Lower Hybrid Launcher Research Articles

Studies of the outer-off-midplane lower hybrid wave launch scenario for plasma start-up on the TST-2 spherical tokamak

Establishment of an efficient central solenoid (CS) free tokamak plasma start-up method may lead to an economical fusion reactor. CS-free start-up using lower hybrid (LH) waves has been studied on the TST-2 spherical tokamak. Plasma current of about a quarter of CS-driven discharges has been obtained fully non-inductively using the outer-midplane and top LH launchers. Recently, an outer-off-midplane LH launcher was developed to achieve higher plasma current by optimizing for core absorption and minimal fast electron losses. Using the (outer-)off-midplane launcher, fully non-inductive plasma current start-up up to about 8 kA was achieved. Coupled ray-tracing and Fokker–Planck simulation was performed on equilibria reconstructed with an extended MHD model. It was found that the experimentally observed plasma current was in reasonable agreement with the numerical simulation. The simulation predicted appreciable orbit losses for the off-midplane launcher driven discharge at the present parameters, which was consistent with the experimentally observed x-ray radiation characteristics. The simulation showed that the current density was saturated for the present off-midplane launcher discharges and higher density and higher LH power was necessary to achieve higher plasma current.

Metallic impurity content behavior during ICRH-heated L-mode discharges in EAST

This study uses the diversity of materials at known locations in the experimental advanced superconducting tokamak to extract, local information on which plasma-surface interaction processes are dominant during ion cyclotron resonance heating (ICRH) (near vs far field effects). Metallic impurity content is indicated by the intensity of impurity line emissions observed with a fast-time-response extreme ultraviolet spectrometer, normalized to line-averaged plasma density, i.e. Iimp/ne. Parametric dependencies are explored over scans of ICRH and LH (lower hybrid) power and for different toroidal phasings between straps, strap power balance, and magnetic configurations. This diversity of behavior is interpreted as the signature of different physical processes. Before 2017, as only the upper divertor region contained tungsten (W), the W content in the core used to increase a lot when moving from lower to upper single null configurations, and was correlated with the total injected power rather than the ICRH power. Molybdenum, covering the part of the inner wall facing one ion cyclotron range of frequencies (ICRF) antenna appear sensitive to this antenna power and phasing benchmarked by modeling suggesting a probable interaction due to residual ICRF waves crossing the plasma. Materials close and magnetically-connected to an active antenna show better correlation with ICRF antenna electrical tuning than those which are far away, or not connected. This is particularly the case with W since 2018, because the limiter tiles of the LH launchers were changed from graphite to tungsten. In these latter conditions, it is shown that W sources at the mid-plane (equatorial plane) contribute to a significant fraction of the core contamination by tungsten (25% in ohmic regime, and more during ICRH).

Nuclear response of additive manufactured GRCop-84 copper for use in Lower hybrid launchers in a fusion environment

Abstract Recent advances in selective laser melting (SLM) 3D printing technology allow additive manufacture of lower hybrid current drive (LHCD) RF launchers from a new material, Glenn Research Copper 84 (GRCop-84), a Cr2Nb (8 at. % Cr, 4 at. % Nb) precipitation hardened alloy, in configurations unachievable with conventional machining. Tensile strength and thermal conductivity are compared to other competing high conductivity copper alloys for fusion use. Swelling and conductivity at 100 DPA when exposed to the Hanford Fast Flux Test Facility neutron spectrum is predicted in the 1–2 % and 65−70% IACS range, respectively, by comparison to copper alloys with similar precipitate and grain size. Self-similar ion irradiation of a GRCop-84 target at 430 °C to 20 DPA resulted in no noticeable void formation in FIB cross sections or TEM lamella.

Electron burst driven by near electric field effects of lower‐hybrid launchers

Hotspot generation by lower‐hybrid (LH) launchers is found to be governed by a resonance in the plasma electric field response to the external drive. The kinetic analysis in 1D‐1V in the parallel direction allows one to compute the amplification effect for small amplitude of the external drive. The resonant Lorentzian response distorts the distribution function. An island structure is formed in the suprathermal part at the phase velocity of the external electrostatic drive. The non‐linear features enhance the plasma response, driving overlap effects between multiple waves at rather low amplitude. The onset of a plateau in the distribution function, with extent reaching one thermal velocity, is already obtained when the standard overlap condition is achieved. The sensitivity of the resonance to plasma parameters and large variation of the amplification magnitude can compensate the fast radial decay of the small‐scale features generated by the LH launchers, which are responsible for the interaction with the cold electrons. This mechanism can trigger hotspot generation further in the scrape‐off layer than otherwise expected.

Analysis of Poloidal Asymmetric Density Behaviors in SOL Induced by 4.6-GHz Lower Hybrid Launcher Power in EAST

On the experimental advanced superconducting tokamak (EAST), a series of striations, including a few strong emissivity striations and several low emissivity striations, were observed in front of the 4.6-GHz lower hybrid (LH) launcher with the visible video camera for the LH power discharge. These striations indicate that LH may create significant poloidal scrape-off layer (SOL) density profile asymmetries in front of the LH launcher. These poloidal asymmetric density behaviors are further confirmed with the edge density measured by two Langmuir probes installed at the top and bottom of the LH launcher. The measured density depends on LH power injection and magnetic field direction. A 2D diffusive convective model was used to study the mechanisms of the observed striations and poloidal asymmetric density. The simulation results qualitatively match with the measured density, indicating these poloidal asymmetric effects are ascribed to the LHW-induced ELH × B t drift.

Characterization of the mutual influence of Ion Cyclotron and Lower Hybrid Range of frequencies systems on EAST

Waves in the Ion Cyclotron (ICRF) and Lower Hybrid (LH) Range of Frequencies are efficient techniques respectively to heat the plasma and drive current. Main difficulties come from a trade-off between good RF coupling and acceptable level of impurities release. The mutual influence of both systems makes such equilibrium often hard to reach [1]. In order to investigate those interactions based on Scrape-Off Layer (SOL) plasma parameters, a new reciprocating probe was designed allying a three tips Langmuir probe with an emissive wire. The emissive filament provides a precise measure of plasma potential [2], which can be used to calibrate Langmuir probe's results. This paper reports on experimental results obtained on EAST, where there are two ICRF antennas and two LH launchers. Among others diagnostics, the new reciprocating probe enabled to evidence the deleterious influence of ICRF power on LHWs coupling in L-mode plasmas. In areas connected with an active ICRF antenna, SOL potentials increase while densities tend to decrease, respectively enhancing impurities release and deteriorating LHWs coupling. This phenomenon has mostly been attributed to RF sheath; the one that forms on top of Plasma Facing Components (PFCs) and causes ExB density convections [3]. From those experiments it seems ICRF has a strong influence on magnetically connected areas, both in the near field – influencing ICRF waves coupling – and in farther locations such as in front of LH grills. Moreover, influence of ICRF on LH system was observed both in L and H modes. Those results are consistent with RF sheath rectification process. Concerning the influence of LHWs on ICRF coupling, nothing was observed in L-mode. Besides during H-mode experiments, LHWs have been identified as having a mitigating effect on ELMs [4], which on average lowers the pedestal, increasing edge densities to the profit of ICRF waves coupling.

Preliminary investigation of the effects of lower hybrid power on asymmetric behaviors in the scrape-off layer in experimental advanced superconducting tokamak

The striations in front of the lower hybrid (LH) launcher have been observed during LH injection by a visible video camera in the Experimental Advanced Superconducting Tokamak. Edge density at the top of the LH launcher tends to be much larger in reversed magnetic field (Bt) than that in the normal Bt. To study the mechanisms of the observations, the diffusive-convective model is employed. Simulations show that the LH power makes the density in scrape-off layer asymmetric in poloidal direction with five density peaks. The locations of the striations are approximately in agreement with the locations of the density peaks in different directions of Bt. Higher LH power strengths the asymmetry of the density and leads to a bad coupling which is in conflict with the experimental results showing a good coupling with a higher power. Furthermore, an ionization term is introduced into this model and the increase of edge density with LH power can be qualitatively explained. The simulations also show that the density peaks in front of the waveguides become clearer when taking into account gas puffing.

Operation and coupling of LH waves with the ITER-like wall at JET

In this paper important aspects of the lower hybrid (LH) operation with the ITER-like wall (ILW) [] at JET are reported. Impurity release during LH operation was investigated and it was found that there is no significant Be increase with LH power. The concentration of W was analysed in more detail and it was concluded that LH negligibly contributes to its increase. No cases of W accumulation in LH-only heating experiments were observed so far. LH wave coupling was studied and optimised to achieve the level of system performance similar to before ILW installation. Measurements by Li-beam were used to study systematic dependencies of the scrape-off layer (SOL) density on the gas injection rate from a dedicated gas introduction module and the LH power and launcher position. Experimental results are supported by SOL transport modelling. Observations of arcs in front of the LH launcher and hotspots on magnetically connected sections of the vessel are reported. Overall, a relatively trouble-free operation of the LH system up to 2.5 MW of coupled radio frequency power in L-mode plasma was achieved with no indication that the power cannot be increased further.

Effects of ICRF power on SOL density profiles and LH coupling during simultaneous LH and ICRF operation on Alcator C-Mod

A dedicated experiment during simultaneous lower hybrid (LH) and ion cyclotron range-of-frequencies (ICRF) operations is carried out to evaluate and understand the effects of ICRF power on the scrape-off-layer (SOL) density profiles and on the resultant LH coupling for a wide range of plasma parameters on Alcator C-Mod. Operation of the LH launcher with the adjacent ICRF antenna significantly degrades LH coupling while operation with the ICRF antenna that is not magnetically connected to the LH launcher minimally affects LH coupling. An X-mode reflectometer system at three poloidal locations adjacent to the LH launcher and a visible video camera imaging the LH launcher are used to measure local SOL density profile and emissivity modifications with the application of LH and LH + ICRF power. These measurements confirm that the density in front of the LH launcher depends strongly on the magnetic field line mapping of the active ICRF antenna. Reflectometer measurements also observe both ICRF-driven and LH-driven poloidal density profile asymmetries, especially a strong density depletion at certain poloidal locations in front of the LH launcher during operation with a magnetically connected ICRF antenna. The results indicate that understanding both LH-driven flows and ICRF sheath driven flows may be necessary to understand the observed density profile modifications and LH coupling results during simultaneous LH + ICRF operation.

Advances in lower hybrid current drive technology on Alcator C-Mod

Lower hybrid current drive (LHCD) is an attractive option for non-inductive tokamak operation due to its high current drive efficiency and ability to drive current off axis. The parameters of the Alcator C-Mod LHCD system (f0 = 4.6 GHz, Bφ ≃ 5.5 T, ) are similar to the proposed LHCD system on ITER. This paper will describe improvements in LHCD technology on C-Mod designed to increase single-pass absorption at high , extend pulse length (to >3 s), and increase power delivered to the plasma (to ∼2 MW). Modelling of lower hybrid (LH) wave propagation indicates that the observed loss of LHCD efficiency at higher can be mitigated by enhancing the single pass power absorption through use of an off mid-plane launcher. The four rows of the launcher are located above the mid-plane (with Ip and Bφ both clockwise viewing from the top down) in order to exploit the poloidal upshift of n‖ as rays propagate from the antenna into the plasma. The transmitter protection system (TPS) was redesigned to model the coolant temperature in real time and shut off the klystron beam voltage if the coolant is close to boiling. The TPS upgrade has been installed and operated on C-Mod for pulses up to 4.5 s into dummy loads and 1.0 s into the plasma. A new movable local LH launcher protection limiter was designed to reduce reflection coefficients across a wide range of launcher positions. Finally, a high power waveguide double-stub tuner is under development to provide feedback controlled load matching to reduce power reflected from the antenna under poor coupling conditions.

Effects of LH power on SOL density profiles and LH coupling on Alcator C-Mod

A swept-frequency X-mode reflectometer has been used to measure the scrape-off-layer (SOL) density profiles with and without lower hybrid (LH) power at three poloidal locations adjacent to the LH launcher for various plasma parameters in order to understand the coupling of LH waves on Alcator C-Mod. LH power has been observed to create significant poloidal SOL density profile asymmetries that are correlated with visible video camera images of emissivity patterns in front of the LH launcher. The observed density profile asymmetries depend on LH power, , magnetic geometry and magnetic field direction. A 2D diffusive–convective model has been used to show that these density profile modifications are consistent with a LH vortex, where LH power drives E × B drifts that then modify the SOL density profile. In particular, the simulations show that the density profile can possibly create a net poloidally averaged density depletion in front of the waveguide rows. A LH slab coupling model is then used to show that the simulated reflection coefficients strongly depend on the poloidal density profile asymmetries. The simulated LH power reflection coefficients agree with the experimental reflection coefficients only after the observed density depletion is included in the model.

Progress of KSTAR 5-GHz Lower Hybrid Current Drive System

A 5-GHz steady-state lower hybrid (LH) current drive (LHCD) system is planned to support steady-state and advanced tokamak operation on the Korea Superconducting Tokamak Advanced Research (KSTAR) experiment. As an initial phase, a pulsed 5-GHz, 500-kW LHCD system has been installed in KSTAR for basic experimental studies of the LH coupling and flux saving in the plasma current ramp-up, prior to long-pulse noninductive operation in KSTAR. A Toshiba-made klystron developed in collaboration with Pohang University of Science and Technology in 2006 is utilized for the initial KSTAR LHCD system. The LH launcher is designed as a fully active waveguide grill type with a parallel refractive index n[parallel] value ranging from 1.8 to 4.3 and with high directivity. In the initial stage, the LH launcher consists of eight columns of four-way power splitters and two columns of dummy waveguides, one on each side. The operational n[parallel] value is fixed at 2.1 but can be adjusted by replacing waveguide components external to the vacuum vessel. Since the target operation pulse duration of the initial LHCD system is 2 s with an output power of 500 kW at the klystron window, the prototype klystron was recently successfully conditioned to a radio frequency power of 514 kW for a maximum pulse duration of 3 s using a matched dummy load (voltage standing wave ratio of 1.16:1). This paper presents the progress of the initial KSTAR LHCD system and the performance test results of the prototype klystron. The research plan aiming at steady-state LHCD operation in KSTAR is also described in this paper.

Observation of Spectral Broadening of Lower Hybrid Waves in Alcator C-Mod

Observations of the spectral broadening of lower hybrid (LH) waves in Alcator C-Mod are presented. We report the dependence of the pump broadening on two plasma parameters: plasma density and magnetic topology. As the plasma density was raised, we observed a significant increase of the pump broadening at 10 dB below the maximum peak. However, at the constant plasma density, when the magnetic topology was changed from the diverted plasma to the inner wall limited plasma, the spectral broadening was reduced. In an attempt to understand the causes of this variation, we report the change of density profiles in front of the LH launcher in two different magnetic topologies. The model analyses indicate that both the scattering process and parametric decay instability can contribute to the spectral broadening. Further experimental investigations are necessary to identify the causality between spectral broadening and the degradation of lower hybrid current drive efficiency at high density plasmas.

Scrape-off layer reflectometer for Alcator C-Mod

A swept-frequency X-mode reflectometer is being built for Alcator C-Mod to measure the scrape-off layer density profiles at the top, middle, and bottom locations in front of both the new lower hybrid launcher and the new ion cyclotron range of frequencies antenna. The system is planned to operate between 100 and 146 GHz at sweep rates from 10 μs to 1 ms, and will cover a density range of approximately 10(16)-10(20) m(-3) at B(0)=5-5.4 T. To minimize the effects of density fluctuations, both differential phase and full phase reflectometry will be employed. Design, test data, and calibration results of this electronics system will be discussed. To reduce attenuation losses, tallguide (TE(01)) will be used for most of the transmission line system. Simulations of high mode conversion in tallguide components, such as e-plane hyperbolic secant radius of curvature bends, tapers, and horn antennas will be shown. Experimental measurements of the total attenuation losses of these components in the lower hybrid waveguide run will also be presented.

Waveguide Splitter for Lower Hybrid Current Drive

We have developed high power four and eight way splitters for a new Lower Hybrid launcher. The motivation for the new launcher was the need to provide more power and reliability to the launcher structure. In addition there was a desire to simplify and increase the reliability of the implementation of the alumina windows. The launcher consists of 64 waveguide apertures powered by 8 klystrons with maximum power of 250 kW each at 4.6 GHz. Hence, it is necessary to split the power from each Klystron into eight separate waveguides. The outputs of the splitter have a difference in power less than 0.1dB and phase less than 2 degree. The design analysis of the splitter was done with the computer code CST. Structure analysis was performed using Ansys. The splitter is fabricated by machining an open cavity into a thick stainless steel plate creating the specified internal geometry. It is machined to a tight tolerance of +/- 0.005″. A fitted lid is then welded on top of the open cavity using electron beam welding. The excess metal is removed with Electro discharge machining (EDM) creating the external geometry. The waveguides are then butt-welded to the splitter. Welding fixtures/parameters are being developed to achieve the desired tolerances. Two methods for attaching the ceramic windows are being evaluated, brazing and electro-forming.

SOL characterization and LH coupling measurements on JET in ITER-relevant conditions

Lower hybrid (LH) current drive experiments have been carried out on JET with an antenna–plasma separatrix distance varying between 0.09 and 0.15 m, and LH power in the range 0–3.2 MW. For different plasma configurations, the electron density ne of the scrape-off layer has been studied by the mean of a reciprocating Langmuir probe magnetically connected to the LH antenna. For pulses in the high confinement regime (H mode) characterized by strong particle bursts in the plasma edge, the edge localized modes (ELMs), profiles of the saturation current (Jsat) are obtained with a sufficient time resolution to distinguish ‘between ELMs’ and during the rise and decay of the ELMs.It is found that gas injection from a valve located near the LH launcher and magnetically connected to it allows one to raise the density and improve the LH coupling. The Jsat profiles indicate quite clearly that this density rise affects mainly the plasma layer in front of the antenna with a typical thickness of 5 cm. The resulting profile can be extremely flat in this region. The effect of the near-launcher gas injection but also of the LH power and the total gas injection on the density at the wall is quantitatively documented. It is shown in particular that with increasing LH power, the required gas injection for obtaining good LH coupling is decreasing, with no saturation obtained so far. Effect of the ELMs on the LH coupling is also discussed. Modelling with the EDGE2D code indicates that such flat profiles of Jsat/ne can be obtained when LH power dissipation is taken into account. Detailed analysis of the heat flux carried by electrons accelerated in the near-field of the antenna confirms the increase in density with gas puff during high LH power coupling.

Effect of gas injection during LH wave coupling at ITER-relevant plasma–wall distances in JET

Good coupling of lower hybrid (LH) waves has been demonstrated in different H-mode scenarios in JET, at high triangularity (δ ∼ 0.4) and at large distance between the last closed flux surface and the LH launcher (up to 15 cm). Local gas injection of D2 in the region magnetically connected to the LH launcher is used for increasing the local density in the scrape-off layer (SOL). Reciprocating Langmuir probe measurements magnetically connected to the LH launcher indicate that the electron density profile flattens in the far SOL during gas injection and LH power application. Some degradation in normalized H-mode confinement, as given by the H98(y,2)-factor, could be observed at high gas injection rates in these scenarios, but this was rather due to total gas injection and not specifically to the local gas puffing used for LH coupling. Furthermore, experiments carried out in L-mode plasmas in order to evaluate the effect on the LH current drive efficiency, when using local gas injection to improve the coupling, indicate only a small degradation (ΔILH/ILH ∼ 15%). This effect is largely compensated by the improvement in coupling and thus increase in coupled power when using gas puffing.

Scrape-off layer reflectometer for Alcator C-Mod

A two-frequency x-mode reflectometer operating from 100 to 146 GHz is deployed on Alcator C-Mod to measure the density profile and fluctuations in the scrape-off layer (SOL) immediately in front of the new J-port ICRF antenna and the new C-port lower hybrid launcher. The reflectometer covers densities from 10(16) to 10(20) m(-3) at 5-5.4 T. To provide the greatest flexibility and capability to deal with density fluctuations approaching 100% peak-to-peak in the SOL, both full-phase and differential-phase measurement capabilities with sweep speeds of approximately 10 micros to >1 ms are implemented. The differential-phase measurement uses a difference frequency of 500 MHz, corresponding to cutoff layer separations ranging from about 0.1 to 1 mm. The reflectometer has six sets of launchers: three on the ICRF antenna and three on the lower hybrid launcher. Both the ICRF antenna and the lower hybrid launcher incorporate reflectometer antennas at their top, bottom, and midplane locations.

Images of the TE Wave Inside Tore Supra's LH PAM Launcher

A new lower hybrid launcher based on the passive active multijunction principle and allowing for the withstand of conditions envisioned in the advanced scenarios for the International Thermonuclear Experimental Reactor (large density variations, thermal loads, neutron fluxes, and mechanical stresses) has been designed for the purpose of heating and current drive of fusion plasmas in the Tore Supra tokamak. A 3-D image of the TE wave inside the full waveguide structure of one of its modules is presented.

Wave-Particle Studies in the Ion Cyclotron and Lower Hybrid Ranges of Frequencies in Alcator C-Mod

This paper reviews the physics and technology of wave-particle-interaction experiments in the ion cyclotron range of frequencies (ICRF) and the lower hybrid (LH) range of frequencies (LHRF) on the Alcator C-Mod tokamak. Operation of fixed frequency (80 MHz) and tunable (40- to 80-MHz) ICRF transmitters and the associated transmission system is described. Key fabrication issues that were solved in order to operate a four-strap ICRF antenna in the compact environment of C-Mod are discussed in some detail. ICRF heating experiments utilizing the hydrogen (H) and helium-3 (3He) minority heating schemes are described, and data are presented demonstrating an overall heating efficiency of 70 to 90% for the (H) minority scheme and somewhat lower efficiency for (3He) minority heating. Mode conversion electron heating experiments in D(3He), D(H), and H(3He) discharges are also reported as well as simulations of these experiments using an advanced ICRF full-wave solver. Measurements of mode-converted ion cyclotron waves and ion Bernstein waves using a phase contrast imaging diagnostic are presented and compared with the predictions of a synthetic diagnostic code that utilizes wave electric fields from a full-wave solver. The physics basis of the LH current profile control program on Alcator C-Mod is also presented. Computer simulations using a two-dimensional (velocity space) Fokker Planck solver indicate that ~200 kA of LH current can be driven in low-density H-mode discharges on C-Mod with ~3 MW of LHRF power. It is shown that this off-axis LH current drive can be used to create discharges with nonmonotonic profiles of the current density and reversed shear. An advanced tokamak operating regime near the ideal no-wall β limit is described for C-Mod, where ~70% of the current is driven through the bootstrap effect. The LH power is coupled to C-Mod through a waveguide launcher consisting of four rows (vertically) with 24 guides per row (toroidally). A detailed description of the LH launcher fabrication is given in this paper along with initial operation results.