Electron Cyclotron Plasma Heating Research Articles

Experimental study of electron cyclotron heating assisted start-up on J-TEXT

Second harmonic X-mode (X2 mode) electron cyclotron heating (ECH) assisted start-up has been studied experimentally on the J-TEXT device to determine the minimum ECH power requirements to assist breakdown and develop a better physical description of the process. Results indicate that the minimum toroidal electric field for a successful start-up on J-TEXT changed from to , and that injecting of X2-mode ECH power can ensure robust breakdown. The critical ECH power for successful start-up was determined to be approximately . At lower powers ECH was observed to cause ionization, but this did not necessarily result in successful start-up. The effects of varying ECH power, pulse-width and toroidal magnetic field on start-up were also investigated. Higher ECH power leads to quicker, stronger ionization and CIII emission, and is beneficial for burn-through. Higher pre-plasma density caused by high ECH power can decrease the required toroidal electric field for ohmic breakdown, while the enhanced CIII emission may not be good for start-up. The characteristics of the pre-plasma formed by ECH prior to application of the loop voltage were also studied. The toroidal magnetic field affects the initial location where pre-plasma forms and this also affected the subsequent tokamak start-up. The analysis of spatial density showed that the pre-plasma can radially develop at a velocity of . Furthermore, it was found that injecting ECH power at the appropriate time with a low power of can achieve similar pre-ionization results to the high-power case. The transition from ECH plasma to ohmic plasma suggests that the ECH assisted start-up modified the process of purely ohmic breakdown.

Stabilization of low-threshold two-plasmon parametric decay instability at oblique incidence of a microwave pump beam onto the plasma

It is shown that with an oblique incidence of a pump microwave in X2-mode electron cyclotron plasma heating (ECRH) experiments, the threshold power of a parametric decay instability (PDI) leading to excitation of two trapped upper hybrid waves increases and the anomalous absorption level decreases. It is proposed to use a small pump beam tilt along the magnetic field to suppress this most dangerous PDI, often observed in the X2-mode ECRH experiments.

Absorption of Microwaves in Different Regimes of Electron Cyclotron Plasma Heating at the L-2M Stellarator

Absorption of Microwaves in Different Regimes of Electron Cyclotron Plasma Heating at the L-2M Stellarator

Isotope effects on transport in LHD

Isotope effects are one of the most important issues for predicting future reactor operations. Large helical device (LHD) is the presently working largest stellarator/helical device using super conducting helical coils. In LHD, deuterium experiments started in 2017. Extensive studies regarding isotope effects on transport have been carried out. In this paper, the results of isotope effect studies in LHD are reported. The systematic studies were performed adjusting operational parameters and nondimensional parameters. In L mode like normal confinement plasma, where internal and edge transport barriers are not formed, the scaling of global energy confinement time (τ E) with operational parameters shows positive mass dependence (M 0.27; where M is effective ion mass) in electron cyclotron heating plasma and no mass dependence (M 0.0) in neutral beam injection heating plasma. The non-negative ion mass dependence is anti-gyro-Bohm scaling. The role of the turbulence in isotope effects was also found by turbulence measurements and gyrokinetic simulation. Better accessibility to electron and ion internal transport barrier (ITB) plasma is found in deuterium (D) plasma than in hydrogen (H). Gyro kinetic non-linear simulation shows reduced ion heat flux due to the larger generation of zonal flow in deuterium plasma. Peaked carbon density profile plays a prominent role in reducing ion energy transport in ITB plasma. This is evident only in plasma with deuterium ions. New findings on the mixing and non-mixing states of D and H particle transports are reported. In the mixing state, ion particle diffusivities are higher than electron particle diffusivities and D and H ion density profiles are almost identical. In the non-mixing state, ion particle diffusivity is much lower than electron diffusivity. Deuterium and hydrogen ion profiles are clearly different. Different turbulence structures were found in the mixing and non-mixing states suggesting different turbulence modes play a role.

Expansion of the Operation Region of Tokamak Plasmas Using the Stationary Direct Current of a Central Solenoid

A stationary direct current of a central solenoid (DCCS) can expand the operation region of tokamak plasmas. For tokamak plasma formation experiments using electron cyclotron heating (ECH) alone, the minimum ECH power and plasma current necessary for tokamak plasma formation were reduced using the DCCS, which was applied in the counter rotational direction to the plasma current. In TST-2, the minimum ECH power necessary for the formation of a tokamak plasma was reduced from 3.3 to 1.6 kW when the DCCS was changed from 0 to 184 A. Simultaneously, the plasma current that was needed to sustain the tokamak plasma configuration was reduced to 0.6 kA.

Decrease in the Mode Purity of Microwave Beams in the L-2M Stellarator Peripheral Plasma

The linear coupling (transformation) of two wave modes in a peripheral layer of inhomogeneous plasma confined by a magnetic field is theoretically analyzed. This transformation is a cause of a decrease in the purity of the X-mode introduced into plasma, which is optimum for electron—cyclotron plasma heating at the second harmonic of gyrofrequency. It is shown that the mode purity defect in radiation power under L-2M stellarator conditions can be no more than 6%. An increase in the mode purity defect because of the arbitrarily large steepness of the electron density profile in the peripheral layer appears insignificant.

Effect of the Pfirsch–Schlüter flow on the inboard/outboard asymmetry of the toroidal flow in LHD

Inboard/outboard asymmetry of the toroidal flow has been observed in the large helical device (LHD), especially when the radial electric field is large. We investigate the effect of the Pfirsch–Schlüter flow on the toroidal flow in LHD plasma. As a result, we find that the Pfirsch–Schlüter flow can be significantly large when the electron root solution of the neoclassical ambipolarity condition is achieved with a large radial electric field. We show that the Pfirsch–Schlüter flow can explain the asymmetry of toroidal flow in the electron cyclotron heating plasma and the perpendicular neutral beam injection plasma of LHD. Furthermore, we estimate the electrostatic potential from the flow asymmetry and obtain a relatively good agreement with experimental results by charge exchange recombination spectroscopy.

On the Possibility of Strong Anomalous Absorption of Microwaves in Experiments on Electron Cyclotron Plasma Heating at the Second Resonance Harmonic

It has been shown that more than two thirds of the pumping power can be transmitted to the upper hybrid waves localized near the local maximum of the plasma density as a result of saturation of the low-threshold two-plasmon decay instability of a microwave beam. This nonlinear effect can significantly change the power deposition profile during electron cyclotron resonance heating of plasma in toroidal traps and, thus, explain its significant broadening, often observed in experiments.

Control of electron-cyclotron instability driven by strong ECRH in open magnetic trap

We discuss the laboratory experiment on a controlled transition from the generation of periodic bursts of electromagnetic radiation into the continuous-wave regime of a cyclotron maser formed in a magnetically confined non-equilibrium plasma (Shalashov A. G. et al., Phys. Rev. Lett., 114 (2018) 205001). The kinetic cyclotron instability of the extraordinary wave of a weakly inhomogeneous magnetized plasma is driven by the anisotropic electron population resulting from electron cyclotron plasma heating in a MHD-stable minimum-B open magnetic trap. In the present communication we focus on a theoretical model that explains the existing data and motivates further experiments.

Design, Test and Analysis of a Gyrotron Cavity Mock-Up Cooled Using Mini Channels

In 2016, we have designed, built, and finally tested at the FE200 facility in Le Creusot, France, a planar mock-up mimicking the water-cooled cylindrical resonance cavity of the European 170-GHz, 1-MW gyrotron to be used for electron cyclotron plasma heating in ITER. The aim of the mock-up is the characterization of the cooling capability of the cavity. A Glidcop target is heated with an electron beam gun with resulting peak heat fluxes relevant for the full-size cavity. Underneath the target surface, whose temperature is monitored by means of a pyrometer, a set of parallel semicircular mini channels, with a diameter of 1.5 mm, allow the flow of pressurized water, entering the mock-up at ~9 bar and 40 °C. Several thermocouples measure the target temperature, at different distances from the heated target surface. The experimental results show that the mock-up is capable to withstand heat fluxes of 21 MW/m2, while the cooling system keeps the heated surface below ~400 °C, for flow conditions comparable to those of the full-size cavity. The test results are used to first calibrate the uncertain model parameters and then, with frozen parameters, to validate a previously developed computational fluid dynamics model, showing good agreement with the experiment. In view of its reliability, this model might eventually be a useful tool for the simulation of the full-size gyrotron cavity operation.

Observation of Poincaré-Andronov-Hopf Bifurcation in Cyclotron Maser Emission from a Magnetic Plasma Trap.

We report the first experimental evidence of a controlled transition from the generation of periodic bursts of electromagnetic radiation into the continuous-wave regime of a cyclotron maser formed in magnetically confined nonequilibrium plasma. The kinetic cyclotron instability of the extraordinary wave of weakly inhomogeneous magnetized plasma is driven by the anisotropic electron population resulting from electron cyclotron plasma heating in a MHD-stable minimum-B open magnetic trap.

A Radiometer for Plasma Diagnostics in a Magnetic Mirror GDT

A radiometer developed at Budker Institute of Nuclear Physics (Siberian Branch, Russian Academy of Sciences) for diagnostics of the efficiency of the electron cyclotron plasma heating in a mirror magnetic trap (GDT) is discussed. The radiometer is based on the superheterodyne principle. The characteristics of the instrument such as the operating range, sensitivity, frequency and time resolution are adapted to the requirements of the experiments. The receiver is equipped with a system for protection against stray radiation from powerful gyrotrons that are used in the electron cyclotron heating experiments. The radiometric measurements of the plasma self-emission carried out at the GDT facility showed that the new information channel helps to interpret the processes of resonant microwave plasma heating more accurately.

Saturation of the two-plasmon parametric decay instability in experiments on the electron cyclotron plasma heating due to the extraordinary pump wave depletion

The saturation level of the low-threshold parametric two-plasmon instability of the extraordinary pump wave decay to two upper hybrid plasmons is analyzed under conditions when the only efficient saturation mechanism is the pump depletion. A closed system of differential equations describing both the instability excitation and saturation is derived. The system is solved numerically and an analytic expression is obtained for the anomalous absorption coefficient of the pump wave caused by the development of this instability. The saturation level of the two-plasmon decay instability and the related anomalous absorption efficiency are estimated from data obtained in experiments on the electron cyclotron resonance heating of the plasma by an extraordinary wave in the TEXTOR tokamak.

Electron cyclotron emission at the fundamental harmonic in GDT magnetic mirror

New electron cyclotron emission (ECE) diagnostics has been installed to facilitate the successful experiment of electron cyclotron plasma heating (ECRH) in a large open magnetic trap GDT at Budker Institute. The particularities of ECE in the vicinity of the ECRH frequency were studied experimentally for a broad range of discharge scenarios. The measured thermal emission has partly validated the existing physical conceptions about microwave plasma heating in the machine. Besides the expected emission of thermal electrons, a clearly resolved non-thermal ECE was observed which unambiguously confirmed the presence of suprathermal electrons driven by high-power microwave heating.

Quasi-optical simulation of the electron cyclotron plasma heating in a mirror magnetic trap

The resonance microwave plasma heating in a large-scale open magnetic trap is simulated taking into account all the basic wave effects during the propagation of short-wavelength wave beams (diffraction, dispersion, and aberration) within the framework of the consistent quasi-optical approximation of Maxwell’s equations. The quasi-optical method is generalized to the case of inhomogeneous media with absorption dispersion, a new form of the quasi-optical equation is obtained, the efficient method for numerical integration is found, and simulation results are verified on the GDT facility (Novosibirsk).

Control of the Radial Energy Deposition Profile in an Open Magnetic Trap During Electron Cyclotron Plasma Heating

We propose a method for controlling the radial profile of electron cyclotron plasma heating in an axisymmetric magnetic mirror by using minor perturbations of the magnetic field of the mirror. The method is based on the analysis of the ray trajectories behavior near the surface of the electron cyclotron resonance. A way to produce such perturbations by supplementing the system with an additional “quadrupole” pair of magnetic coils is also proposed. The possibility to improve the coupling of radiation with the plasma in an open trap is demonstrated, as well as the possibility to control the energy deposition profile by means of small variations of the current in the additional coils for two basic scenarios of electron cyclotron plasma heating, specifically, longitudinal launching of microwave radiation to the magnetic mirror region and trapping of obliquely launched radiation by the inhomogeneous magnetized-plasma column.

Construction of a plasma confinement device TOKASTAR-2 with tokamak-helical hybridization concepts

In the research of confinement of torus plasma using magnetic fields, a concept of superposition of the external helical magnetic field to tokamak plasma has been widely proposed. One of the objectives of this concept is to improve the stability of plasmas, for example, to suppress plasma current disruption. A tokamak-helical hybrid plasma confinement device TOKASTAR-2 has recently been constructed to investigate the effect of helical field to tokamak plasma. First plasma was just produced in June, 2009. TOKASTAR-2 machine has Ohmic heating central coils, eight toroidal field coils and two outboard helical field coil segments. The major radius Rp and the averaged minor radius <ap> of the toroidal plasma with nearly circular cross-section is expected to be approximately 0.1 m and 0.04 m, respectively. The toroidal magnetic field Bt is typically 0.1 T. The plasma start-up experiment using the electron cyclotron heating (ECH) by the radio frequency wave with the frequency of 2.45 GHz and the injected power of 2.0 kW is in progress in this machine as the pre-ionized phase of tokamak operation. In this paper, the overview of TOKASTAR-2 device and present states of the study are presented. According to the preliminary experiment, we confirmed the effect of outer vertical or helical magnetic field application to suppress bursting density fluctuation in the pre-ionized ECH plasmas.

Electron Cyclotron Heating Program and Electron Cyclotron Emission Diagnostics on the EAST and HT-7 Superconducting Tokamaks

A program of electron cyclotron heating (ECH) with 4 MW at 140 GHz has been launched for developing scenarios of stable high performance through control of the pressure and current density profiles on the EAST tokamak. Several electron cyclotron emission (ECE) diagnostics are under development as important components of the research program on EAST. The smaller HT-7 tokamak is equipped with a multichannel superheterodyne radiometer and an ECE imaging system. Physics issues including fluctuations driven by electron and ion modes, low frequency zonal flows, magnetic reconnection mechanisms, etc. were investigated on HT-7 using these two systems, which have been moved to EAST after some modifications. New systems, including a 32-channel ECE system and an ECE imaging system of 24(radial) × 16(vertical) channels, are under development. These new systems are designed for the ECH plasma regimes and provide long-range correlation measurements of plasma turbulence. A grating polychromator ECE system has been installed for measurement of the Te profile covering the whole operational range of toroidal magnetic field on EAST.

Calculation of a Hyperbolic Corrugated Horn Converting the TEM00 Mode to the HE11 Mode

Corrugated waveguide transmission lines are in use to transmit high power mm-wave radiation from gyrotrons to the plasma for electron cyclotron plasma heating in tokamaks such as ITER. The coupling efficiency of the gyrotron output radiation formed as a quasi-Gaussian beam to the waveguide mode is a critical issue. A hyperbolic corrugated horn serves as a converter of the TEM00 Gaussian mode to the HE11 mode of a corrugated waveguide. We report the design of a hyperbolic horn for application in the ITER transmission line at 170 GHz. The theoretical conversion efficiency of the horn is higher than 0.995.

Observation of Space and Energy Distributions of High Energy Electrons Produced in ECH Plasmas of LHD

Hard x-ray spectrum has been studied with x-ray pulse-height analyzer (PHA) in Large Helical Device (LHD). The hard x-ray emissions have been observed in high temperature plasmas heated by electron cyclotron heating (ECH). The radial distribution of the hard x-ray emissions is quantitatively derived from the line-integrated spectrum through an Abel inversion. When the bulk electron density ranges at 2.0 × 1018 m−3 , the hard x-ray emissions are measured to be the most intense. From an analysis on the spectrum it is quantitatively proved that the energy-distribution function of the suprathermal electrons produced by ECH remarkably deviates from that of thermal electrons. The relaxation time of the high energy electrons is estimated to be 70 ms from the exponential decay rate after turning off the ECH pulse.