T. Shimozuma

Spectrum response and analysis of 77 GHz band collective Thomson scattering diagnostic for bulk and fast ions in LHD plasmas

A collective Thomson scattering (CTS) diagnostic was developed and used to measure the bulk and fast ions originating from 180xa0keV neutral beams in the Large Helical Device (LHD). Electromagnetic waves from a gyrotron at 77xa0GHz with 1xa0MW power output function as both the probe and electron cyclotron heating beam. To clarify the diagnostic applicability of the gyrotron in the 77xa0GHz frequency band, we investigated the dependence of the probe and receiver beam trajectories in plasmas with high electron densities of (4–5)xa0×xa01019xa0m−3 and low electron densities of (1–2)xa0×xa01019xa0m−3. At high density, a stray radiation component was observed in the CTS spectrum whereas it was negligibly small at low density. The CTS spectrum was measured and analysed after the in situ beam alignment using a beam scan. Qualitatively, the CTS spectrogram shows consistent response to ion temperatures of 1–2xa0keV for electron densities of (1–2)xa0×xa01019xa0m−3 and electron temperatures of 2–4xa0keV. The measured CTS spectrum shows an asymmetric shape at the foot of the bulk-ion region during the injection of 180xa0keV fast ions. This shape is explained by the fast-ion distribution in the velocity space (v‖, v⊥) based on Monte Carlo simulation results. The analysis method of the CTS spectra is used to evaluate the ion temperature and fast-ion velocity distribution from the measured CTS data.

High-power pulsed gyrotron for 300 GHz-band collective Thomson scattering diagnostics in the Large Helical Device

A high-power pulse gyrotron was developed to generate a probe wave for 300 GHz-band collective Thomson scattering (CTS) diagnostics in the Large Helical Device. In this frequency range, avoiding mode competition is critical to realizing high-power and stable oscillation with a narrow frequency bandwidth. A moderately over-moded cavity was investigated to ensure sufficient isolation of a desired mode from neighbouring modes, and to achieve high power output simultaneously. A cavity with the TE14,2 operation mode, a triode electron gun with an intense laminar electron beam, and an internal mode convertor were designed to construct a prototype tube. It was experimentally observed that oscillation of the TE14,2 mode was strong enough for mode competition, and provided high power with sufficient stability. The oscillation characteristics associated with the electron beam properties were compared with the numerical characteristics to find an optimum operating condition. As a result, single-mode operation with maximum output power of 246 kW was demonstrated at 294 GHz with 65 kV/14 A electron beam, yielding efficiency of ~27%. The radiation pattern was confirmed to be highly Gaussian. The duration of the 130 kW pulse, which is presently limited by the power supply, was extended up to 30 µs. The experimental results validate our design concept and indicate the potential for realizing a gyrotron with higher power and longer pulse toward practical use in 300 GHz CTS diagnostics.

Initial result of collective Thomson scattering using 77 GHz gyrotron for bulk and tail ion diagnostics in the Large Helical Device

The collective Thomson scattering (CTS) technique has been utilized with the backscattering configuration in the collective scattering regime to diagnose the velocity distribution functions in the Large Helical Device (LHD). The receiver was equipped with 16 channels and the first test has been carried out using the eight channels for scattered radiation and these channels cover a few GHz frequency shift from the 76.95 GHz probe beam. During the discharge, the electron density and temperature at the central region of the LHD are 1×1019m−3, and 1.0 keV, respectively. The probing beam with rectangular wave modulation is injected by 50 Hz in order to be distinct from the background electron cyclotron emission (ECE). The scattered radiation is resolved successfully at each channel of CTS receiver system. The detected signals of bulk ion and electron components are by a factor of 10 ~ 102 larger than the background ECE signal. We found that the measured spectra are in reasonably agreement with the theoretical spectra calculated by using the reliable measured electron temperature and density for input parameters. The CTS receiver system will be improved to obtain more accurate velocity distributions in high temperature plasmas.

Extension of high Te regime with upgraded electron cyclotron resonance heating system in the Large Helical Device

Enhancement of the output power per gyrotron has been planned in the Large Helical Device (LHD). Three 77-GHz gyrotrons with an output power of more than 1u2009MW have been operated. In addition, a high power gyrotron with the frequency of 154 GHz (1 MW/5u2009s, 0.5u2009MW/CW) was newly installed in 2012, and the total injection power of Electron cyclotron resonance heating (ECRH) reached 4.6u2009MW. The operational regime of ECRH plasma on the LHD has been extended due to the upgraded ECRH system such as the central electron temperature of 13.5u2009keV with the line-averaged electron density ne_firu2009=u20091u2009×u20091019 m−3. The electron thermal confinement clearly improved inside the electron internal transport barrier, and the electron thermal diffusivity reached neoclassical level. The global energy confinement time increased with increase of ne_fir. The plasma stored energy of 530u2009kJ with ne_firu2009=u20093.2u2009×u20091019 m−3, which is 1.7 times larger than the previous record in the ECRH plasma in the LHD, has been successfully achieved.

Mode Content Analysis for ECH Transmission Lines by Burn Pattern and Nonlinear Optimization

Abstract In the system of electron cyclotron heating, highly overmoded, corrugated circular waveguides are used. To analyze propagating mode content in the waveguide, burn patterns of the thermal paper placed on the waveguide aperture are observed at several positions. Theoretical burn patterns are obtained by taking into account a nonlinear grayscale response of the thermal paper to the calculated power profiles. We have developed a new method of mode analysis by nonlinear optimization, which is based on an iterative error reduction of differences between observed and theoretical patterns. To examine the status of polarization, the transformation between hybrid modes and linearly polarized (LP) modes is derived. The method is applied to the 82.7-GHz transmission line connected with the gyrotron. The propagating wave is linear polarized and consists of [approximately]4% of the LP11 odd mode, [approximately]95% of the LP01 mode, and [approximately]1% of other modes. The calculated burn pattern is similar to the observed one, like a plateau. By using both center of power and weighted averages of the perpendicular wavenumber in these profiles, offset and tilting angles of an injecting electromagnetic beam to the waveguide entrance are inferred. These are verified to be consistent with the results by the coupling code of a Gaussian beam with hybrid modes.

Electron Bernstein wave heating by electron cyclotron wave injection from the high-field side in LHD

In the Large Helical Device (LHD), evident electron Bernstein wave (EBW) heating was successfully performed. The experiment was carried out using the electron cyclotron heating (ECH) system that was upgraded by installation of high-power, long-pulse 77 GHz gyrotrons. The EBW heating was achieved by a mode conversion from injected EC wave to EBW, by the so-called slow-XB technique where an X-mode wave is injected to the plasma from the high magnetic field side. The specific magnetic configuration of LHD provides a good opportunity to realize the slow-XB technique, which is generally difficult for tokamaks. With the slow-XB technique, increases in kinetically evaluated electron energy Wpe and electron temperature Te were observed in overdense plasmas. An electron heating in the so-called super dense core plasma in LHD, which is characterized with an internal diffusion barrier and a steep density gradient at the plasma core, was successfully demonstrated in the plasma core region where the central electron density ne0 of 17 × 1019 m−3 was about 1.2 times higher, at the beginning of the EC-wave injection, than the left-hand cut-off density of applied 77 GHz EC waves.

Electron cyclotron beam measurement system in the Large Helical Device.

In order to evaluate the electron cyclotron (EC) heating power inside the Large Helical Device vacuum vessel and to investigate the physics of the interaction between the EC beam and the plasma, a direct measurement system for the EC beam transmitted through the plasma column was developed. The system consists of an EC beam target plate, which is made of isotropic graphite and faces against the EC beam through the plasma, and an IR camera for measuring the target plate temperature increase by the transmitted EC beam. This system is applicable to the high magnetic field (up to 2.75 T) and plasma density (up to 0.8 × 10(19) m(-3)). This system successfully evaluated the transmitted EC beam profile and the refraction.

Progress of microwave collective Thomson scattering in LHD

Microwave collective Thomson scattering (CTS) by using a 77 GHz gyrotron is routinely working in LHD and the improvements of the system is now underway. The targets of this diagnostic are measurements of energetic fast ion distribution and ion ratio. In the present system, 800kW 77 GHz gyrotron is injected horizontally and scattered radiation is received changing scattering angle. The system works with existence of electron cyclotron resonance layer. Thus, most of the power is absorbed at the layer like beam damping and stray radiation dramatically decreases. Gyrotron is modulated at 40 Hz, then, background ECE, which is signal in gyrotron off phase, is subtracted from scattered signal in gyrotron on phase. The perturbation of electron temperature due to the gyrotron injection is almost negligible. Temporal evolution of CTS spectrum is obtained by 32ch filter bank receiver through discharge and fine spectrum is obtained by 10 GHz sampling fast digitizer for 80 ms. Change of the width and asymmetry of CTS spectrum is observed after turning off of tangentially injected neutral beam (NB). This is qualitatively consistent with reduction of fast ion density. Preliminary data of ion ratio between hydrogen and helium are also obtained.The Pixel Detector of the ATLAS experiment has shown excellent performance during the whole Run-1 of LHC. Taking advantage of the long shutdown, the detector was extracted from the experiment and brought to surface, to equip it with new service quarter panels, to repair the modules and to ease installation of the Insertable B-Layer (IBL). The IBL is a fourth layer of pixel detectors, and has been installed in May 2014 between the existing Pixel Detector and a new smaller radius beam-pipe at a radius of 3.3 cm. To cope with the high radiation and pixel occupancy due to the proximity to the interaction point, a new read-out chip and two different silicon sensor technologies (planar and 3D) have been developed. Furthermore, the physics performance will be improved through the reduction of pixel size while, targeting for a low material budget, a new mechanical support using light weight staves and CO2 based cooling system have been adopted. An overview of the refurbishing of the Pixel Detector and the IBL project as well as the experience in its construction will be presented, focusing on adopted technologies, module and stave production, qualification of assembly procedure, integration of staves around the beam pipe and commissioning of the detector.

Improved notch filter for microwave plasma diagnostics in 70 GHz range

A notch filter in 70 GHz range is developed to eliminate undesirable signals for plasma diagnostics using millimeter waves. We require a robust and narrow band characteristic for the notch filter. Since the previous notch filter which we reported contains the spurious notches, the plungers are improved to reduce the spurious notches. The lossy disks infill the gaps between the plungers and the cavities. The insert of the lossy disks minimizes the spurious notches around 76–78 GHz, while the insertion loss increases in the frequency range from 77.5 to 80 GHz. The plungers with differential screws are also used to make the tuning fine. For further performance increase, based on the same cavity mode, the design of coupling slits is modified to coupling holes. The simulated results in the case of coupling holes predict the characteristic of less than 50 MHz at −3 dB bandwidth and less than −50 dB with two cavities and −100 dB with eight cavities at the maximum attenuations.

Collective Thomson scattering of 77 GHz high power ECRH beam in LHD

The collective Thomson scattering (CTS) is one of the most promising methods for evaluating the ion velocity distribution function.