Turbulence in edge and core transport barriers: new experimental results and modeling

In this paper, recent progressive studies on experimental analysis and theoretical models for turbulence phenomena around the transport barriers in high-performance magnetic confined fusion plasma are reviewed. The linkage of radial electric fields and turbulence, the importance of radial electric field curvature, and observations of spatiotemporal turbulence structures are described with related theoretical models

Q-band high-performance notch filters at 56 and 77 GHz notches for versatile fusion plasma diagnostics

A six-pole Q-band waveguide filter with a notch frequency above the Q-band has been developed for plasma diagnostics. The previous paper [Nishiura et al., J. Instrum. 10, C12014 (2015)] reported that the notch frequency exists within the standard band. In this study, the newly required notch filter extends the function, which prevents a thorny wave from being mixed into an instrument beyond the standard bandwidth of the waveguide. The mode control technique for cavities realizes a deep and sharp filter shape for Q-band notch filters with 56 and 77 GHz notches, respectively. The former filter has an attenuation more than 50 dB at 56.05 GHz and a bandwidth of 1.1 GHz at −3 dB. The latter filter has an attenuation more than 55 dB at 76.95 GHz and a bandwidth of 1.6 GHz at −3 dB. The electron cyclotron emission imaging and the electron cyclotron emission (ECE) diagnostics for the Q-band implemented a pair of the fabricated filters and demonstrated the ECE measurement successfully in the intense stray radiation from a 56 GHz gyrotron

Improvements of CO2 Laser Heterodyne Imaging Interferometer for Electron Density Profile Measurements on LHD

After installation of CO2 laser (wavelength 10.6 μm) heterodyne imaging interferometer (CO2 HI) in 2001, continuous developments have been carried out to improve the measurements capability and stability of operation. The CO2 HI works almost without phase jumping at high electron density (> 1 × 1020 m-3), where the existing far infrared laser (wavelength 118.9 μm) interferometer suffers from fringe jump due to the reduction of signal intensity caused by refraction. However a second interferometer is required to compensate mechanical vibration. A YAG laser (wavelength 1.06 μm) heterodyne imaging interferometer (YAG HI) is presently used for the vibration compensation. In the 10th LHD experimental campaign (2006?2007), sixty four channels of CO2 HI to measure electron density profile and ten channels of YAG HI to measure mechanical vibration are working. A measurement example of a pellet fuelled high-density discharge is reported

Conceptual Design of Electron Density Measurement System for DEMO-Relevant Helical Plasmas

Electron density measurement remains indispensable to control fueling on a DEMO reactor. For steady-state operation of the DEMO reactor, density measurement should be highly reliable and accurate. A dispersion interferometer and a Faraday polarimeter are free from measurement errors caused by mechanical vibrations. Hence combination of the two diagnostics yields a suitable system for density measurement on future steady-state fusion reactors. A wavelength around 1 ?m is one of the desirable candidates in terms of the fringe shift and the Faraday rotation angle, the variety of optical components, and the efficiency of frequency doubling for the dispersion interferometer. This paper presents a conceptual design for the dispersion interferometer and Faraday polarimeter with a 1 ?m light source

Onset of instability with collapse observed in relatively high density and medium beta regions of LHD

Edge MHD instabilities with pressure collapse are found in relatively high beta and low magnetic Reynolds number regions with a magnetic axis torus outward-shifted configuration of the large helical device (LHD), and characteristics and onset conditions of the instability are investigated. The instability has a radial structure with an odd parity around the resonant surface, which is different from that of the interchange instability typically observed in the LHD. The onset condition dependence on the magnetic axis location shows that the onset beta increases as the magnetic axis location moves more torus inwardly, and the instability appears only in limited configurations where the magnetic axis is located between 3.65 and 3.775 m. In such configurations, the resonant surface location is close to an index of the plasma boundary. This fact suggests that the distance between the resonant surface location and the plasma boundary plays an important role in the onset, and a possibility that the instability is driven by an external mode

Developments of frequency comb microwave reflectometer for the interchange mode observations in LHD plasma

We have upgraded the multi-channel microwave reflectometer system which uses a frequency comb as a source and measure the distribution of the density fluctuation caused by magneto-hydro dynamics instability. The previous multi-channel system was composed of the Ka-band, and the U-band system has been developed. Currently, the U-band system has eight frequency channels, which are 43.0, 45.0, 47.0, 49.0, 51.0, 53.0, 55.0, and 57.0 GHz, in U-band. Before the installation to the Large Helical Device (LHD), several tests for understanding the system characteristics, which are the phase responsibility, the linearity of output signal, and others, have been carried out. The in situ calibration in LHD has been done for the cross reference. In the neutral beam injected plasma experiments, we can observe the density fluctuation of the interchange mode and obtain the radial distribution of fluctuation amplitude

Direct observation of mass-dependent collisionless energy transfer via Landau and transit-time damping

The energy transfer from wave to particle occurs in collisionless plasma through the interaction between particle and wave, associated with the deformation of ion velocity space from Maxwell-Boltzmann distribution. Here we show the direct observation of mass-dependent collisionless energy transfer via Landau and transit-time damping in a laboratory plasma. The Landau and transit-time damping are confirmed by the bipolar velocity-space signature of the ion velocity distribution function, measured by fast charge exchange spectroscopy with a time resolution less than ion-ion collision time. The excellent agreement between the resonant phase velocity evaluated from the bipolar velocity-space signature and the wave’s phase velocity, estimated from the frequency of the magnetohydrodynamics oscillation measured with the plasma displacement is clear evidence for the Landau damping. The energy transfer from solitary wave to fully ionized carbon impurity ions is larger than that of bulk ions 2-3 times due to heavier mass

Non-contact and real-time measurement of heart rate and heart rate variability using microwave reflectometry

In this paper, we present noncontact and noninvasive vital signal detection using a microwave reflectometer. Elimination of noise components due to random movement of human subjects has been the biggest issue for microwave measurement. Appropriate filtering, amplitude control of the reflectometer signal, and cross correlation among multiple reflectometers together with new algorithms have enabled motion artifact elimination, signal peak detection, and data processing for various parameters related to heart rate (HR) and heart rate variability (HRV). We focus here on the real time measurements of instantaneous HR and HRV for practical use. The evaluation by microwave reflectometry is completely noninvasive and feasible even through clothing, which is extremely effective for health maintenance in daily life as well as for preventing sudden death related to, for example, coronary heart disease and ventricular arrhythmia

Exhaust of turbulence cloud at the tongue shaped deformation event

Exhaust of turbulence cloud at the tongue-shaped deformation which triggers MHD bursts is observed in the Large Helical Device in the low density plasma with significant contribution of trapped particles injected by perpendicular neutral beam injection. The exhaust of turbulence cloud is characterized by the abrupt large increase of turbulence amplitude in the frequency range of 150–500 kHz measured with Doppler reflectometer at the edge region of the plasma (). The increase of turbulence amplitude is significantly large and is by one order of magnitude. This abrupt increase of turbulence level is transient and disappears within one milli-second (typically  ~600 μs). In contrast, the turbulence level slightly inside the plasma edge () decreases by a factor of 2 after the MHD bursts

Observation of distorted Maxwell-Boltzmann distribution of epithermal ions in LHD

A distorted Maxwell-Boltzmann distribution of epithermal ions is observed associated with the collapse of energetic ions triggered by the tongue shaped deformation. The tongue shaped deformation is characterized by the plasma displacement localized in the toroidal, poloidal, and radial directions at the non-rational magnetic flux surface in toroidal plasma. Moment analysis of the ion velocity distribution measured with charge exchange spectroscopy is studied in order to investigate the impact of tongue event on ion distribution. A clear non-zero skewness (3rd moment) and kurtosis (4th moment –3) of ion velocity distribution in the epithermal region (within three times of thermal velocity) is observed after the tongue event. This observation indicates the clear evidence of the distortion of ion velocity distribution from Maxwell-Boltzmann distribution. This distortion from Maxwell-Boltzmann distribution is observed in one-third of plasma minor radius region near the plasma edge and disappears in the ion-ion collision time scale