Diamagnetic Loop Research Articles

Design and development of the magnetic diagnostic systems for the first operational phase of the SMART tokamak.

A set of magnetic diagnostics has been designed, manufactured, and calibrated for the first operational phase of the small aspect ratio tokamak. The sensor suite comprises of Rogowski coils; 2D magnetic probes; and poloidal, saddle, and diamagnetic flux loops. A set of continuous Rogowski coils has been manufactured for the measurement of plasma current and induced eddy currents in conductive elements. A set of flux loops and magnetic probes will be used as input for the reconstruction of the magnetohydrodynamic equilibrium. The quantity and position of these sensors have been verified to be sufficient with synthetic equilibrium reconstructions using the equilibrium fitting code and baseline scenarios computed with the Fiesta code. These sensors will also be used as input for the real-time control system, and magnetic probes will be used for the detection of plasma instabilities. The calibration procedure for the magnetic probes is described, and the results are shown. The signal conditioning and data acquisition systems are described.

Design and measurements of the diamagnetic loop in Aditya-U tokamak

A diamagnetic diagnostic system is designed, prepared and installed at Aditya Upgrade Tokamak. Also, the raw data is acquired in a specified acquisition system with suitable front-end electronics. The entire system consists of separate loops and is first calibrated in the presence of all coils, which conduct electricity during the plasma discharge. Then, the change in the toroidal magnetic field due to the plasma is measured using the diamagnetic system, and hence, several plasma parameters including poloidal beta, stored energy, etc. are estimated. The measurements were then repeated during various tests of the tokamak plasma, and several interesting facts were observed. The present paper covers the entire concept of designing magnetic diagnostics – from basic considerations to final data interpretation.

Development of a diamagnetic loop in KAIMIR

We developed a diamagnetic loop for the estimation of plasma stored energy in the KAIST Magnetic Mirror magnetic mirror device [Oh et al., J. Plasma Phys. 90, 975900202 (2024)]. Diamagnetic loops are used to estimate the plasma stored energy from measurements of the diamagnetic flux in plasma with an applied external magnetic field. However, diamagnetic flux measurements are accompanied by the vacuum flux, which generally exceeds the diamagnetic flux by over 10 000 times. Therefore, it is critical to attain a high signal-to-noise ratio with minimized noise in diamagnetic flux measurements. In this study, we employed a novel method to reduce background noise and improve the signal-to-noise ratio. Using two identical loops with opposite polarities, we successfully removed parasitic capacitive noise from the external insulation while amplifying the inductive signal two times. To eliminate the vacuum flux, we utilized two coaxial loops with different radii positioned at the same axial location. Results obtained from six paired loops confirmed the successful removal of the vacuum flux. The plasma stored energy was also found to agree well with Langmuir probe measurements, which verifies the diamagnetic flux measurements using the developed loop.

Surface Spin Polarization in the Magnetic Response of GeTe Rashba Ferroelectric

We experimentally investigate magnetization reversal curves for a GeTe topological semimetal. In addition to the known lattice diamagnetic response, we observe narrow magnetization loop in low fields, which should not be expected for non-magnetic material. The diamagnetic hysteresis loop is unusual, so the saturation level is negative in positive fields, and the loop is passed clockwise, in contrast to standard ferromagnetic behavior. We show, that the experimental hysteresis curves cannot be obtained from standard ferromagnetic ones by adding/subtracting of any linear dependence, or even by considering several interacting magnetic phases. The latter possibility is also eliminated by the remanence plots technique (Henkel or \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\delta M$$\\end{document} plots). We explain our results as a direct consequence of the correlation between ferroelectricity and spin-polarized surface states in GeTe, similarly to magnetoelectric structures.

Enhanced confinement induced by pellet injection in the stellarator TJ-II

Enhanced confinement is observed in neutral beam injector (NBI)-heated hydrogen discharges made in the stellarator TJ-II after the injection of a single cryogenic fuel pellet into the plasma core. In addition to the expected increase in electron density, ne, in the core after pellet injection (PI), the plasma diamagnetic energy content is seen to rise, with respect to similar discharges without PI, by up to 40%. Furthermore, the energy confinement time, τEdiag, as determined using a diamagnetic loop, is enhanced when compared to predictions obtained using the International Stellarator Scaling law [H. Yamada et al., Nucl. Fusion 45, 1684 (2005)] and the triple product, ne · Ti · τEdiag, exhibits a clear bifurcation point toward an improved confinement branch as compared to the branch product predicted by this scaling law. In general, once such a pellet-induced enhanced confinement (PiEC) phase has been established, it is characterized by steepened radial density gradients, by more negative plasma potential in the core, more negative radial electric fields, Er, across a broad plasma region, as well as by reductions in density and plasma potential fluctuations in the density gradient region. In addition, experimental observations show increased peaking of core radiation losses, this pointing to edge/core plasma decoupling. In parallel, neoclassical simulations of reference and PiEC plasmas predict increased particle and energy confinement times during a PiEC phase together with a more negative Er profile. Qualitative rather than quantitative agreement with experimental parameters is found, indicating that turbulence seems to play a significant role here. In summary, single cryogenic pellet injection facilitates the achievement of an enhanced operational regime that was previously not observed in NBI-heated discharges of the TJ-II.

An axisymmetric mirror device for studying confinement and instability.

We describe a magnetic mirror device, namely, the Keda Mirror with AXisymmetricity (KMAX), which aims to study new approaches to confine and stabilize the mirror plasma as well as basic plasma research. KMAX consists of one central cell, two side cells, and two end chambers at two ends of the device. For the central cell, the mirror-to-mirror distance is 5.2m, while the central cylinder is 2.5m in length and 1.2m in diameter. The plasmas are generated by two washer guns located in the end chambers, which subsequently flow into the central cell and merge there. The density in the central cell is usually adjusted by changing the magnetic field strength inside the side cell, and it ranges from 1017 to 1019m-3, depending on the experimental requirement. Ion cyclotron frequency heating with two 100 kW transmitters is routinely used to heat up the ions. Plasma controls mainly rely on configuring the magnetic geometry and rotating magnetic fields to improve the confinement and suppress instability. Routine diagnostics, such as probes, interferometers, spectrometers, diamagnetic loops, and bolometers, are also reported in this paper.

Current quench and vessel currents characterisation at the COMPASS tokamak

The characterisation of plasma current quench and understanding of its underlying physical processes play a crucial role when designing large fusion devices such as ITER. For the first time, an extensive analysis of the COMPASS tokamak disruption database is presented. A unique set of magnetic diagnostics allows the investigation of local toroidal and poloidal vessel currents, including currents flowing along the open magnetic field lines from the plasma to the vacuum vessel (VV) (i.e., halo currents). Area-normalised current quench times are in agreement with the ITPA 1.67 ms m−2 lower limit. Extremely fast I p quench rates of up to 0.6 MA ms−1 are observed during runaway electron campaigns at COMPASS, which are under the ITPA lower limit. Vertical movement of the plasma column is accelerated by the I p quench during major disruptions. Toroidal vessel currents of around 2% – 4% of the predisruptive plasma current are observed during I p quench. Net poloidal eddy currents are obtained by Mirnov coils and diamagnetic loop, reaching 3% of . Using the Mirnov coils it is shown that the halo current magnitude grows and its poloidal profile broadens with increasing plasma current I p . Geometric features of the VV structure and in-vessel component positions on the poloidal vessel current measurements are discussed.

Noninvasive beam-size measurements for linear induction accelerators

Diamagnetic loops (DML) can be used as a noninvasive method for the measurement of beam size in electron beam accelerators that use solenoidal magnetic transport. The loop fundamentally measures the magnetic flux excluded by a diamagnetic object. A comprehensive theory relates the rms beam radius to the excluded flux measured by the DML. We have built and calibrated a DML apparatus. Recently, this DML has been used to measure the size of the electron beam near the final focus of a flash-radiography linear induction accelerator. The results are in agreement with predictions from transport code simulations. In this article, we review and summarize the construction, calibration, and electron-beam testing of this DML.

Upgrade of the magnetic diagnostic system for restart of HT-6M operation

The HT-6M tokamak at the Thailand Institute of Nuclear Technology has been restarted. In order to ensure the smooth breakdown of plasma and obtain plasma discharge parameters, optimization of the poloidal field coils and upgrade of the magnetic diagnostics are described in this article. A perfect null field (stray field in the main chamber < 10 G) is obtained using an ohmic heating field. To obtain important information about the plasma, an external magnetic diagnostics system is designed and calibrated, including a Rogowski coil (measuring plasma current), a magnetic probe (measuring external field), diamagnetic loops (measuring β p) and so on. In order to realize high-frequency signal measurement and transmission, a series of frequency responses with the magnetic probe and transmission line are tested. Later, to verify the null field, a fitting code is developed to reconstruct the stray field in the vacuum chamber based on magnetic probe measurements and flux loops. The results show that the error is within 1.5%. This indicates the accuracy of the magnetic measurement system and ensures the preparation for the breakdown of plasma.

The measurements by diamagnetic loops in EAST

The measurement of the diamagnetic flux for poloidal beta and energy content of EAST based on diamagnetic loops is described in this article. The error sources can be well compensated with the diamagnetic loop and the compensation loop signals. And the maximum error of vacuum flux under experiments without plasmas is about 0.11 mWb, which meet the typical experimental requirements for high βp discharges (βp∼2.5). For ohmic discharges, the stored energy determined from the diamagnetic flux measurement Wdia is in good agreement with that from the equilibrium reconstruction Wmhd. After the auxiliary heating power are injected into plasmas, the increment of the energy content is observed to verify the heating effects of different heating systems. With the upgrade of 2021, the recent campaign has shown a good response between the diamagnetic flux measurements and rapid stored energy drop induced by edge localized modes (ELMs). The preliminary experimental results indicate that plasma anisotropic degree decreases with increasing density under operation conditions with high density and high heating power.

Diagnostic Potentials of Diamagnetic Measurements with Two Loops in Tokamaks

The magnetic flux through the diamagnetic loop consists of two parts: one is produced by the poloidal current in the plasma and another by the external currents. Separation of these parts is the focus of the study, which is fully analytical. The system is assumed to be axially symmetric. Then the “inductances” in the flux–current relations are expressed via easily tractable integrals. With such precisely defined coefficients, the theoretical possibility of perfect separation by means of two independent simultaneous measurements is demonstrated. The desired result can be obtained with quantities measured by either double-loop method or with a single diamagnetic loop and a correction coil. This conclusion is more optimistic that those in the generally accepted concepts. An additional step is proposed to determine, with the same magnetic data, the poloidal current induced in the vacuum vessel wall. This current can be useful for evaluation of the disruption force on the wall. The proposed algorithms can be used in existing tokamaks with two loops and in the ITER as well.

Progress on the small modular stellarator SCR-1: new diagnostics and heating scenarios

This work presents updates in the diagnostics systems, magnetohydrodynamics (MHD) calculations and simulations of microwave heating scenarios of the small modular Stellarator of Costa Rica 1 (SCR-1). Similarly, the design of a flexible bolometer and magnetic diagnostics (a set of Mirnov coils, Rogowski coils and two diamagnetic loops) are introduced. Furthermore, new MHD equilibrium calculations for the plasma of the SCR-1 device were performed using the VMEC code including the poloidal cross-section of the magnetic flux surfaces at different toroidal positions, profiles of the rotational transform, magnetic well, magnetic shear and total magnetic field norm. Charged particle orbits in vacuum magnetic field were computed by the magnetic field solver BS-SOLCTRA (Vargas et al. In 27th IAEA Fusion Energy Conference (FEC 2018), 2018. IAEA). A visualization framework was implemented using Paraview (Solano-Piedra et al. In 23rd IAEA Technical Meeting on the Research Using Small Fusion Devices (23rd TM RUSFD), 2017) and compared with magnetic mapping results (Coto-Vílchez et al. In 16th Latin American Workshop on Plasma Physics (LAWPP), 2017, pp. 43–46). Additionally, simulations of microwave heating scenarios were performed by the IPF-FDMC full-wave code. These simulations calculate the conversion of the ordinary waves to extraordinary waves and allow us to identify the location where the conversion takes place. Finally, the microwave heating scenarios for the $330^{\circ }$ toroidal position are presented. The microwave heating scenarios showed that the O–X–B mode conversion is around 12–14 %. It was possible to identify the spatial zone where the conversion takes place (upper hybrid frequency).

Characterization of diamagnetism inside an ECR thruster with a diamagnetic loop

The plasma-induced magnetic field in an electron cyclotron resonance plasma thruster is measured non-intrusively by means of a diamagnetic loop that encloses the plasma flow. The calibration process is described, and parasitic currents in the thruster walls and plasma oscillations are identified as the dominant sources of uncertainty. The integrated magnetic flux is seen to depend on the applied power and less significantly on the mass flow rate. The effect of the diamagnetic loop radius is also studied by testing two loops of different diameters. To estimate the perpendicular electron pressure in the plasma from the loop measurements, two plasma beam models, 1D and 2D, are used. While both models give similar results for the small loop, they differ significantly for the large loop, showing the relevance of 2D effects when a large diamagnetic loop is used.

Time evaluation of diamagnetic loop and Mirnov oscillations in a major plasma disruption and comparison with a normal shot

In this paper, plasma disruption was investigated in IR-T1 tokamak. Moreover, the time evaluation of plasma parameters such as plasma current, Ip; loop voltage, Vloop; power heating; and safety factor was illustrated. The results of diamagnetic loop and Mirnov oscillations in a major disruption were compared with their results in a normal shot. In addition, plasma disruption in different tokamaks was investigated and the results were compared with the IR-T1 tokamak.

Extended theory of diamagnetic measurements with account of the wall currents in tokamaks

Effect of the poloidal current induced in the tokamak vessel wall during transient events is explicitly incorporated into the formulas for the diamagnetic signal. The analysis is based on the equation linking this current to two drivers, the evolving plasma inside and the varying toroidal field produced by currents outside. The wall itself is considered resistive and serves as a damper. The main part of the described analytical approach is applicable to any situation of interest, but the emphasis is put on the case with diamagnetic loop placed on the inner side of the wall, as it is planned in ITER [R. Fresa, et al., Fusion Eng. Des. 100, 133 (2015)]. Such a loop will actually measure the voltage equal to the poloidal current induced in the wall multiplied by the wall poloidal resistivity. With this current as a main detectable quantity, a refined algorithm is needed for determining the plasma energy from the magnetic measurements. Theory-based recommendations are given to improve the diagnostics, especially for the rapid events like thermal quenches and ELM (edge localized mode) crashes.

Third harmonic ICRF heating in LHD hydrogen experiments

The ion cyclotron range of frequencies (ICRF) heating power injection in the hydrogen experiment in LHD was demonstrated after the upgrade of ICRF antennas. The ICRF wave couples and accelerates the energetic particles injected by perpendicular-NBIs with 40 keV. The simulation by the MORH code shows the existence of energetic particles around the ICRF third harmonic resonance layers. As the result of ICRF heating power deposition, the beta value increased by 0.2% in absolute beta mainly due to the increased energetic particle content. The increase of energetic ions particularly around 60 keV, which should be accelerated by the ICRF heating, is observed. The ICRF heating efficiency was approximately 30%–50%, estimated from the break-in-slope analysis at the turn off timing of ICRF power from the stored energy measured by diamagnetic loops. This increase of the stored energy is mostly the contribution of the increased energetic particles. The heating efficiency increases as the density increases.

Diamagnetic energy measurement during the first operational phase at the Wendelstein 7-X stellarator

The magnetic diagnostic system at the Wendelstein 7-X stellarator includes three diamagnetic loops to measure magnetic flux changes in the plasma. Their signals are directly related to the plasma energy. The diagnostic design with respect to materials, component cooling and data acquisition is built to be fully steady-state capable within the harsh environment of a fusion plasma device. During the first operational phase, two diamagnetic loops have been put into operation, each of them close to one of the up-down symmetric main planes of the plasma column with a bean-shaped and triangular-shaped cross-section, respectively. Both loops measured reliable energies in accordance to theoretical expectations. The triangular-shaped diamagnetic loop is equipped with four compensation coils. They are used to compensate errors during the energy measurement due to small fluctuations of externally driven currents in the main superconducting magnetic field coils and eddy currents in the adjacent vacuum vessel and thereby increase the time-resolution allowing to measure fast changes of the plasma energy. The diamagnetic flux measurements agree well with corresponding estimations of diamagnetic signals using three-dimensional Biot–Savart calculations. A consistency check for the diamagnetic energy is performed by a reconstruction of the associated Pfirsch–Schlüter current distribution and a comparison of predicted signals with measurements of an arrangement of eight plasma encircling Rogowski coil segments. Additionally, the measured diamagnetic energy is compared to kinetic energy calculations based on density and temperature measurements performed by the Thomson scattering diagnostic and the x-ray imaging crystal spectrometer diagnostic. The resulting energy confinement times are similar to predictions of empirical scaling laws, like ISS04. For upcoming operational periods of Wendelstein 7-X, the diamagnetic energy measurement will be used to generate an interlock signal, which will turn off the main plasma heating systems in case of a sudden, unwanted plasma collapse.

Effect of the toroidal magnetic field the on energy and fast particle confinement in the Globus-M spherical tokamak

The experiments with auxiliary heating by neutral beam injection (NBI) were carried out before the spherical tokamak Globus-M was upgraded to Globus-M2. The aim of the experiments was to determine the dependence of the confinement time on toroidal magnetic field. The total stored energy was obtained by diamagnetic loops, and verified with ASTRA modelling based on kinetic measurements. The absorbed heating power was estimated using 3D fast ion tracking modeling. The obtained dependence of the energy confinement time is similar to the MAST and NSTX results but contradicts the conventional IPB98(y,2) scaling. The toroidal Alfvén eigenmodes study was performed in a set of discharges with NBI at the early stage of the discharge. The experiments have shown that fast particle losses decrease with the increase of the toroidal magnetic field and plasma current.

Characterization of a medium-sized washer-gun for an axisymmetric mirror.

A new medium-sized washer gun is developed for a plasma start-up in a fully axisymmetric mirror. The gun is positioned at the east end of the Keda Mirror with AXisymmetricity facility and operated in the pulsed mode with an arc discharging time of 1.2 ms and a typical arc current of 8.5 kA with 1.5 kV discharge voltage. To optimize the operation, a systematic scan of the neutral pressure, the arc voltage, the bias voltage on a mesh grid 6 cm in front of the gun and an end electrode located on the west end of mirror, and the mirror ratio was performed. The streaming plasma was measured with triple probes in the three mirror cells and a diamagnetic loop in the central cell. Floating potential measurements suggest that the plasma could be divided into streaming and mirror-confined plasmas. The floating potential for the streaming plasma is negative, with an electric field pointing inwards. The mirror-confined plasma has a typical lifetime of 0.5 ms.

Progress of the magnetic sensor development for JT-60SA

In order to obtain information for control of JT-60SA plasma and physics research on the plasma, many types of magnetic sensors are developed. We have designed a new architecture of magnetic sensors and connection methods for JT-60SA taking installation and maintenance into account. The former include a newly designed Mirnov coil, a Rogowski coil and a diamagnetic loop. They enable us to make simple and robust sensors. The latter include a newly developed connector between a sensor and a mineral insulation cable, and a connection box. An unsealed connection box enables easy installation in a vacuum vessel; however, it should have a withstand voltage greater than 1 kV at intermediate gas pressures. We successfully achieved a high withstand voltage for the unsealed connection box in the gas pressure region around the Paschen minimum voltage. After R&D of JT-60SA magnetic sensors, almost all manufacturing steps are complete. This paper reports the manufacturing and tests of the magnetic sensors for JT-60SA.