High Magnetic Field Side Research Articles

Non-uniformity of the radial electric field for tokamak plasma on the low and the high toroidal magnetic field side

This paper presents a relativistic mechanism that can create a non-uniformity of a radial electric field on the low side and the high toroidal magnetic field side of tokamak plasmas. This mechanism is associated with the significant current velocity of electrons in the plasma for many experimental modes of tokamak operation. It is important that electric fields arise without the separation of charges in the plasma. The traditional approach to determining radial electric fields in tokamak plasmas gives a uniform radial electric field on magnetic surfaces inside the plasma, while there are no radial electric fields outside the plasma. The presented mechanism leads to the appearance of an electric field outside the plasma with a current, and a non-uniformity of the radial electric field on magnetic surfaces appears inside the plasma. The proposed mechanism can explain the experimentally measured non-uniformity of C+6 carbon ion toroidal rotation velocities in the Tore-Supra tokamak plasma.

Simulations of Reflectometer Response to ITER Plasma Perturbations Caused by Alfvén Modes

The KINX and VENUS codes were used for simulation of the baseline inductive and steady-state scenarios of the ITER tokamak operation. The perturbations of plasma electron density and magnetic field caused by the Alfvén modes were calculated in the flux coordinates for these scenarios. The perturbation fields obtained were converted into the engineering coordinates in order to calculate the propagation of probe electromagnetic radiation of the reflectometer using the two-dimensional full-wave TAMIC RτX code in the expected geometry of the experiment. The calculations performed show that for the baseline inductive scenario, in the case of reflection of the extraordinary wave at the lower cutoff frequency from the high magnetic field side, the electric field relative perturbations of the reflected reflectometer signal correspond to the margin of linear range of the diagnostics operation or even go out of this range. It was found that in a number of scenarios, not only the electron density perturbations, but also the magnetic field perturbations significantly contribute to the total signal perturbations that makes even more difficult the further data interpretation. Another possible problem is the narrow frequency range of probing frequencies where the Alfvén mode can be observed. In addition to simulating the reflection of electromagnetic waves from plasma, it was analyzed also the possibility of measuring the Alfvén modes parameters when the extraordinary wave pass through the plasma in the transparency window between the upper and lower cutoff frequencies of the extraordinary wave (refractometry). It is shown that at the fundamental frequency, the phase perturbations range from 3 to 60 degrees, which makes it impossible to use the amplitude-modulated refractometer for analyzing signals. The “synthetic diagnostics” approach was used, which showed itself well for simulating the operation of reflectometers at plasma facilities.

Designing an upgrade of ohmic heating system for the QUEST spherical tokamak

An upgraded Ohmic heating (OH) system with a bipolar circuit is designed to achieve plasma current over 100 kA ideally without the effect of eddy current in the QUEST spherical tokamak. IGBT (Insulated Gate Bipolar Transistor) stacks work as switches to change the polarity of OH primary current up to ±8 kA. A main target of the new OH is to generate over-dense plasmas for the 8.56 GHz electron cyclotron (EC) wave. Heating with electron Bernstein waves can be conducted in such over-dense plasmas. The effect of eddy current flowing in the vacuum vessel, which is generated by the current swing through the central solenoid, on null point is investigated by analysis of magnetic field and flux surfaces. In the initial phase, eddy current flowing through the vacuum chamber shifts the null point to the high magnetic field side where multiple EC resonance layers are located.

Electrically induced breakdown of the quantum Hall effect at different Hall bar widths: Visualizing the edge- and bulk-dominated regimes within a quantum Hall plateau

We present systematic investigations of the electrically induced breakdown of the integer quantum Hall effect (QHE) in (Al,Ga)As-based Hall bars of different widths ranging from 7 to 70 $\ensuremath{\mu}\mathrm{m}$. It is striking that, for the narrow Hall bars, the threshold values for the applied voltage that induces the breakdown of the longitudinal zero-resistance state differ appreciably between the upper and lower sides of a quantum Hall (QH) plateau. When moving from the low magnetic field side of the QH plateau to higher magnetic fields, the threshold rises in a strongly superlinear manner until---at the expected integer value for the Landau level (LL) filling factor in the bulk of the two-dimensional electron system (2DES)---the threshold value abruptly drops to low values for the rest of the plateau. With increasing widths of the Hall bars, the zero-resistance state extends slightly further to higher magnetic field values. The threshold values on the low-magnetic field side are almost independent of the Hall bar width, whereas the threshold values on the high magnetic field side scale linearly with the Hall bar width. These observations correspond perfectly with the microscope picture of the QHE where the biased current flows in a dissipationless manner in electrically incompressible regions of the 2DES of locally the same LL filling factor, driven by the respective drop of the Hall voltage over the width of these incompressible regions. Owing to the self-consistent evolution of the electrically incompressible/compressible landscape within the 2DES as a function of magnetic field, a transition from an edge- to a bulk-dominated QH regime is described within a QH plateau. This microscopic picture was derived from sophisticated, long-lasting scanning probe experiments measuring Hall potential profiles on typically 15-$\ensuremath{\mu}$m-wide Hall bars. Conversely, performing systematic electrically induced breakdown measurements like those presented here would allow us to identify the presence of edge- and bulk-dominated regimes of the QHE in a wider variety of samples.

Advanced ASDEX Upgrade pellet guiding system design.

Cryogenic pellet injection will be the prime candidate to fuel future fusion power plants. In order to harvest optimum fueling performance, it is essential to inject pellets from the magnetic high field side of the tokamak. The pellet launching system of the tokamak ASDEX Upgrade injects cryogenic hydrogen pellets with a speed of up to 1000 m/s from the magnetic high field side via curved guiding tubes. Pellets passing the guiding tube are sliding on a gas cushion, generated by the Leidenfrost effect. The actual track has a rectangular cross section and is composed of a series of ellipses in order to generate the required 270° looping type turn; the path length is 17 m. The last part of this track is marked by strong geometrical constraints from the vacuum vessel port. The previous design was composed of a sequence of three sections of ellipses too, tangentially constant but discontinuous with regard to the curvature. It had been in operation for almost 20 years. Its steps in the curvature are supposed to limit the system performance. A novel and advanced geometry concept, adopting a method well-known from civil engineering (e.g., for the railroad track design), has been applied to develop an improved design. It relies on clothoid shape sections keeping the track curvatures continuous and, thus, provides a smooth transition between all the elements. The new design presented improves the pellet launching system performance on ASDEX Upgrade and provides knowledge for an advanced design of pellet guiding tubes in future fusion devices.

3D structure of density fluctuations in the T-10 tokamak and new approach for current profile estimation

The paper is focused on the new systematic measurements of the 3D spatial distributions of the amplitudes, the radial correlation lengths and the long-range correlations along the magnetic field lines for the different turbulence types. The density fluctuations were measured by the heterodyne correlation reflectometry (CR) using the plasma probing with ordinary mode. CR data was supported by recent experiments with the measurements of the perturbation properties using an heavy ion beam probe (HIBP). The new reflectometer antenna array in T-10 tokamak consists of sets of horns distributed at four places toroidally and poloidally over the torus. The experiments confirmed previously found strong poloidal asymmetry of the amplitude for the broadband (BB) and quasi-coherent (QC) fluctuations. It was found that amplitude of density fluctuations is uniform poloidally for the stochastic low frequency (SLF) fluctuations. The radial correlation were measured at four poloidal angles to reveal the poloidal dependence of the radial correlation length for the different fluctuation types. The significant decrease of the radial correlation lengths towards the high magnetic field side was observed for the QC and the SLF fluctuations. The long-range correlations along the field lines were measured by the reflectometers in two cross-sections separated by 1/4 of the torus. The reflectometers had the same probing frequency thus provide reflection from the same magnetic surface. The measurements were carried out at the low and the high field sides with two currents and two magnetic configurations with simultaneous reversal of the toroidal field and plasma current. The positions of the resonance radius were calculated also using 3D tracing of the magnetic field line and demonstrated good agreement with experiment ones. These results allow to propose the new approach for the current profile measurements in tokamaks.

Measurements of electron density profile by frequency modulated continuous wave reflectometer in the T-10 tokamak using high magnetic field side probing and extraordinary mode lower cutoff.

A new, previously unused combination of the high magnetic field side probing and the lower extraordinary mode cutoff was used in the T-10 tokamak for the electron density profile measurements by using a frequency modulation continuous wave reflectometer. This scheme has a significant advantage for large fusion machines where flat electron density profiles and high electron temperatures are expected. The reflectometer design is based on a common scheme with voltage controlled oscillators, active multipliers, and in-phase/quadrature signal detection. Iterative procedures were applied to calibrate a microwave source frequency and compensate the dispersion of probing signals in the conventional rectangular waveguides used as the transmission lines. About 0.2% stability of the beating frequency was reached after the calibration procedure had been finished. An advanced finite-difference scheme was proposed for the reconstruction of the electron density profile to reduce errors and improve the overall performance of the algorithm. Reconstructed electron density profiles demonstrate good agreement with conventional T-10 diagnostic data. Some specific technical and physical aspects of the diagnostic were also discussed on the basis of T-10 operation experience.

Fast synthetic X-mode Doppler reflectometry diagnostics for the full-f global gyrokinetic modeling of the FT-2 tokamak

The fast linear (Born approximation) version of the X-mode Doppler reflectometry (DR) synthetic diagnostics is developed in the framework of the ELMFIRE global gyrokinetic modeling of the FT-2 tokamak ohmic discharge. The DR signal frequency spectra and the dependence of their frequency shift and shape on the probing antenna position are computed and shown to be similar to those measured in the high magnetic field side probing DR experiment at the FT-2 tokamak. The fluctuation poloidal velocities are determined using the DR experiment and synthetic diagnostics and shown to be within 15%. However, the computed and measured dependences of the DR signal power on the antenna position appear to be different presumably due to underestimation of the small-scale trapped electron mode turbulence component in the measurement region by the code.

Turbulence measurements on the high and low magnetic field side of the DIII-D tokamak.

In this paper, we address the challenging question of measuring turbulence levels on the high magnetic field side (HFS) of tokamak plasmas. Although turbulence measurements on the HFS can provide a stringent constraint for the turbulence model validation, to date only low magnetic field side (LFS) measured turbulence has been used in validation studies. To address this issue, an eight channel Correlation Electron Cyclotron Emission (CECE) system at DIII-D was modified to probe both LFS and HFS. In contrast to the second harmonic extraordinary mode electron cyclotron resonance emission that is typically used in CECE, we show that it is possible to probe the HFS using fundamental O-mode electron cyclotron resonance emission. The required hardware modifications for the HFS measurements are presented here, and the potential issues in this measurement are discussed.

Pellet-Injector Technology—Brief History and Key Developments in the Last 25 Years

High-speed injection of solid fuel was first proposed in 1954 as a possible solution to the problem of transporting fresh fuel across the confining magnetic fields into the plasma of a fusion reactor. While it took a few decades, the use of cryogenic pellets (typically H2 and D2) on fusion experiments became common place; most tokamaks and stellarators are now equipped with a pellet injector(s). These devices operate at low temperatures (~10 to 20 K) and most often use a simple light gas gun to accelerate macroscopic-size pellets (~0.4- to 6-mm diameter) to speeds of ~100 to 1000 m/s. Before the advantages of pellet injection from the magnetic high-field side (HFS) of a tokamak were recognized in 1997, development focused on increasing the pellet speed to achieve deeper plasma penetration and higher fueling efficiency. The HFS injection technique typically dictates slower pellets (~100 to 300 m/s) to survive transport through the curved guide tubes that route the pellets to the plasma from the inside wall of the device. Two other key operating parameters for plasma fueling are the pellet-injection repetition rate and time duration—a single pellet is adequate for some experiments and a steady-state injection rate of up to ~50 Hz is appropriate for others. In addition to plasma fueling, cryogenic pellets have often been used for particle transport and impurity studies in fusion experiments (most often with neon pellets). During the past two decades, a few new applications for cryogenic pellets have been developed and used successfully in plasma experiments: (1) one for edge-localized mode mitigation, (2) one for plasma disruption mitigation (requires large pellets that are shattered before injection into the plasma), and (3) another in which pure argon pellets are used to trigger runaway electrons in the plasma for scientific studies. In this paper, a brief history and the key developments in this technology during the past 25 years are presented and discussed.

Improvement of Flux Pinning Properties for Hafnium-Doped Gd123 Thin Films Fabricated by a Fluorine-Free MOD Method

In order to enhance the flux pinning force density $F_{p}$ in magnetic fields for REBa2Cu3O $_{y}$ (RE123)-coated conductors, Hf and Zr doping are the very valid way of introducing the perovskite nanostructure as artificial pinning centers (APCs) into the superconductors. However, the studies for Hf-doped RE123 thin films fabricated by the fluorine-free metal organic deposition (FF-MOD), which is the easiest production process, have not been done yet. In this study, we have successfully fabricated Hf-doped FF-MOD GdBa2 Cu3O $_{y}$ (Gd123) thin films on LaAlO3 substrates and investigated their flux pinning properties. Critical temperature for Gd123 thin films indicated around 92 K, and $T_{c}$ varied little by Hf doping. Hf 10 mol% doped film achieved high critical current densities of 2.72 mA⋅cm −2 at 77.3 K under 0 T, and 0.27 mA⋅cm−2 at 77.3 K under 1 T. With increasing Hf-doping amount, $F_{p}$ gradually increased, and the peak of $F_{p}$ shifted to the high magnetic field side. The elementary pinning force and the effective pinning center density also increased. We believe that effective APCs, probably BaHfO3 are introduced into FF-MOD Gd123 thin films by Hf doping.

Structural Analysis of High-Field-Side RF Antennas During a Disruption on the Advanced Divertor eXperiment (ADX)

The Advanced Divertor and RF tokamak eXperiment (ADX) is a compact, high-field device proposed by the MIT Plasma Science and Fusion Center and collaborators, which will address critical gaps in world fusion research on the pathway to fusion energy. In addition to developing and testing new divertor concepts at reactor level magnetic field strengths and power densities, ADX will test new antenna concepts for lower hybrid current drive (LHCD) and ion cyclotron range of frequency (ICRF) heating systems. In particular, ADX will be purpose-built to allow antennas to be positioned on the high magnetic field side of the torus, i.e., on the inner wall. With antennas placed at this location, plasma-wall interactions are greatly reduced and favorable RF wave physics projects to dramatic improvements in current drive efficiency and current profile control as well as very effective scenarios for RF heating and flow drive. Initial designs for the high-field-side LHCD and ICRF antennas have been completed and are analyzed to determine the loads induced during a full-current plasma disruption. While locating antennas at the inner wall is beneficial from an RF standpoint, it exposes them to a higher toroidal field which, when combined with the eddy currents caused by a disrupting plasma, will lead to higher loads. Using COMSOL Multiphysics, a model of the ADX vessel and coils is created to predict the magnetic fields, eddy currents, and loads acting on the antennas during a disruption. Structural models are then run to predict the stresses and to provide guidance for design improvement, such as determining where structural reinforcements may be necessary.

Experimental Study of the Propellant Gas Load Required for Pellet Injection with ITER-Relevant Operating Parameters

An existing pipe gun test facility at ORNL was used for an experimental study of propellant gas loads required for ITER-relevant pellet injection, with the key objective of determining the minimal amount of gas required for optimal pellet speeds. Two pellet sizes were tested, with nominal 4.4 and 3.2 mm diameters comparable to pellets planned for fueling and ELM pacing in ITER, respectively. A novel scheme was used to freeze solid pellets from room temperature gas; this facilitated operations at higher temperatures (14.5 to 16.5 K, similar to those planned for extruder operations for ITER pellet injectors) and thus lower pellet breakaway pressures and gas loads. Most of the single-shot D2 pellet tests were carried out with a relatively low H2 propellant gas load of ~0.0133 bar-L. Some limited testing was also carried out with a mixed propellant gas that consisted mostly of D2, which is more representative of the gas that will be used for ITER pellet injection. In testing it was found that this reference gas load resulted in pellet speeds in close proximity to a speed limit (~300 m/s) previously determined in a series of tests with D2 pellets shot through a mock-up of the curved guide tubes planned for the ITER installation (for pellet fueling from the magnetic high-field side). The equipment, operations, and test results are presented and discussed, with emphasis on the relevance for ITER operations.

High-power and long-pulse operation of TE31,11 mode gyrotron

The ITER electron cyclotron system is designed to inject a 20MW RF beam by using twenty-four 170GHz/1MW gyrotrons. JAEA is currently developing a gyrotron having a high-order mode (TE31,11) to reduce the heat load in the cavity resonator and achieve an output power greater than 1MW. The measured radiation profile at the front of the diamond window agreed with the results of the calculation. In order to suppress RF loss in the equatorial and upper port launchers, a high-quality HE11 mode is required at the exit of the matching optics unit (MOU). An HE11 mode purity of 96% was achieved by finely adjusting the two mirrors in the MOU. During the oscillation start-up phase, mode competition with counter-rotating TE29,12 mode was observed on the higher magnetic field side which caused arcing and pressure increase in the gyrotron. To avoid the counter-rotating TE29,12 mode from being excited, a start-up scenario that controls the voltage between the anode and cathode electrodes at the initial phase of operation was introduced, which was able to achieve a stable start-up of TE31,11 mode. A 1.2MW output power having a total electric efficiency of 43% was obtained in high-power experiments. In steady-state operation, a 1000s oscillation length and output power of 0.51MW was achieved.

Quasi-optics design of the dual-array ECE imaging system on the EAST Tokamak

A novel large aperture quasi-optical imaging system is designed for the new dual-array electron cyclotron emission (ECE) imaging (ECEI) instrument on the EAST tokamak. The zoom doublet scheme is used in the microwave imaging system on a super-conducting tokamak for the first time, and the focal plane can reach the high magnetic field side region even in the narrowest zoom configuration. The best spatial resolution in the vertical direction is 1.1 cm and the maximum vertical coverage can reach 80 cm. The field curvature is largely reduced in the narrow zoom configuration by the parabolic correction of a single lens surface. The imaging performance is fully characterized in the laboratory, and the characterized beam patterns show good agreements with the Gaussian beam specifications in the simulation results of the design.

An analytic nonlinear toroidal equilibrium with flow

A nonlinear solution to the Grad–Shafranov equation (Cicogna et al 2010 Phys. Plasmas 17 102506) is extended to plasmas with incompressible flows parallel to the magnetic field. The solution can describe either crescent-shaped or D-shaped equilibria having hollow current density profiles but safety factors monotonically increasing from the magnetic axis to the plasma boundary. Application of a sufficient condition for linear stability implies that this condition is satisfied in a plasma area located on the high magnetic field side. Stabilization is related to the variation of the magnetic field perpendicular to the magnetic surfaces. The area in which the condition is satisfied depends on the plasma shape but is insensitive to the amplitude and shear of the flow.

Mode conversion and absorption of fast waves at high ion cyclotron harmonics in inhomogeneous magnetic fields

The propagation and absorption of high harmonic fast waves is of interest for non-inductive current drives in fusion experiments. The fast wave can be coupled with the ion Bernstein wave that propagates in the high magnetic field side of an ion cyclotron harmonic resonance layer. This coupling and the absorption are analyzed using the hot plasma dispersion relation and a wave equation that was converted from an approximate dispersion relation for the case where λi=k⊥2ρi2/2≳1 (where k⊥ is the perpendicular wave number and ρi is the ion Larmor radius). It is found that both reflection and conversion may occur near the harmonic resonance layer but that they decrease rapidly, giving rise to a sharp increase in the absorption as the parallel wave number increases.

A repetitive pellet injection system for steady state fuelling in EAST superconducting tokamak

A new hydrogen/deuterium pellet injector has been developed for Experimental Advanced Superconducting Tokamak (EAST). The pellet injector based on a screw extruder is able to fire pellets (∅2mm×2mm; frequency 1–10Hz and velocity 150–300m/s) in steady state mode with reliability greater than 95%. An injection line was designed for pumping propellant gas and for diagnostic purpose also. A guide tube for magnetic high-field side (HFS) injection was developed and theoretical calculation has been done. After successful engineering commissioning, the injection system served at EAST 2012 campaign and first experimental results were obtained.

Broadband Continuously Frequency Tunable Gyrotron for 600 MHz DNP-NMR Spectroscopy

A broadband continuously frequency tunable gyrotron with a triode-type magnetron injection gun was developed as power source for analysis of protein structures. The TE7,3 oscillation mode was selected to avoid mode competitions in the high magnetic field side. Axial modes of the TE7,3,−10 were sequentially excited by changing the cavity magnetic field, and frequency tuning of about 4 GHz around 395 GHz was observed with output power greater than 50 W. The frequency also varied about 1 GHz as the anode–cathode voltage varied. Thus, the broadest tuning bandwidth in the 400 GHz band gyrotrons was achieved.

The enhanced ASDEX Upgrade pellet centrifuge launcher

Pellets played an important role in the program of ASDEX Upgrade serving both for investigations on efficient particle fuelling and high density scenarios but also for pioneering work on Edge Localised Mode (ELM) pacing and mitigation. Initially designed for launching fuelling pellets from the magnetic low field side, the system was converted already some time ago to inject pellets from the magnetic high field side as much higher fuelling efficiency was found using this configuration. In operation for more than 20 years, the pellet launching system had to undergo a major revision and upgrading, in particular of its control system. Furthermore, the control system installed adjacent to the launcher had to be transferred to a more distant location enforcing a complete galvanic separation from torus potential and a fully remote control solution. Changing from a hybrid system consisting of PLC S5/S7 and some hard wired relay control to a state of the art PLC system allowed the introduction of several new operational options enabling more flexibility in the pellet experiments. This article describes the new system architecture of control hardware and software, the operating procedure, and the extended operational window. First successful applications for ELM pacing and triggering studies are presented as well as utilization for the development of high density scenarios.