional dispersion relation of a bounded whistler waves (Chen 1991 Plasma Phys. Control. Fusion 33 339) is order of magnitudes longer than the length of the plasma in this device which seems to suggest that such helicon device is impossible. Surprisingly, the wavelength calculated by the unbounded whistle wave dispersion formula in turn suggests the occurrence of a half wavelength resonance.

Helicon antenna radiation patterns in a high-density hydrogen linear plasma device

Antenna radiation patterns in the vicinity of a helicon antenna are investigated in hydrogen plasmas produced in the MAGPIE linear plasma device. Using a uniform cold-plasma full-wave code, we model the wave physics in MAGPIE and find good agreement with experimental wave measurements. We show for the first time which antenna elements in a helicon device couple most strongly to the plasma and discuss the physical mechanism that determines this effect. Helicon wavefields in the near field of the antenna are best described in terms of the group velocity and ray direction, while far from the antenna, helicon wavefields behave like plane waves and are best described in terms of eigen-modes. In addition, we present recent 2D axis-symmetric full-wave simulations of the 120 kW helicon source in ProtoMPEX [Rapp et al., IEEE Trans. Plasma Sci. 44(12), 3456–3464 (2016); Caughman et al., J. Vac. Sci. Technol. Vac. Surf. Films 35, 03E114 (2017); and Goulding et al., Fusion Sci. Technol. 72(4), 588–594 (2017)] (ne∼ 5 × 1019 m−3, B0∼ 70 mT, and f= 13.56 MHz) where the antenna radiation patterns are evident, and we provide an interpretation of the numerical results using the ideas developed in this paper.

RFEA Measurements of High-Energy Electrons in a Helicon Plasma Device with Expanding Magnetic Field

In the inductively coupled plasma of the Njord helicon device we have, for the same parameters as for which an ion beam exists, measured a downstream population of high-energy electrons emerging from the source. Separated measurements of energetic tail electrons was carried out by Retarding Field Energy Analyzer (RFEA) with a grounded entrance grid, operated in an electron collection mode. In a radial scan with the RFEA pointed toward the source, we found a significant population of high-energy electrons just inside the magnetic field line mapping to the edge of the source. A second peak in high-energy electrons density was observed in a radial position corresponding to the radius of the source. Also, throughout the main column a small contribution of high-energy electrons was observed. In a radial scan with a RFEA biased to collect ions a localized increase in the plasma ion density near the magnetic field line emerging from the plasma near the wall of the source was observed. This is interpreted as a signature of high-energy electrons ionizing the neutral gas. Also, a dip in the floating potential of a Langmuir probe is evident in this region where high-energy electrons is observed.

Negative hydrogen ion production in a helicon plasma source

In order to develop very high energy (>1 MeV) neutral beam injection systems for applications, such as plasma heating in fusion devices, it is necessary first to develop high throughput negative ion sources. For the ITER reference source, this will be realised using caesiated inductively coupled plasma devices, containing either hydrogen or deuterium discharges, operated with high rf input powers (up to 90 kW per driver). It has been suggested that due to their high power coupling efficiency, helicon devices may be able to reduce power requirements and potentially obviate the need for caesiation due to the high plasma densities achievable. Here, we present measurements of negative ion densities in a hydrogen discharge produced by a helicon device, with externally applied DC magnetic fields ranging from 0 to 8.5 mT at 5 and 10 mTorr fill pressures. These measurements were taken in the magnetised plasma interaction experiment at the Australian National University and were performed using the probe-based laser photodetachment technique, modified for the use in the afterglow of the plasma discharge. A peak in the electron density is observed at ∼3 mT and is correlated with changes in the rf power transfer efficiency. With increasing magnetic field, an increase in the negative ion fraction from 0.04 to 0.10 and negative ion densities from 8 × 1014 m−3 to 7 × 1015 m−3 is observed. It is also shown that the negative ion densities can be increased by a factor of 8 with the application of an external DC magnetic field.

Localized electron heating and density peaking in downstream helicon plasma

Localized electron temperature and density peaking at different axial locations in the downstream helicon plasma have been observed in a linear helicon device with both geometrical and magnetic expansion. The discharge is produced with an right helical antenna powered by a RF source operating at 13.56 MHz. Axial wave field measurement shows the presence of damped helicon waves with standing wave character folded into it even at low densities ( m). The measured helicon wavelength is just about twice the antenna length and the phase velocity is almost the speed required for electron impact ionization. These experimental observations strongly advocate the Landau damping heating and density production by the helicon waves, particularly in low density plasma such as ours. The electron temperature maximizes at 35–45 cm away from the antenna center in our experiments indicating a local source of heating at those locations. Different mechanisms responsible for this additional heating at a particular spatial location have been discussed for their possible roles. Further downstream from the location of the maximum electron temperature, a density peak located 55–65 cm away from the antenna is observed. This downstream density peaking can be explained through pressure balance in the system.

A segmented multi-loop antenna for selective excitation of azimuthal mode number in a helicon plasma source.

A flat type, segmented multi-loop antenna was developed in the Tokai Helicon Device, built for producing high-density helicon plasma, with a diameter of 20 cm and an axial length of 100 cm. This antenna, composed of azimuthally splitting segments located on four different radial positions, i.e., r = 2.8, 4.8, 6.8, and 8.8 cm, can excite the azimuthal mode number m of 0, ±1, and ±2 by a proper choice of antenna feeder parts just on the rear side of the antenna. Power dependencies of the electron density ne were investigated with a radio frequency (rf) power less than 3 kW (excitation frequency ranged from 8 to 20 MHz) by the use of various types of antenna segments, and n(e) up to ~5 × 10(12) cm(-3) was obtained after the density jump from inductively coupled plasma to helicon discharges. Radial density profiles of m = 0 and ±1 modes with low and high rf powers were measured. For the cases of these modes after the density jump, the excited mode structures derived from the magnetic probe measurements were consistent with those expected from theory on helicon waves excited in the plasma.

Development of very small-diameter, inductively coupled magnetized plasma device

In order to miniaturize a high-density, inductively coupled magnetized plasma or helicon plasma to be applied to, e.g., an industrial application and an electric propulsion field, small helicon device has been developed. The specifications of this device along with the experimental results are described. We have succeeded in generating high-density (~10(19) m(-3)) plasmas using quartz tubes with very small diameters of 10 and 20 mm, with a radio frequency power ~1200 and 700 W, respectively, in the presence of the magnetic field less than 1 kG.

Interpretation of Ion Velocity Distributions Measured with a Grounded Retarding Field Energy Analyzer (RFEA) in an Inductively Coupled Helicon Plasma

AbstractA Retarding Field Energy Analyzer (RFEA) with a grounded surface in a plasma will be surrounded by a sheath. The ion distribution function (IDF) measured at the probe will be distorted as compared to the IDF in a plasma unaffected by the sheath.In the present study we investigate whether a practical method can relate the measured distribution function to the undisturbed IDF, thus allowing for derivation of useful plasma parameters from the measured IDFs.We study the IDF in the sheath by 3D numerical simulations of a spherical RFEA for parameters similar to the Njord helicon device at the University of Tromsø. By comparison with the numerical results, we critically assess the results from simple 1D models that are often used in the analysis of the RFEA signal. With the PIC simulations we also show that the acceptance angle of the probe has a significant effect on the low energy part of the measured IDF. This result is compared with real probe measurements from the Njord device (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

On the Measurement of Subsonic Flow in a Capacitively Coupled Helicon Plasma Source

Plasma parameters and the subsonic flow from a capacitively coupled, cylindrical plasma source of the Njord helicon device are investigated by means of a Mach probe and a retarding field energy analyzer (RFEA). 13.56 MHz and 600 W RF power is inserted into the argon working gas under low-pressure conditions and moderate magnetic field. By means of a downstream field coil, the magnetic field is shaped from a purely expanding field to a configuration with more parallel field lines. It is shown that the downstream plasma density along the outer rim of the source increases significantly and there is a sudden increase by nearly 20 V in the plasma potential already after a moderate increase in the downstream magnetic field. The investigation of the flow indicates that current ratios derived from the Mach probe result in an apparent flow in the direction towards the source, while the current bratios derived from the RFEA indicate a flow in the direction away from the source. PIC simulations demonstrate that the acceptance angle of the probes, being nearly 180o for the Mach probe, and about 45o for the RFEA, can critically affect the current ratios and hence the subsonic flow measured by the probes in the weakly magnetized plasma in our device. The first section in your paper

Magnet Configuration and Experimental Analysis of Helicon Source for Space Magnetoplasma Propulsion

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <?Pub Dtl=""?>The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is one of the high performance electric propulsion devices for future interplanetary applications. Helicon sources can produce high-density plasmas with high-ionization efficiency suitable for VASIMR, which usually use two electromagnets to produce the required magnetic field configuration. How to optimal design the solenoids and how to create the requisite B-field are the key factors to affect the characteristics and performance of the helicon plasma. In this paper, the magnets were designed and fabricated, and an experimental helicon source was presented. Two solenoids situated around the cylindrical quartz tube were used to create an expanding magnetic field of about 250 G in the center decreasing to a few tens of Gauss in the downstream diffusion region. The antenna was powered by a radio-frequency (RF) system of 13.56 MHz, which maximum changeable power is 500 W. Our helicon device was installed with vacuum chamber that is pumped down to a base pressure of about 0.001 Pa using cryopumps. Langmuir probe is mounted on to measure the plasma parameters. It is found that, when increasing the radio-frequency power or varying the magnetic field, both the plasma density and operating mode of helicon-wave ion are also changed. Permanent magnets (PM) and High temperature superconducting (HTS) magnets are also considered to use in the helicon source, and relative analyses are discussed. The details of further research are to investigate the relationship that the applied magnets affect on the efficiency and performance of the helicon plasma. </para>

Negative Ions in Helicon Oxygen Discharges

AbstractThe negative oxygen ion density is studied experimentally in the high‐density helicon device VINETA by laserinduced photodetachment. The measurements indicate a negative ion density being higher than the electron density in contrast to previous experiments in helicon oxygen discharges. The dependency of the relative negative ion density on the oxygen gas pressure and rf‐power is in good qualitative agreement with observations in low temperature RF inductive and capacitive discharges, but differ quantitatively by one order of magnitude. To understand the basic physics in the helicon discharges, the measurements are compared to a zero dimensional reaction rate model of the electronegative oxygen discharge. The particle densities are calculated via rate equations estimated fromcollisional cross sections assumingMaxwellian electron energy distribution functions. The model shows that the electron temperature is the most sensitive parameter governing the formation of negative oxygen ions. Additionally, the differences to previous experimental measurements in rf discharges can be explained by different electron temperature and plasma losses to the walls (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Plasma Profiles in a Cylindrical Helicon Discharge with Converging Magnetic Source Field

AbstractDetailed Langmuir probe measurements of the plasma density, plasma potential, and electron temperature in the cylindrical helicon device VINETA [C. M. Franck et al., Phys. Plasmas 9(8), 2002] are presented. The probe measurements cover the entire radial‐axial plane including the helicon antenna region. Two magnetic field configurations are compared: homogeneous magnetic field in the helicon antenna region and magnetic field gradient at the transition between the antenna and the discharge chamber. It is demonstrated that the converging magnetic field leads to the formation of steep plasma density and potential increase, which is strongly correlated with the position of the maximum field gradient. No electron heating detached from the helicon source is observed. The measurements strongly suggest that the plasma profiles are the direct result of volume reduction due to magnetic field convergence, which leads to the formation of a single sheath‐like layer in front of the helicon source. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Radial propagation of structures in drift wave turbulence

The formation and propagation of spatiotemporal fluctuation structures in weakly developed drift-wave turbulence in a linearly magnetized helicon device is investigated. Turbulent density fluctuations in the far edge plasma display an intermittent character with large-amplitude positive density bursts. Their peak amplitudes correspond to the time-averaged density in the maximum radial plasma pressure gradient. The conditional average technique is applied to reconstruct the dynamics of turbulent coherent structures in the azimuthal plane. The formation of turbulent structures is closely linked to a quasicoherent m=1 drift wave mode, which is generally observed in the radial density gradient region in the weakly developed turbulent state. It is demonstrated that every positive high amplitude density burst in the plasma edge is due to the radial propagation of a turbulent structure. The typical scale size of the turbulent structures is 4ρs and their lifetime exceeds the eddy turnover time by orders of magnitude, thereby characterizing them as coherent structures. Although the turbulent structures propagate mainly azimuthally in the direction of the E×B drift they are observed to have a radial velocity, which is typically 10% of the ion sound speed.

Propagating structures in drift-wave turbulence

The formation and propagation of spatiotemporal fluctuation structures in drift-wave turbulence in a linearly magnetized helicon device is investigated. Structures are extracted by correlation analysis. Their typical lifetime exceeds the eddy turnover time by one order of magnitude. The radial correlation analysis yields radial propagation of turbulent fluctuations with velocities up to 50% of the ion sound speed. The two-dimensional analysis of the entire azimuthal cross-sections reveals that the propagation is purely azimuthal and that the radial speeds are an artefact of the one-dimensional measurement of a multi-dimensional propagation.

Plasma performance in very large helicon device

A large volume (75 cm in diameter and 486 cm in axial length) helicon-plasma device has been developed, and the plasma characteristics in the device have been investigated. With the use of a large (43 cm in outer diameter) spiral antenna located just outside a quartz-glass window at the end of the vacuum chamber, the device is capable of producing high-density plasma (gas species used so far are argon (Ar) and helium (He)) of n e ≥ 10 12 cm − 3 with moderate rf input power of less than 1 kW, showing excellent discharge efficiency. With this device it is possible to control the radial density profile employing the following two techniques: (1) changing the magnetic-field configuration near the antenna and (2) changing the antenna radiation-field patterns.

Density profile control in a large diameter, helicon plasma

Plasmas with peaked radial density profiles have been generated in the world’s largest helicon device, with plasma diameters of over 70 cm. The density profiles can be manipulated by controlling the phase of the current in each strap of two multistrap antenna arrays. Phase settings that excite long axial wavelengths create hollow density profiles, whereas settings that excite short axial wavelengths create peaked density profiles. This change in density profile is consistent with the cold-plasma dispersion relation for helicon modes, which predicts a strong increase in the effective skin depth of the rf fields as the wavelength decreases. Scaling of the density with magnetic field, gas pressure, and rf power is also presented.

Calculation of reactor impedance for helicon discharge

We solve the Maxwell-Boltzmann equations and calculate reactor impedance for helicon device with the Nagoya type III antenna using a finite, width modeling of antenna current and inspect the dependence of various components of impedance on discharge parameters. The total resistance is divided into various components such as electron-ion contributions and axial-perpendicular field contributions. Moreover, the total resistance can be approximately expressed as the sum of TG and helicon contributions in a wide range of parameters.