High Power Helicon Research Articles

First high-power helicon results from DIII-D

Abstract More than 0.6 MW of rf power at 476 MHz has been coupled to DIII-D plasmas by launching helicon (whistler) waves with a traveling-wave antenna (comb-line) in the fast-wave polarization (Van Compernolle et al 2021 Nucl. Fusion 61 116034) which resulted in the observation of electron heating of the core plasma with single-pass absorption based on ray-tracing in L-mode discharges. The coupling performance of the 1.5 m wide 30-element comb-line traveling-wave antenna has been consistent with expectations based on the 2015–2016 experiments on DIII-D with a low-power 12-element prototype (Pinsker et al 2018 Nucl. Fusion 58 106007). The conditioning process that was necessary to carry out high-power experiments is discussed; rf-specific impurities have not been observed. Parametric decay instabilities have been observed and are being investigated as a potential edge absorption mechanism (Porkolab et al 2023 AIP Conf. Proc. 2984 070004).

Advancement of Langmuir probe-based laser photo-detachment technique for negative ion density measurement in a high-power helicon plasma source.

In the pursuit of precise diagnostics for measuring negative ion density in a helicon plasma source (HPS), a new approach utilizing a radio frequency (RF) broadband transformer-based Langmuir probe is developed specifically for laser photo-detachment (LPD) analysis. This inductively coupled LPD technique is useful for high power RF systems in which capacitive RF noise is in the same scale as the pulsed photo-detachment signal. The signal acquired by this transformer-based probe is compared against the conventional Langmuir probe-based LPD technique, revealing a remarkable enhancement in signal fidelity through an improved signal-to-noise ratio (SNR) achieved by the RF broadband transformer methodology. In addition, the localized hydrogen negative ion density measurements obtained through this probe are harmoniously aligned with the line-averaged negative ion density derived from the cavity ringdown spectroscopy (CRDS) technique. These concurrence measurements highlight the RF broadband transformer-based approach's accuracy in capturing localized negative ion density during helicon mode operation in an HPS setup. Furthermore, the correlation of negative ion density values with RF input exhibits a consistent trend in tandem with background plasma density. Notably, both CRDS and LPD measurements ascertain negative ion densities ranging from ∼5 to 6×1016m-3 under an RF power of 500-700W and a pressure of 8 × 10-3mbar, all under the influence of a 55G axial magnetic field. These specific parameters represent the optimal operational configuration for effective negative ion production with the present experimental HPS setup. Due to its better SNR, the RF broadband transformer-based Langmuir probe emerges as a useful tool for LPD diagnostics, particularly in the presence of pervasive RF noise.

Grafting Amino Groups onto Polyimide Films in Flexible Copper-Clad Laminates Using Helicon Plasma.

Polyimide (PI) films are widely used in electronic devices owing to their excellent mechanical and electrical properties and high thermal and chemical stabilities. In particular, PI films play an important role in flexible printed circuit boards (FPCBs). However, one challenge currently faced with their use is that the adhesives used in FPCBs cause a high dielectric loss in high-frequency applications. Therefore, it is envisioned that PI films with a low dielectric loss and Cu films can be used to prepare two-layer flexible copper-clad laminates (FCCLs) without any adhesive. However, the preparation of ultra-thin FCCLs with no adhesives is difficult owing to the low peel strength between PI films and Cu films. To address this technical challenge, an FCCL with no adhesive was prepared via high-power helicon wave plasma (HWP) treatment. Field-emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) were tested. Also, the surface roughness of the PI film and the peel strength between the PI film and Cu film were measured. The experimental results show that the surface roughness of the PI film increased by 40-65% and the PI film demonstrated improved adhesion (the peel strength was >8.0 N/cm) with the Cu film following plasma treatment and Cu plating.

The high-power helicon program at DIII-D: gearing up for first experiments

Helicon current drive (CD), also called fast wave CD in the lower hybrid range of frequencies, has long been regarded as a promising CD tool for reactor grade plasmas. A newly installed MW-level system at DIII-D will be the first test of this technology in reactor-relevant plasmas, in the sense that full single-pass absorption is expected. A 30-module traveling wave antenna has been installed and optimized in-vessel in early 2020. The linear electromagnetic characteristics of the unloaded module array have been extensively tested both on the bench and in the vessel at instrumentation power levels. Excellent performance has been achieved, ∼2% reflected power and ∼1.5% dissipated power per module in air, in a 10 MHz band around 476 MHz. Stripline feeds on both ends of the antenna allow either co or counter CD. The installation of a 1.2 MW klystron and associated high-power electronics was completed in Fall 2020. Commissioning of the antenna is ongoing. An important goal of this experiment is to validate the helicon CD physics basis using an extensive set of new and upgraded diagnostics.

Ion heating in the PISCES-RF liquid-cooled high-power, steady-state, helicon plasma device

Radio frequency (RF) driven helicon plasma sources are commonly used for their ability to produce high-density argon plasmas (n > 1019 m−3) at relatively moderate powers (typical RF power < 2 kW). Typical electron temperatures are <10 eV and typical ion temperatures are <0.6 eV. A newly designed helicon antenna assembly (with concentric, double-layered, fully liquid-cooled RF-transparent windows) operates in steady-state at RF powers up to 10 kW. We report on the dependence of argon plasma density, electron temperature and ion temperature on RF power. At 10 kW, ion temperatures >2 eV in argon plasmas are measured with laser induced fluorescence, which is consistent with a simple volume averaged 0D power balance model. 1D Monte Carlo simulations of the neutral density profile for these plasma conditions show strong neutral depletion near the core and predict neutral temperatures well above room temperatures. The plasmas created in this high-power helicon source (when light ions are employed) are ideally suited for fusion divertor plasma-material interaction studies and negative ion production for neutral beams.

Depletion of atomic hydrogen in a high power helicon discharge

Depletion of the ground state atomic hydrogen density has been directly measured using two-photon laser-induced fluorescence in a high-density helicon plasma. The depletion is correlated with the plasma pressure becoming increasingly higher than the neutral gas fill pressure. Spatially resolved measurements show depletion of atomic hydrogen in the centre of the discharge chamber. Temporally resolved measurements display a replenishment of atomic hydrogen in the plasma afterglow at high plasma densities in comparison to the typical two-step decay at lower plasma densities.

Design of multipactor-suppressed high-power VFT for helicon current drive in KSTAR

A helicon wave was tested for coupling at low power in the KSTAR, and high-power coupling has also been tried. When RF is applied to the antenna system, the reflected power gradually increases on a sub-millisecond timescale. A slower process than usual arcing suggests that a multipactor discharge causes the reflection. It is essential to mitigate or eliminate the multipactor discharge to apply a high-power helicon wave to the tokamak plasmas. A new VFT is fabricated following a design that focuses on the reduced total RF electric field and zero axial electric field on TiN-coated alumina and conductor surfaces. The unmatched impedance caused by the 1-cm-thick alumina is compensated by the series matching element positioned in the pressurized section. Particle in cell codes are used to predict the effects of secondary electrons caused by electrons emitted in the high-power RF transmissions. Different coating conditions are compared, and it is calculated that the designed VFT is free from multipacting. Thermomechanical stress and deformation due to the RF loss is also analyzed. The RF characteristics of the fabricated VFT agrees well with the design. Currently, a megawatt-level high-power test with the KSTAR helicon antenna in a vacuum is being prepared.

The Materials Plasma Exposure eXperiment: Status of the Physics Basis Together With the Conceptual Design and Plans Forward

The Materials Plasma Exposure eXperiment (MPEX) is a linear plasma device that will address the plasma–material interaction (PMI) science for future fusion reactors and will enable testing of plasma-facing components (PFCs). It is designed as a steady-state device eventually delivering an ion fluence of up to 1031 m−2 to the target. The device will be designed to handle neutron-activated materials. These capabilities should push the technical readiness level of PFCs up to six for some end-of-life aspects. In order to achieve the relevant plasma conditions, as they are expected in future fusion reactor divertors, MPEX will utilize a novel plasma source concept. This plasma source concept uses a high-power helicon (200 kW, 13.56 MHz), an electron cyclotron heating (ECH) system, which will heat electrons via electron Bernstein wave (EBW) heating (about 400 kW, 70 GHz), and an ion cyclotron heating (ICH) system (400 kW, 6–9 MHz). The physics basis for this plasma source concept including the heating physics and transport is based on experiments on Proto-MPEX. An overview of the experimental results and the physics basis will be given. The physics basis directly translates into functional requirements of MPEX and the conceptual design. The status of the conceptual design of MPEX will be shown.

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.

The Material Plasma Exposure eXperiment: Mission and conceptual design

• New linear plasma device MPEX will address plasma material interaction gaps for fusion. • MPEX will have increased capabilities over existing linear plasma devices and will be world-leading when completed. • MPEX will expose apriori neutron irradiated material samples for fusion reactor divertor relevant heat and particle fluxes. • The conceptual design of MPEX is finished. Mastering Plasma Material Interactions (PMI) is key for obtaining a high performance, high duty-cycle and safe operating fusion reactor. Numerous gaps exist in PMI which have to be addressed before a reactor can be built. In particular the lack of data at high ion fluence, fusion reactor divertor relevant plasma conditions and neutron displacement damage requires new experimental devices to be able to develop plasma facing materials and components. This has been recognized in the community and the U.S. fusion program is addressing this need with a new linear plasma device—the Material Plasma Exposure eXperiment (MPEX). MPEX will be a superconducting linear plasma device with magnetic fields of up to 2.5 T. The plasma source is a high-power helicon source (200 kW, 13.56 MHz). The electrons will be heated via Electron Bernstein Waves with microwaves using multiple 70 GHz gyrotrons (up to 600 kW in total). Ions will be heated via ion cyclotron heating in the so-called “magnetic beach heating” scheme in the frequency range of 6−9 MHz (up to 400 kW in total). An overview of the conceptual design and the project/design requirements is given.

Latest Results from Proto-MPEX and the Future Plans for MPEX

The Prototype Material Plasma Exposure eXperiment (Proto-MPEX) is being used to qualify the plasma source and heating systems for the Material Plasma Exposure eXperiment (MPEX). The MPEX will address important and urgent research needs on plasma material interactions for future fusion reactors. In MPEX, plasma-facing components (nonirradiated and a priori neutron irradiated) will be exposed to plasma conditions as they are expected in future fusion reactors. The MPEX, a steady-state device enabled by superconducting magnets, will be able to break into new ground by assessing plasma-facing materials and components at an ion fluence level in the range of 1030 to 1031 m−2. To achieve the relevant plasma conditions, high-density plasmas (>4 × 1019 m−3) are produced with a high-power helicon source. The so-produced low-temperature helicon plasma is then additionally heated with waves in the ion cyclotron resonance frequency and electron cyclotron resonance frequency domains. Proto-MPEX has achieved all key parameters (source ne, source Te, source Ti, target Te, target Ti, target ion flux, and target heat flux) within a factor of 2 of the design requirements of MPEX, albeit not simultaneously. These parameters were achieved with a total installed heating power of 330 kW, which is less than half of the planned heating power in the MPEX (800 kW). An overview of the latest results from Proto-MPEX is given. These results are shown in relationship to the MPEX system goals. Remaining necessary research and development tasks are discussed. The MPEX is currently in the conceptual design phase. The status of the design and an overview of the system requirements are presented.

A high-power helicon antenna for the DIII-D tokamak and its electromagnetic aspects

A comb-line antenna to demonstrate efficient off-axis non-inductive current drive from the absorption of toroidally directed very high harmonic fast waves is being designed and built for DIII-D. The antenna consists of a toroidal array of 30 modules, spanning 1.7 m on the outer vessel wall just above the tokamak midplane. This antenna will be fed with 1 MW of RF power at 476 MHz through a stripline (SL) feed on both sides inside the vacuum vessel. COMSOL Multiphysics, a commercial finite element analysis software, was used to perform the RF analysis and the induced force analysis due to plasma disruption events on the whole antenna system. Results on the RF performance of the SL and the RF coupling into and between the modules, and the SL and modules’ RF losses will be presented. In addition, the disruption induced current and forces are shown, and a method to mitigate them is presented.

Design and Analysis of an Actively Cooled Window for a High-Power Helicon Plasma Source

The material's plasma exposure experiment (MPEX) is a steady-state, linear plasma facility proposed to provide plasma conditions that simulate the first wall and divertor of the next-step fusion devices. MPEX will use an actively cooled, high-power helicon antenna as a plasma source. The antenna is located outside of the vacuum region, and during operation, a significant percentage of the antenna power is deposited on the plasma facing surface of the antenna window. The window thus must be actively cooled so that thermal stresses do not become excessive. The design presented includes two cylindrical windows with an annular water passage in-between in order to allow active cooling. A ceramic-to-metal seal is proposed in order to produce a water-to-vacuum seal that will not outgas and produce impurities. Experimental results from an uncooled window on a short-pulse prototype experiment are modeled using finite-element (FE) results in order to correlate with the heat flux profile. The design of the actively cooled window assembly is presented with computational fluid dynamics and FE analyses to confirm that the hardware can function with a 200-kW antenna within performance limits.

High density negative hydrogen ion production in a high power pulsed helicon discharge

Temporospatially resolved measurements of negative hydrogen ion density and related parameters are observed in a 40 ms pulsed high power helicon plasma source with a magnetic mirror configuration. Exceptionally high negative ion densities of are observed on the far side of the magnetic mirror during the early stages of the pulse. It is proposed that the temporal evolution of the negative ion density into the steady-state is primarily associated with neutral dynamics and depletion. This interpretation is further supported by a 2D-axisymmetric model.

Operational domain of Proto-MPEX

The Prototype-Material Plasma Exposure eXperiment (Proto-MPEX) is a linear plasma device to test the plasma source concepts for the future Material Plasma Exposure eXperiment (MPEX). MPEX is planned to address plasma material interactions under fusion reactor divertor conditions as will occur in devices like DEMO, including the plasma exposure of a-priori neutron irradiated materials. This requires a novel linear plasma device with a plasma source that can produce high electron densities along with heating systems to obtain high electron and ion temperatures. In Proto-MPEX a high-power helicon source was tested together with electron heating (28 GHz microwaves) and ion cyclotron heating (8.5–12 MHz). Electron densities in excess of 1 × 1020 m−3 were produced with the helicon antenna. Electron temperatures can reach 20 eV and are typically below 6 eV at high densities (>5 × 1019 m−3). Ion heating has been demonstrated at low electron densities (<2 × 1019 m−3) with ion temperatures of up to 13 eV. Maximum heat fluxes of 14 MW/m2 have been obtained in low density discharges (∼ 5 × 1018 m−3), while heat fluxes are typically only 2–3 MW/m2 in high density discharges (>2 × 1019 m−3).

Progress in the Development of a High Power Helicon Plasma Source for the Materials Plasma Exposure Experiment

Proto-MPEX is a linear plasma device being used to study a novel RF source concept for the planned Material Plasma Exposure eXperiment (MPEX), which will address plasma-materials interaction (PMI) for nuclear fusion reactors. Plasmas are produced using a large diameter helicon source operating at a frequency of 13.56 MHz at power levels up to 120 kW. In recent experiments the helicon source has produced deuterium plasmas with densities up to ~6 × 1019 m–3 measured at a location 2 m downstream from the antenna and 0.4 m from the target. Previous plasma production experiments on Proto-MPEX have generated lower density plasmas with hollow electron temperature profiles and target power deposition peaked far off axis. The latest experiments have produced flat Te profiles with a large portion of the power deposited on the target near the axis. This and other evidence points to the excitation of a helicon mode in this case.

A High Power Helicon Antenna Design for DIII-D

A new antenna design for driving current in high beta tokamaks using electromagnetic waves, called Helicons, will be experimentally tested for the first time at power approaching 1 megawatt (MW) in the DIII-D Tokamak. This method is expected to be more efficient than current drive using electron cyclotron waves or neutral beam injection, and may be well suited to reactor-like configurations. A low power (100 watt (W)) 476 megahertz (MHz) “comb-line” antenna, consisting of 12 inductively coupled electrostatically shielded, modular resonators, was tested in DIII-D and showed strong coupling to the plasma without disturbing its characteristics or introducing metal impurities.The high power antenna consists of 30 modules affixed to back-plates and mounted on the outer wall of the vacuum vessel above the mid-plane. The antenna design follows a similar low power antenna design modified to minimize RF loss. Heat removal is provided by water cooling and a novel heat conducting path using pyrolytic graphite sheet. The CuCrZr antenna modules are designed to handle high eddy current forces. The modules use molybdenum Faraday shields that have the plasma side coated with boron carbide to enhance thermal resistance and minimize high Z impurities. A RF strip-line feed routes the RF power from coaxial vacuum feed-throughs to the antenna. Multipactor analysis of the antenna, strip line, and feed-through will be performed. A 1.2 MW, 476 MHz klystron system, provided by the Stanford Linear Accelerator (SLAC) will provide RF power to the new antenna. A description of the design of the high power antenna, the RF strip-line feeds, and the vessel installation will be presented.

Ion heating and flows in a high power helicon source

We report experimental measurements of ion temperatures and flows in a high power, linear, magnetized, helicon plasma device, the Resonant Antenna Ion Device (RAID). Parallel and perpendicular ion temperatures on the order of 0.6 eV are observed for an rf power of 4 kW, suggesting that higher power helicon sources should attain ion temperatures in excess of 1 eV. The unique RAID antenna design produces broad, uniform plasma density and perpendicular ion temperature radial profiles. Measurements of the azimuthal flow indicate rigid body rotation of the plasma column of a few kHz. When configured with an expanding magnetic field, modest parallel ion flows are observed in the expansion region. The ion flows and temperatures are derived from laser induced fluorescence measurements of the Doppler resolved velocity distribution functions of argon ions.

Development of a high power Helicon system for DIII-D

A mechanism for driving current off-axis in high beta tokamaks using fast electromagnetic waves, called Helicons, will be experimentally tested in the DIII-D tokamak. This method is calculated to be more efficient than current drive using electron cyclotron waves or neutral beam injection, and it may be well suited to reactor-like configurations [1,2]. A low power (100W) 476MHz “combline” antenna, consisting of 12 inductively coupled, electrostatically shielded, modular resonators [3], was recently installed in DIII-D. Initial operation showed that the plasma operating conditions were achieved under which helicon waves can be launched. Plasma operations also showed that the location of the antenna has not reduced the performance of, or introduced excessive impurities into, the discharges produced in DIII-D.The development of a high power (1MW) Helicon system is underway. This antenna consists of 30 modules mounted on the inside of the outer wall of the vacuum vessel slightly above the midplane. Carbon tiles around the antenna protect the antenna from thermal plasma streaming along field lines. A 1.2MW, 476MHz klystron system will be transferred from the Stanford Linear Accelerator to DIII-D to provide the RF input power to the antenna.A description of the design and fabrication of high power antenna and the RF feeds, the klystron and RF distribution systems, and their installation will be presented.

Parametric Instabilities During High Power Helicon Wave Injection on DIII-D

High power helicon (whistler) waves at a frequency of 0.47 GHz are being considered for efficient off-axis current generation in high performance DIII-D plasmas and in K-Star [3]. The need for deploying helicon waves for current profile control has been noted in previous publications since penetration to the core of reactor grade plasmas is easier than with lower hybrid slow waves (LHCD) which suffer from accessibility limitations and strong electron Landau absorption in fusion grade high temperature plasmas. In this work we show that under typical experimental conditions in present day tokamaks with 1 MW of RF power coupled per antenna, the associated perpendicular electric fields of the order of 40 kV/m can drive strong parametric decay instabilities near the lower hybrid layer. The EXB and polarization drift velocities which are the dominant driver of the PDI can be comparable to the speed of sound in the outer plasma layers, a key measure of driving PDI instabilities. Here we calculate growth rates and convective thresholds for PDIs, and we find that decay waves into hot ion lower hybrid waves and ion cyclotron quasi modes dominate in the vicinity of the lower hybrid layer, possibly leading to pump depletion. Such instabilities in future reactor grade high temperature plasmas are less likely.