Helicon Wave Propagation Research Articles

Experimental and simulation study of argon helicon discharge in multiple plasma simulation linear device (MPS-LD)

Abstract To obtain a high-parameter plasma in the target region of a multiple plasma simulation linear device and to realize the experimental simulation environment of tokamak divertor plasma, experimental and numerical simulations of argon helicon discharge are carried out. Langmuir probes are used to diagnose the electron density (ne ) in the source and target regions with different experimental parameters (magnetic field, radio frequency power, puffing flow rate). A three-dimensional discharge model is developed using drift-diffusion equations of electron density and electron energy with the aid of COMSOL. Helicon discharge with a long straight plasma beam and a bright blue core is experimentally achieved. The simulation and experimental results are compared, validating the model. The corresponding spatial ne distribution is obtained, and the dependence of ne on the main experimental parameters is confirmed. The energy conversion relationship between the helicon and plasma is found. Helicon waves prefer to transfer energy to the plasma in the source region, where ne is significantly increased. This results in a strong ne gradient, which acts as a barrier to prevent the propagation of helicon waves. Therefore, localized standing helicon waves are formed, which limits the increase in plasma density in the target region. By increasing the magnetic field strength (B < 1500 G) and RF power (P < 1500 W), ne in the source region can be increased, but they have little effect on ne in the target region.

Influence of Magnetic Field Configuration in Helicon-Negative Ion Source on Increase of the Efficiency of NNBI Heating System

The influence of the magnetic field configuration on the performance of a helicon-based negative ion source is investigated with simulation experiments. Using COMSOL Multiphysics software, a three-dimensional simulation model for a negative ion source, based on a helicon plasma source, is presented in two magnetic field configurations: uniform and nonuniform configurations. The helicon plasma source employed a Nagoya-type antenna to apply radio-frequency (RF) power at a frequency of 13.56 MHz. The injected gas is hydrogen with a flow of 10 standard cubic centimeters per minute. Using a three-dimensional model, helicon wave propagation in the presence of a magnetic filter and the energy absorption mechanism in the helicon system are investigated. In this context, in the presence of the two magnetic field configurations, the influence of the important parameters’ working pressure and RF power on the optimization of negative ion production under volume mode is studied. Six electromagnetic coils at the same current are used for producing the magnetic field in both cases of uniform and nonuniform configurations. The variation of the electron density and electron temperature, in both regions of driver and expansion, are calculated and represented with respect to the different power and the gas pressure. The simulation results of the negative ion density in the expansion region for the uniform and nonuniform magnetic field configurations are compared. The results indicate that at the same applied current of coils, the negative ion density in the presence of the nonuniform magnetic field is about 1.75 times higher than the negative ion density of the uniform case. Moreover, the results show that the negative ion density is decreased by decreasing the magnetic field of the driver region in the nonuniform cases.

A novel Doppler backscattering (DBS) system to simultaneously measure radio frequency plasma fluctuations and low frequency turbulence.

A novel quadrature Doppler Backscattering (DBS) system has been developed and optimized for the E-band (60-90GHz) frequency range using either O-mode or X-mode polarization in DIII-D plasmas. In general, DBS measures the amplitude of density fluctuations and their velocity in the lab frame. The system can simultaneously monitor both low-frequency turbulence (f < 10MHz) and radiofrequency plasma density fluctuations over a selectable frequency range (20-500MHz). Detection of high-frequency fluctuations has been demonstrated for low harmonics of the ion cyclotron frequency (e.g., 2fci ∼ 23MHz) and externally driven high-frequency helicon waves (f = 476MHz) using an adjustable frequency down conversion system. Importantly, this extends the application of DBS to a high-frequency spectral domain while maintaining important turbulence and flow measurement capabilities. This unique system has low phase noise, good temporal resolution (sub-millisecond), and excellent wavenumber coverage (kθ ∼ 1-20cm-1 and kr ≲ 30cm-1). As a demonstration, localized internal DIII-D plasma measurements are presented from turbulence (f ≤ 5MHz), Alfvenic waves (f ∼ 6.5MHz), ion cyclotron waves (f ≥ 20MHz), as well as fluctuations around 476MHz driven by an external high-power 476MHz helicon wave antenna. In the future, helicon measurements will be used to validate GENRAY and AORSA modeling tools for prediction of helicon wave propagation, absorption, and current drive location for the newly installed helicon current drive system on DIII-D.

Helicon full-wave modeling with scrape-off-layer turbulence on the DIII-D tokamak

Helicon waves have been recently proposed as an off-axis current drive actuator due to their expected high current drive efficiency in the mid-radius region in high beta tokamaks. This paper focuses on a numerical study to better understand effects of scrape-off-layer (SOL) turbulence on helicon wave propagation and absorption on the DIII-D tokamak using a recently developed helicon full-wave model with turbulent density inputs from synthetic single wavelength SOL turbulence and first-principles HERMES multi-wavelength turbulence models. With both input turbulence models, three key effects are observed: the helicon wave can scatter to undesirable locations in the plasma, large helicon wave electric fields can form in localized regions near the SOL turbulence, and the helicon wave can mode convert to slow waves in the SOL. This is shown to cause helicon wave refraction to undesirable locations and strong helicon wave absorption in the SOL resulting in significantly less helicon wave power in the core plasma. Using synthetic SOL turbulence, the simulations additionally show that high amplitudes and long wavelengths greater than a few cm on average have the largest effect on modifying the helicon wave propagation and absorption; the modeling predicts, for example, that approximately 60% of helicon power can be absorbed in the SOL for ñ/n ∼ 0.8 and lambda_perp ∼ 0.05 m. Several potential physical mechanisms that may explain the interaction of helicon waves with SOL turbulence in these simulations are discussed.

Optimization of helicon wave off-axis current drive in CFETR tokamak

The China Fusion Engineering Testing Reactor (CFETR) is a tokamak reactor and aims to eventually reach DEMO relevant fusion power level of 1 GW. A lower single null configuration of the divertor design is proposed for the machine. To obtain the high performance operation of the plasma, both the hybrid and steady-state operating scenarios are suggested. In these scenarios, the plasma beta of 〈βe〉∼1.6% has been generally reached. In the paper, helicon wave propagation and current drive in CFETR are studied through GENRAY/CQL3D code. An analysis of the wave damping factor in CFETR plasma indicates that the electron Landau damping is dominant for the core plasma. According to the dimensionless parameter ξe and βe, a strong wave damping region is found. Based on the GENRAY simulation, the single pass power absorptions are generally obtained in both scenarios. Scanning of generated current on the launched poloidal angle indicates that high current drive efficiency is produced at θ∼300. For helicon wave with frequency f=700 MHz, calculations of the current drive show that off-axis current drive about 30 kA/MW and 50 kA/MW are obtained in the hybrid and steady-state scenarios respectively. Simultaneously, the lower plasma density and high temperature promote the high current generation. Moreover, the obtained current drive and its peak position are shown to be sensitive to the launched n//. Finally, the helicon wave with frequency f=700 MHz and launched n//∼2.0–3.0 is proposed to be a promising scheme for effective off-axis current drive in CFETR.

Negative ion characterization in a helicon plasma source for fusion neutral beams by cavity ring-down spectroscopy and Langmuir probe laser photodetachment

Negative ions are characterized in the helicon plasma source resonant antenna ion device (RAID) at the Swiss plasma center by means of cavity ring-down spectroscopy (CRDS) and Langmuir probe (LP)-assisted laser photodetachment. A high density and axially homogeneous plasma column is produced via a RF antenna able to sustain the propagation of helicon waves in a steady state regime. An electron density in plasma at 0.3 Pa and 3 kW of input power is measured in the center of the plasma column by LP and microwave interferometry. The electron temperature profile is peaked on axis reaching and decreasing to 1.5 eV at . Thus, a hot core region forms where molecules are rovibrationally excited (), and a cold edge, where low energy electrons can attach to and produce ions by dissociative attachment. In this work we use LP-assisted laser photodetachment and CRDS diagnostics to measure and radial density profiles and how they depend on source parameters. We show that negative ions are distributed on a shell of 0.06 m radius with a peak value of ∼ in plasma. These results suggest that, although substantial technical development is needed, helicon plasmas could be considered as a possible candidate as sources of negative ions for future NBIs.

First B-dot measurements in the RAID device, an alternative negative ion source for DEMO neutral beams

As a new concept of Neutral Beam Injectors (NBI) for DEMO-like reactors, SIPHORE (IRFM, CEA, in France) expects to extract negative deuterium ions and photo-neutralize the accelerated D-. The Swiss Plasma Center (SPC) of EPFL is involved in this project by developing an innovative helicon source, which could provide the adequate D2 negative ion blade-shaped plasma, in terms of density and homogeneity along the axial direction. In the Resonant Antenna Ion Device (RAID), the test bed, a helicon wave is sustained by a resonant antenna plasma source at 13.56 MHz (input power ≤ 10 kW), connected to a cylindrical vacuum chamber (1.5 m long, 0.4 m diameter) and is surrounded by 6 Helmholtz coils, providing a DC magnetic field up to 800 G on axis. To characterize the helicon wave propagation, RAID has been recently equipped with a three-axis magnetic probe (B-dot). The paper describes the RAID experiment and its helicon source, including a 3D characterization of density and temperature, together with the B-dot design and calibration. It presents measurements of helicon wave propagation; in typical H2 plasmas (0.3 Pa), preliminary results show a helicon wave right-handed polarized with a wavelength of approximately 240 mm.

Collisional damping of helicon waves in a high density hydrogen linear plasma device

In this paper, we investigate the propagation and damping of helicon waves along the length (50 cm) of a helicon-produced 20 kW hydrogen plasma ( 1–2 1019 m−3, 1–6 eV, H2 8 mTorr) operated in a magnetic mirror configuration (antenna region: 50–200 G and mirror region: 800 G). Experimental results show the presence of traveling helicon waves (4–8 G and 10–15 cm) propagating away from the antenna region which become collisionally absorbed within 40–50 cm. We describe the use of the WKB method to calculate wave damping and provide an expression to assess its validity based on experimental measurements. Theoretical calculations are consistent with experiment and indicate that for conditions where Coulomb collisions are dominant classical collisionality is sufficient to explain the observed wave damping along the length of the plasma column. Based on these results, we provide an expression for the scaling of helicon wave damping relevant to high density discharges and discuss the location of surfaces for plasma-material interaction studies in helicon based linear plasma devices.

Observation of low magnetic field density peaks in helicon plasma

Single density peak has been commonly observed in low magnetic field (&amp;lt;100 G) helicon discharges. In this paper, we report the observations of multiple density peaks in low magnetic field (&amp;lt;100 G) helicon discharges produced in the linear helicon plasma device [Barada et al., Rev. Sci. Instrum. 83, 063501 (2012)]. Experiments are carried out using argon gas with m = +1 right helical antenna operating at 13.56 MHz by varying the magnetic field from 0 G to 100 G. The plasma density varies with varying the magnetic field at constant input power and gas pressure and reaches to its peak value at a magnetic field value of ∼25 G. Another peak of smaller magnitude in density has been observed near 50 G. Measurement of amplitude and phase of the axial component of the wave using magnetic probes for two magnetic field values corresponding to the observed density peaks indicated the existence of radial modes. Measured parallel wave number together with the estimated perpendicular wave number suggests oblique mode propagation of helicon waves along the resonance cone boundary for these magnetic field values. Further, the observations of larger floating potential fluctuations measured with Langmuir probes at those magnetic field values indicate that near resonance cone boundary; these electrostatic fluctuations take energy from helicon wave and dump power to the plasma causing density peaks.

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.

Experimental investigation of interaction of very low frequency electromagnetic waves with metallic nanostructure

The interaction of low-frequency electromagnetic waves with metallic nanostructure consisting of nanoparticles has been investigated. The existence of helicons in metallic nanostructure is predicted based on the enhancement of effective mass of the electrons in metallic mesostructures in low-frequency electromagnetic field. The enhancement of the effective mass of electrons subjected to low-frequency electromagnetic field results in the suppression of the cyclotron frequency. When the excitation frequency approaches the cyclotron frequency, helicons can be observed, which in typical metals occur at MHz frequencies. With the decrease in the cyclotron frequency in nanostructures, the helicons can be observed at low frequencies. We present an experimental setup to detect and image helicon resonances in metallic nanostructures at low frequencies and room temperature conditions. The approach is based on modifying an existing atomic force microscope to image surface topography and the magnetic field images of helicon waves simultaneously in metallic nanoparticles. The magnetic field images of the helicon waves and different resonance modes are presented for platinum nanoparticles deposited using through thin film ablation. The contrast in the magnetic field images is explained based on the generation of resonance modes due to helicon wave propagation in the nanostructure. The features in the helicon resonance images are examined as a function of the particle size and frequency of the electromagnetic wave.

Plasma control by modification of helicon wave propagation in low magnetic fields

By making use of nonuniform magnetic fields, it is shown experimentally that control of helicon wave propagation can be achieved in a low pressure (0.08 Pa) expanding plasma. The m=1 helicon waves are formed during a direct capacitive to wave mode transition that occurs in a low diverging magnetic field (B0<3 mT). In this initial configuration, waves are prevented from reaching the downstream region, but slight modifications to the magnetic field allows the axial distance over which waves can propagate to be controlled. By changing the effective propagation distance in this way, significant modification of the density and plasma potential profiles can be achieved, showing that the rf power deposition can be spatially controlled as well. Critical to the modification of the wave propagation behavior is the magnetic field strength (and geometry) near the exit of the plasma source region, which gives electron cyclotron frequencies close to the wave frequency of 13.56 MHz.

Comment on “Plasma ionization by annularly bounded helicon waves” [Phys. Plasmas 13, 063501 (2006)

The neoclassical calculation of the helicon wave theory contains a fundamental flaw. Use is made of a proportional relationship between the magnetic field and its curl to derive the Helmholtz equation describing helicon wave propagation. However, by the fundamental theorem of Stokes, the curl of the magnetic field must be perpendicular to that portion of the field contributing to the local curl. Re-examination of the equations of motion indicates that only electromagnetic waves propagate through a stationary region of constant pressure in a fully ionized, neutral medium.

Density jump in helicon discharges

Helicon discharges characteristically exhibit a sharp jump from low to high density as the radiofrequency power is raised. This is usually explained by the transition from an inductively coupled plasma mode to a helicon mode when the dispersion relation for helicon wave propagation is satisfied at a critical power or magnetic field. Experiments have suggested a different mechanism for the sudden jump, a mechanism that depends on overcoming the parasitic circuit losses. This effect is analysed computationally, and agreement with measurements is obtained.

Transition from unstable electrostatic confinement to stable magnetic confinement in a helicon reactor operating with Ar∕SF6 gas mixtures

Two types of instabilities were previously identified in inductive discharges having an expanding chamber when negative ions are present: (i) the source instability, occurring in the neighborhood of the capacitive-to-inductive (E to H) transition, and (ii) the downstream instability, which was shown to be the periodic formation and propagation of double layers. These unstable double layers were found over the entire parameter space (pressure/power) of interest, and they were born at the interface of the source and diffusion chambers. They acted as an internal electrostatic barrier separating a low-electronegativity, high-electron-density plasma upstream (in the source) and a high-electronegativity, low-electron-density plasma downstream. In this paper we have investigated the effect of adding a static axial magnetic field, classically used to increase the confinement and the plasma heating via helicon wave propagation. This had the following consequences: (i) the unstable double layers, and therefore the axial electrostatic confinement, were suppressed in a large part of the parameter space, and (ii) the magnetic confinement leads to a radially stratified plasma, the center being a low-electronegativity, high-density plasma and the edges being essentially an ion-ion plasma.

Comparing Experiments with Modeling to Optimize Light Ion Helicon Plasma Sources for VASIMR

The ability to obtain high plasma densities with high fractional ionization using readily available, low-cost components makes the helicon a candidate plasma source for many applications, including plasma rocket propulsion, fusion component testing, and materials processing. However, operation of a helicon can be a sensitive function of the magnetic field strength and geometry as well as the driving frequency, especially when using light feedstock gases such as hydrogen or helium. In this paper, we compare results from a coupled radio frequency (RF) and transport model with experiments in the axially inhomogeneous Mini-Radio Frequency Test Facility (Mini-RFTF). Experimental observations of the radial shape of the density profile can be quantitatively reproduced by iteratively converging a high-resolution RF calculation including the RF parallel electric field with a transport model using reasonable choices for the transport parameters. The experimentally observed transition into the high density helicon mode is observed in the model, appearing as a nonlinear synergism between radial diffusion, the RF coupling to parallel electric fields that damp near the plasma edge, and propagation of helicon waves that collisionally damp near the axis of the device. Power deposition from various electric field components indicates that inductive coupling and absorption in the edge region can reduce the efficiency for high-density operation. These findings can be used to optimize helicon discharges for use in Variable Specific Impulse Magnetoplasma Rocket (VASIMR) designs, and estimates for the helicon power required to perform ion cyclotron heating experiments in the Mini-RFTF are given.

Comparing experiments with modeling for light ion helicon plasma sources

The ability to obtain high plasma densities with high fractional ionization using readily available, low-cost components makes the helicon a candidate plasma source for many applications, including plasma rocket propulsion, fusion component testing, and materials processing. However, operation of a helicon can be a sensitive function of the magnetic field strength and geometry as well as the driving frequency, especially when using light feedstock gases such as hydrogen or helium. In this paper, results from a coupled rf and transport model are compared with experiments in the axially inhomogeneous Mini-Radio Frequency Test Facility [Goulding et al., Proceedings of the International Conference on Electromagnetics in Advanced Applications (ICEAA 99), Torino, Italy, 1999 (Litografia Geda, Torino, 1999), p. 107] (Mini-RFTF). Experimental observations of the radial shape of the density profile can be quantitatively reproduced by iteratively converging a high-resolution rf calculation including the rf parallel electric field with a transport model using reasonable choices for the transport parameters. The experimentally observed transition into the high density helicon mode is observed in the model, appearing as a nonlinear synergism between radial diffusion, the rf coupling to parallel electric fields that damp near the plasma edge, and propagation of helicon waves that collisionally damp near the axis of the device. Power deposition from various electric field components indicates that inductive coupling and absorption in the edge region can reduce the efficiency for high-density operation. The effects of absorption near the lower hybrid resonance in the near-field region of the antenna are discussed. Ponderomotive effects are also examined and found to be significant only in very low density and edge regions of the Mini-RFTF discharge.

Measurements of helicon wave propagation and Ar II emission

Wave magnetic field, optical and Langmuir probe measurements are carried out to examine fast and thermal electron contributions to plasma ionization in a helicon plasma source. For the optical measurements, an Ar II line (443 nm) with a particularly short lifetime (7 ns) is chosen to resolve the excitation rate within a radio frequency period (73.7 ns). Information is then obtained regarding the acceleration processes and their action on the electron distribution that caused the excitation. Spatio-temporal measurements of 443 nm peak emission show that the emission is modulated at the source frequency. The peak count phase of the modulation propagates along the plasma at a comparable speed as the local helicon wave phase velocity. Computer modeling utilizing lab data is carried out to examine wave field effects on electron acceleration and ionization contributions arising from non-Maxwellian fast electrons.

Characteristics of radio frequency wave propagation in bounded plasma under the various magnetic field configurations

Detailed characteristics of radio frequency (rf) waves with pulsed modes in the whistler wave range were studied in a cylindrical rf-produced plasma, where the plasma boundary lay in the intermediate regime between infinite whistler wave propagation and bounded geometry helicon wave propagation. Excited magnetic field amplitudes and phases with three components in two-dimensional space were measured for different experimental conditions. Three magnetic field configurations were used and the diameter of the excitation loop antenna was also varied. Numerical calculation by the finite element method, which has been demonstrated to be a powerful means for this analysis, showed good agreement with the observed results, satisfying the dispersion relation and wave structures of helicons in the semisteady state and also satisfying the dispersion of whistlers with a short pulsed mode. The excited waves propagated nearly along the magnetic field lines within a small angle of less than 10°. Furthermore, in the low (high) collisionality regime, domination of standing (propagating) waves was found from the wave analysis.

Wave propagation and absorption simulations for helicon sources

A two-dimensional (2-D), finite-difference computer code is developed to examine helicon antenna coupling, wave propagation, collisionless Landau, and collisional heating mechanisms. The code calculates the electromagnetic wave fields and power absorption in an inhomogeneous, cold, collisional plasma. The current distribution of the launching antenna, which provides the full antenna spectra, is included in the model. An iterative solution that incorporates warm plasma thermal effects has been added to the code to examine the contribution of collisionless (Landau) wave absorption by electrons. Detailed studies of the wave fields and electron heating profiles at low magnetic fields (B0&amp;lt;100 G), where both Trivelpiece–Gould (TG) and helicon (H) modes are present, are discussed. The effects of the applied uniform magnetic field (B0=10–1000 G), 2-D (r,z) density profiles (ne0=1011–1013 cm−3), neutral gas pressures of 1–10 mTorr and the antenna spectrum on collisional and collisionless wave field solutions and power absorption are investigated. Cases in which the primarily electrostatic (TG) surface wave dominates the heating and the power is absorbed near the edge region and cases in which the propagating helicon wave transports and deposits its energy in the core plasma region are examined.