Helicon Plasma Source Research Articles

Radical probe system for in situ measurements of radical densities of hydrogen, oxygen, and nitrogen

In this study, an in situ catalytic radical probe system together with a software platform is developed to measure concentrations of reactive species in low-temperature plasmas with high spatial resolutions. The radical probes can be used to determine radical densities of hydrogen, nitrogen, and oxygen independently, in pairs and often simultaneously in any continuous plasma source in a vacuum environment. The basic principle and advantage of a probe array is the capability to distinguish between different gas species due to several sensitive elements acting as recombination catalysts. Radical densities of hydrogen, nitrogen, and oxygen were measured in a helicon plasma source. Generally, it is observed that radical densities increase with respect to pressure and power. Additionally, the electron density and electron temperature were measured by Langmuir probes. The electron density increased with increasing power and pressure. Electron temperature increased with power but decreased with increasing pressure. The key to getting absolute numbers of radical densities is based on knowing the recombination coefficient of the given gas on the catalytic surface. The probe system measures densities in a broad range of reactive species’ concentrations varying from about 1013 to 1015 cm−3.

Direct experimental comparison of krypton and xenon discharge properties in the magnetic nozzle of a helicon plasma source

Helicon plasma sources currently represent an active field of research in the domain of low-temperature plasmas due to several interesting characteristics for in-space propulsion applications. This work reports direct comparison of krypton and xenon plasma properties through spatially resolved measurements performed in the near-field plume of a sub-kilowatt-class 13.56 MHz helicon source. The set of developed and employed intrusive diagnostics is comprehensively described. The magnetic components of the plasma rf field are inferred using a 3D B-dot probe along the reactor axial direction with and without the externally applied DC magnetic field. Plasma floating potential fluctuations are measured using a capacitive probe to draw design requirements for a rf-compensated Langmuir probe. The latter is used to perform axial and radial measurements of plasma density, electron temperature, and plasma potential. A four-grids retarding potential analyzer is used to infer the ion kinetic energy downstream the source exit. The effect of magnetic field magnitude on ions and electrons properties is especially investigated.

Prediction of Axial Variation of Plasma Potential in Helicon Plasma Source Using Linear Regression Techniques

Analytical expressions are used frequently for the determination and analysis of plasma parameters. Instead of relying on analytical expressions, the proposed method uses regression techniques supplemented with experimental data for the selected parameters (plasma potential). In the machine learning domain, this is equivalent to the creation of the training data set, building and training the model, and authenticating the result over a range of desired physical parameters. An experimental dataset is built using two axially movable Triple Langmuir Probe (TLPs) which measure the electron temperature, electron density, and electric potential of a plasma. The presented work is a first step towards developing an inclusive model with detailed kinetic simulations capable of characterizing the HELicon Experiment for Negative ion source (HELEN-I) with a single driver. Plasma potential is measured at different axial locations (z) by keeping pressure fixed at 6 mTorr.

Electron population properties with different energies in a helicon plasma source

The characteristics of electrons play a dominant role in determining the ionization and acceleration processes of plasmas. Compared with electrostatic diagnostics, the optical method is independent of the radio frequency (RF) noise, magnetic field, and electric field. In this paper, an optical emission spectroscope was used to determine the plasma emission spectra, electron excitation energy population distributions (EEEPDs), growth rates of low-energy and high-energy electrons, and their intensity jumps with input powers. The 56 emission lines with the highest signal-to-noise ratio and their corresponding electron excitation energy were used for the translation of the spectrum into EEEPD. One discrete EEEPD has two clear different regions, namely the low-energy electron excitation region (neutral lines with threshold energy of 13–15 eV) and the high-energy electron excitation region (ionic lines with threshold energy ≥19 eV). The EEEPD variations with different diameters of discharge tubes (20 mm, 40 mm, and 60 mm) and different input RF powers (200–1800 W) were investigated. By normalized intensity comparison of the ionic and neutral lines, the growth rate of the ionic population was higher than the neutral one, especially when the tube diameter was less than 40 mm and the input power was higher than 1000 W. Moreover, we found that the intensities of low-energy electrons and high-energy electrons jump at different input powers from inductively coupled (H) mode to helicon (W) mode; therefore, the determination of W mode needs to be carefully considered.

A large-area planar helicon plasma source with a multi-ring antenna on Linear Experimental Advanced Device (LEAD)

A planar helicon plasma source with a four-ring antenna has been developed on Linear Experimental Advanced Device (LEAD) in Southwestern Institute of Physics. The diameter of the largest antenna ring is 320 mm. This source is located outside of the vacuum chamber without any vacuum interface and injects radio frequency power into the chamber through a 340-mm-diameter quartz window. A low power threshold of 150 W for electromagnetic-mode to wave-mode transfer is experimentally confirmed. A large volume plasma with a density of over 1019 m−3 and a plasma generation efficiency (total number of electrons diveded by input power) of over 30× 1013 W−1 indicate the high performance of this large-area helicon plasma source, promising LEAD a suitable device for fundamental plasma physics and plasma material interaction research.

Testing and analysis of steady-state helicon plasma source for the Material Plasma Exposure eXperiment (MPEX)

Preparing for next-step fusion facilities will require developing materials that can withstand the high ion and neutron fluences that will be present in the divertor region. These fluences are inaccessible in current toroidal devices. The Material Plasma Exposure eXperiment (MPEX) is a steady-state linear plasma device, currently undergoing conceptual design, that proposes to reach ion fluences as high as 1031 m−2. It will also be able to receive neutron irradiated samples to examine the multivariate effects of plasma material interactions. A target exchange chamber will be employed so that the MPEX target can be removed and placed in a separate diagnostic station without leaving vacuum. To operate in steady-state, the MPEX plasma will be confined using superconducting magnets, with active cooling for all plasma-interacting and plasma-facing components. The plasma source will be a high-power (200 kW) helicon antenna, which will be placed outside of the vacuum chamber. The radio frequency-transparent window for this antenna must be water cooled and must have a very low dielectric coefficient to limit the dielectric losses. The water-to-vacuum seal should not be an elastomer seal, to limit impurities at the plasma source. It is proposed to use a ceramic-to-metal joint. A prototype water-cooled helicon antenna window and assembly have been manufactured and tested in long-pulse conditions up to 10 kW in the Controlled Shear De-correlation eXperiment at the University of California, San Diego. Thermal results have been correlated with computational fluid dynamics simulation.

The Effect of Key Parameters on Power Absorption in Helicon Plasma Sources

In recent years, helicon plasma sources have attracted much attention from scientific centers and industry. In this regard, the power coupling is of particular importance for the production of dense plasma in these systems. Therefore, in order to achieve maximum efficiency in these types of plasma sources, it is necessary to analyze the key parameters affecting plasma power absorption such as magnetic field (50–700 G), gas pressure (1–20 mTorr), radiofrequency power (800–2200 W), and RF frequency (6.28, 13.56, 20.34, and 27.12 MHz) applied to Nagoya antenna. Finding the optimal range of these parameters is one of the key parameters in the design and construction of helicon plasma sources. Therefore, in this article, we investigate the parameters affecting the power absorption mechanisms in these types of plasma sources using a 3-D simulation of the helicon plasma source with the finite-element method. Our findings indicate that absorption mechanisms have different effects on the absorption power due to the option of the most optimal plasma parameters’ value. The results show that changes in absorption power in terms of plasma density are nonlinear and show changes in the mode from E to H and eventually to W. Because of the domination of collisionless mechanism, the absorbed power increases in the pressures lower than 2 mTorr. In addition, at pressures above 2 mTorr, it increases due to the collisional mechanism. Plasma density increases linearly as the magnetic field intensity increases. The density peak is observed at the vacuum chamber edge in low magnetic fields (in the range of 65 G) which represents the Trivelpiece-Gould (TG) mode formation.

Beam Plasma Characteristics of a Helicon Plasma Source Measured by a Spatially Resolved Optical Emission Spectroscope

Beam plasma directly forms the thrust of single-stage helicon plasma thrusters and its properties are the input parameters of the two-stage helicon thruster. Therefore, the spatial distribution of the beam plasma is vital for both single-stage and two-stage helicon thrusters. Using an optical emission spectroscope with a radial resolution of 5 mm, the spatial distribution of the beam plasma parameters and its variation with different input radio frequency (RF) powers (100–1000 W) and diameters of the discharge tube (20–60 mm) was investigated. Results show that the electron density decreases with both axial and radial distances, but the electron temperature remains nearly constant. The electron density increases with increasing input RF powers because of an increase of power deposition when the electron temperature remains constant. The radial profiles of electron density are flat, and the maximum electron densities located in the center line decrease with increasing diameters of the discharge tube, which is due to the decrease in the power density.

Spectral measurements of inductively coupled and m = +1, −1 helicon discharge modes of the constructed plasma source

In the present context, the industrial type of Amirkabir helicon plasma source has been introduced that was designed and constructed at the Helicon Plasma Laboratory of Amirkabir University of Technology with the aim of using it in material processing applications. Helicon plasma in two m = +1 and m = −1 modes of operation was studied, and also its application was compared with the inductively coupled plasma (ICP) mode in this experimental work. This study was performed by employing two techniques including optical emission spectroscopy and imaging using a camera with polarizing filters, in which the images and spectra of ICP and m = +1, −1 helicon plasma modes were recorded under the experimental conditions. The effects of the device operational parameters on the argon plasma emission spectra were investigated in the wavelength range of 350–950 nm. It was observed from the comparison of the plasma spectra that the ionization rate increases significantly for the plasma helicon mode than ICP and also for m = +1 helicon mode of operation than m = −1. In this work, the values of device operational parameters such as the RF power delivered to the half-helix antenna, external magnetic field intensity, and the injected gas flow rate were varied in the range of 400–900 W, 100–300 mT, and 1–10 SCCM in the experiment, respectively. In addition, the optimum values of RF power, magnetic field intensity, and the injected gas flow rate for achieving the maximum ionization rate were, respectively, obtained as 900 W, 300 mT, and 3 SCCM.

Application of Thomson scattering to helicon plasma sources

The possibility of performing electron density and temperature measurements in a high power helicon plasma is a crucial issue in the framework of the AWAKE (Advanced WAKefield Experiment) project, which demonstrates acceleration of particles using $\text{GeV}~\text{m}^{-1}$ electric fields in plasmas. For AWAKE, a helicon is currently envisaged as a candidate plasma source due to its capability for low electron and ion temperature, high electron density and production of an elongated plasma column. A plasma diagnostic to accurately determine the electron density in AWAKE regimes would be a valuable supporting tool. A demonstration Thomson scattering (TS) diagnostic was installed and successfully tested on the resonant antenna ion device (RAID) at the Swiss Plasma Center of Ecole Polytechnique Fédérale de Lausanne. RAID produces a helicon plasma column with characteristics similar to those of the AWAKE helicon source, and is therefore an optimal testbed for application to the AWAKE device. The spectrometer employed in RAID is based on polychromators which collect the light scattered by plasma electrons in spectrally filtered wavelength regions. Results from TS on RAID demonstrate conditions of electron density and temperature respectively of $n_{e}=1.10\,(\pm 0.19)\times 10^{19}~\text{m}^{-3}$ and $T_{e}=2.3\,(\pm 0.6)~\text{eV}$ in a steady-state discharge in an Ar plasma with 5 kW of RF power. If the same polychromator system is used for AWAKE, where the electron density attained is $2\times 10^{20}~\text{m}^{-3}$ , the contribution to measurement error due to coherent scattering is ${\sim}2.5\,\%$ . Presented here are details of the TS diagnostic and the first tests in RAID, and the expectations for the system when employed on the AWAKE device.

Artificial neural network based predictive negative hydrogen ion helicon plasma source for fusion grade large sized ion source

Ion source operation and control is a challenging task, and the manual search of the optimized input parameter by the operator becomes a time consuming and cumbersome process. HELicon Experiment for Negative Ion Source (HELEN-I) with single driver developed at the Institute for Plasma Research (IPR) for the production negative hydrogen ion involves a multitude of interacting systems (such as Radio Frequency (RF) power, gas feed pressure, magnetic fields). This paper presents an Artificial Intelligence (AI) driven multiple-input and multiple-output model for HELEN-I to get optimized values of ion saturation current (mA) and high plasma density of order 1018/m3. The developed Artificial Neural Network (ANN) based model predicts the outputs of the HELEN-I and is further utilizes a heuristic global optimization Particle Swarm Optimization (PSO) algorithm to select optimal source parameters to predict the desired outputs without conducting real-time experiments. The experimental studies further validate the results obtained by the proposed approach.

Experimental Investigation of the Effects of Magnetic Mirrors on Plasma Transport in the Prototype Material Plasma Exposure Experiment

The Prototype Material Plasma Exposure eXperiment (Proto-MPEX) at Oak Ridge National Laboratory is developing a helicon radio frequency (RF) source for plasma production along with microwave electron heating and RF ion heating. Proto-MPEX has several magnetic mirrors that affect plasma transport. The magnetic field near the helicon source is ~ 0.05 T, and the peak magnetic field in Proto-MPEX is ~ 1.8 T, which results in a large magnetic well for particles sourced at the helicon transporting toward the target region. Keeping the helicon source field and the target field constant, experiments have been conducted by methodically changing the magnetic field, at a local axial location, within the source and the target. Plasma flow measurements using Mach probes have shown a decrease in the flow as a function of the magnetic mirror ratio. Plasma flows were further investigated with ion cyclotron resonant heating (ICH). Results with the addition of ICH measured upstream of the antenna have shown a decrease in Mach number as a function of mirror ratio when compared to helicon discharges. Such flow behavior demonstrated slowing down of the plasma due to the increased reverse plasma flow toward the helicon plasma source. However, in high magnetic fields, when the resonant magnetic field moved away from the ICH antenna, no effects on the plasma flow due to ICH were observed.

The effect of external magnetic field on the collisional power absorption in helicon plasma sources

The paper reports on the effects of uniform and non-uniform external magnetic field on the power absorption in a helicon plasma source, which are computationally investigated by the CST Microwave Studio code. RF (13.56 MHz) power deposition was studied using one designs of antennas, namely, the Nagoya type-III. Argon was used as the plasma working gas at the operating pressure of 15 mTorr. We have focused on the collisional power absorption utilizing WKB approximation to describe the plasma inhomogeneity. We put discrete plasma density layers in CST. The obtained results show that the radial inhomogeneity has different effects on the power absorption at the uniform and the non-uniform external magnetic fields. It is found that at uniform external magnetic fields (i.e., B0 = 200 G), there is a specific density (nc) ranging from 1 × 1018 m−3 to 5 × 1018 m−3, before and after which the radial inhomogeneity decreases and increases the absorbed power, respectively. On the other hand, at uniform external magnetic fields (i.e., B0 = 900 G), the inhomogeneity has no regular effect on the power absorption in various plasma densities. In addition, for a given plasma density (e.g., n = 1018 m−3), as the magnetic field increases, the radial inhomogeneity effect on the power absorption would decrease for the Nagoya type-III.

Study on negative ion production by electronegative gases in a helicon source

A helicon plasma source (HeliPS) experimental set-up is developed to perform experiments in electronegative gases. Characterization of HeliPS is done in argon and oxygen discharge by varying the working pressure, magnetic field strength as well as radio frequency power at 13.56 MHz thereby identifying the optimum parameters for its operation. After characterization, experiments have been performed in electronegative gases such as oxygen and hydrogen to understand the effect of electron affinity in producing negative ions in such plasmas. To enhance the negative ion production, both magnetic filter and magnetic cage, used innovatively in such a system, shows interesting results on their influence in electronegative plasma production.

Characterization and Control of an Ion-Acoustic Plasma Instability Downstream of a Diverging Magnetic Nozzle

The study and control of resonant instabilities in magnetised plasmas is of fundamental interest over a wide range of applications from industrially relevant plasmas to plasma sources for spacecraft propulsion. In this work electrostatic probes were employed to measure a 4-20 kHz instability in the ion saturation current downstream of an electric double layer (DL) in an expanding helicon plasma source. The amplitude and frequency of the instability were found to vary in inverse proportion to the operating argon gas pressure (0.2-0.6 mTorr) and in direct proportion to the applied rf power (100-600 W) and applied solenoid current (3-8 A). A spatially resolved characterisation of the maximum instability amplitude determined two radial maxima, corresponding to the locations of most positive radial ion density gradient. Control and inhibition of the instability were achieved through the application of a kHz voltage amplitude modulation to the 13.56 MHz radio-frequency (rf) power supplied to the helicon antenna. Through the application of voltage amplitude modulations in the frequency range 2-12 kHz the instability was reduced by up to 65 %, exhibiting a greater reduction at higher applied modulation frequencies. This effect is described through a variation in the radial ion density gradient via asymmetrically attenuated ion acoustic density perturbations induced by the applied voltage modulation. The application of voltage amplitude modulations has been demonstrated as a potential control mechanism for density gradient driven instabilities in magnetised plasmas.

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.

Minimal multi-target plasma sputtering tool

A multi-target plasma sputtering tool is developed for a recently progressed new concept of a semiconductor fab system called ‘Minimal Fab’. A frequency-tuning compact helicon plasma source is attached on the side wall of the compact vacuum chamber and the plasma is transported along curved magnetic field lines to a target surface, which is mounted on a rotational target holder and negatively biased by a dc power supply. The target material is sputtered by the ions accelerated by the sheath potential drop and deposited on the substrate facing the target. The target material can be changed by rotating the holder; thereby the multi-layered metallic film can be formed in the single chamber. Substrate heating induced by secondary electrons emitted from the target surface and accelerated by the sheath is successfully inhibited by a magnetic filter near the substrate. The performance of the sputtering reactor designed as a laboratory model is shown here. The sputtering reactor and all of the components required for the sputtering tool are successfully contained within a compact frame of Minimal Fab (29.5 cm in width, 45 cm in depth, and 144 cm in height) via the development of the compact reactor, especially the compact rf plasma source.

3D-VIRTUS: Equilibrium condition solver of radio-frequency magnetized plasma discharges for space applications

We present 3D-VIRTUS (3-Dimensional adVanced fluId dRifT diffUsion plaSma solver), a numerical tool conceived for the evaluation of the equilibrium conditions of Helicon plasma sources, that consists of the Electro-Magnetic Module, and the FLUID Module. The first evaluates the power deposited into the plasma by the antenna that drives the discharge, and it is based on a solid numerical tool (i.e., ADAMANT). The power deposition is used by the second module to solve for the macroscopic transport of charged and neutral species by means of a fluid approach, and assuming valid the Drift-Diffusion approximation. The set of continuity, momentum, energy, and Poisson equations are solved numerically through the Finite Volume Method, and are implemented in OpenFOAM. The two modules are iterated until a converged solution is obtained. This approach allows the self-consistent evaluation of the local plasma parameters (e.g., plasma density) at equilibrium in Helicon sources that feature (i) arbitrary-shaped plasma regions, and antennas, (ii) realistic magneto-static fields generated by electromagnets or permanent magnets. The numerical accuracy of the FLUID Module has been assessed as a function of the time, and space discretization. The FLUID Module, and the 3D-VIRTUS code have been independently validated against other well-established numerical approaches and experimental measurements. Finally, 3D-VIRTUS has been exploited to investigate Helicon plasma discharges for space propulsion applications for magneto-static field values from 0 G up to 750 G.

Investigation of the plasma parameters of a laboratory argon plasma source using a collisional radiative model with the comparison of experimental and simulated spectra

Plasma spectroscopy is a common method for diagnosing the radiative behaviour and predicting the key properties of a plasma. Plasma spectroscopy may be merged with plasma modelling and simulation to perform numerical diagnostics. A new Collisional Radiative (CR) model is developed for argon plasma at low temperature and low pressure. In the developed model, rather than grouping the energy levels into lumped sets, all the energy levels up to a maximum level were modeled for both the argon neutrals and the argon single ions. With the gathered data of collisional and radiative rate coefficients from LANL (Los Alamos National Laboratory) and NIST (National Institute of Standards and Technology), this model calculates the time-varying population densities of 101 atomic and 222 ionic energy levels. The model considers the emissions of 146 Ar-I lines (in 86.68–2397.3 nm range) and 303 Ar-II lines (in 71.8–6985.3 nm range), and gives the simulated spectra over the optical and near-optical region. The simulated radiation emission spectra were then compared with the measured spectra of a laboratory helicon plasma source for the prediction of the plasma parameters such as electron temperature and electron density.

Whistler modes excited by magnetic antennas: A review

A review of a deceptively simple topic is presented, i.e., the excitation of whistler modes by antennas. It includes the knowledge of antennas and of the waves and their coupling. This review will show how the research in the last few decades has advanced and become a refined and complex topic which covers nonlinear effects, instabilities, nonuniform fields, whistler modes with orbital angular momentum, wave field topologies, etc. This review is mainly focused on experimental work in laboratory plasmas, but the findings will be related to research on whistler waves in space plasmas, helicon plasma sources in the laboratory, and significant findings of other research groups. This review starts with antenna properties such as radiation patterns, radiation efficiencies, and the topology of the emitted wave packets. Next, the propagation of whistler modes in highly nonuniform ambient magnetic fields will be presented. Even in the linear regime, new phenomena have been discovered such as the reflection of whistler modes from strong magnetic field gradients or the eigenmodes of waves on circular magnetic field lines. Important nonlinear effects of whistler modes are presented. Whistler instabilities will be briefly reviewed although this is a broad topic by itself. Examples of magnetic reconnection in the Hall parameter regime will be shown. This review will also discuss the advances of wave diagnostics in dedicated laboratory plasmas, the difficulties of diagnostics in high power laboratory plasmas, and the limiting multipoint diagnostics in space plasmas.