Effects of Plasma Radiation on the Thomson Scattering Diagnostic Installed on the Large Helical Device

Recently we modified the Thomson scattering diagnostic (TS) installed on LHD so that DC levels (VDC) of all avalanche photodiodes (APD) used for detecting scattered light can be registered every 1 ms, which enabling us to make validity check on TS data taken under very intense plasma radiation. In the line of this task, we first examined how the pulse-performance of an APD degrades as the intensity of continuous light (JDC) incident to the APD increases. We found two effects are involved in deteriorating the pulse-performance of the APD: (1) the responsivity of the APD to a pulsed light drops as JDC increases, causing a systematic errors on the deduced electron temperature (Te) and density (ne); (2) the frequency response of the APD and the following circuit drops as JDC increases, which deforms the pulse shape. The bias voltage applied to the APD (Vb) has large influence on these behaviors, showing the best overall performance for a high JDC around Vb ? 0.5Vr, where Vr is the recommended voltage giving responsivity of 675 kV/W at 1060 nm. Considering these effects together, we set a conservative validity criterion for the pulse APD performance in term of the VDC: VDC < 0.5 V. The Vb = 0.5 Vr setup gives much reliable Te-profiles without a collapse in Te-profile for a much wider range of plasma radiation intensity. With this criterion, we check the validity of Te- and ne-profiles of two example data

Raman and Rayleigh Calibrations of the LHD YAG Thomson Scattering

We have carried out absolute calibrations of the LHD YAG Thomson scattering system by using Raman scattering and Rayleigh scattering in order to verify the applicability of Rayleigh calibration in the LHD Thomson scattering, and make a comparative study of Raman and Rayleigh calibrations. In the LHD Thomson scattering device, Rayleigh calibration is expected to give more reliable calibration factors. For the Rayleigh calibration, additional Rayleigh channel was installed into 20 polychromators. The other 124 polychromators without Rayleigh channel were calibrated by only Raman scattering. In the Raman calibration, pure gaseous nitrogen was introduced into the LHD vacuum vessel whereas the Rayleigh calibration was made by using air as target gas. The calibration factors obtained from the Raman and Rayleigh calibrations show good agreements. Uncertainties in the calibration factors obtained from the Raman and Rayleigh calibrations are discussed

On-Demand Density Correction Using Steady-State Plasmas in the LHD Thomson Scattering

In order to measure reliable electron densities of fusion plasmas by using Thomson scattering system, both accurate absolute calibration and long-term stability in the system are required. Even if slight misalignment of some optics occurs, it may cause large errors in measured densities. We propose a new method to obtain correctionfactors to the errors originated from misalignment by using steady-state plasma discharges. In addition to the datacorrection, realignment of the laser beam can be applied als

Detection of electron temperature anisotropy by an x-ray crystal spectrometer in the Large Helical device

ORCID　 0000-0002-1021-0322We examine Ar XVI spectra measured by an x-ray crystal spectrometer to estimate electron temperature anisotropy in high-temperature electron cyclotron heated plasmas in the Large Helical Device (LHD). We calculate the atomic structure and electron impact excitation cross section between magnetic sublevels of Ar XVI. By assuming a bi-Maxwellian electron distribution function and considering the mount parameters of the x-ray spectrometer in LHD, we model the expected intensity ratio q/r as a function of Te⊥B and Te∣∣B, where ⊥B and ∣∣B denote perpendicular and parallel components to the magnetic axis of toroidal plasma, respectively. The calculation results show that the intensity ratio of q and r, which are formed by inner-shell excitation from the ground state of Ar15+, is sensitive to electron temperature anisotropy. We apply the calculation results to the LHD experiments. The ratio of q/r changes with variations in electron density and collision frequency. In the core and low νe region, Te⊥B is predominant, and the electron temperature becomes isotropic above νe > 104 Hz. By combining electron temperature measurements from Thomson scattering and radial profile of Ar15+ ions estimated using the extreme ultraviolet spectrometer, local values of Te⊥B/Te∣∣B are quantitatively estimated using the q/r ratio. The derived Te⊥B/Te∣∣B is compared with collision frequency, radial electric field, and effective helical ripple, and the experimental results are explained qualitatively.journal articl

Multi-functional Diagnostic Method with Tracer-encapsulated Pellet Injection

In order to obtain a better understanding of impurity transport in magnetically confined plasmas, a Tracer-Encapsulated Soild PELlet (TESPEL) has been developed. The essential points of the TESPEL are as follows: the TESPEL has a double-layered structure, and a tracer impurity, the amount of which can be known precisely, is embedded as an inner core. This structure enables us to deposit the tracer impurity locally inside the plasma. From experiences of developing the TESPEL production technique and its injection experiments, it became clear that various plasma properties can be studied by the TESPEL injection. There are not only impurity transport in the plasma but also transport both outside and inside of the magnetic island O-point, heat transport and high-energy neutral particle flux. Therefore, the TESPEL injection has a favorable multi-functional diagnostic capability. Furthermore a Tracer-Encapsulated Cryogenic PELlet (TECPEL) has been also developed. The TECPEL has an advantage over the TESPEL in terms of no existence of carbons in the outer layer. The TECPEL injector was installed at LHD in December 2005, and the preliminary injection experiments have been carried out

Impurity emission characteristics of long pulse discharges in Large Helical Device

Line spectra from intrinsic impurity ions have been monitored during the three kinds of long-pulse discharges (ICH, ECH, NBI). Constant emission from the iron impurity shows no preferential accumulation of iron ion during the long-pulse operations. Stable Doppler ion temperature has been also measured from Fe XX, C V and C III spectra

Nd:YAG laser Thomson scattering diagnostics for a laboratory magnetosphere

A new Nd:YAG laser Thomson scattering (TS) system has been developed to explore the mechanism of high-beta plasma formation in the RT-1 device. The TS system is designed to measure electron temperatures (Te) from 10 eV to 50 keV and electron densities (ne) of more than 1.0 × 1017 m−3. To measure at the low-density limit, the receiving optics views the long scattering length (60 mm) using a bright optical system with both a large collection window (260-mm diameter) and large collection lenses (300-mm diameter, a solid angle of ∼68 × 10−3 str). The scattered light of the 1.2-J Nd:YAG laser (repetition frequency: 10 Hz) is detected with a scattering angle of 90° and is transferred via a set of lenses and an optical fiber bundle to a polychromator. After Raman scattering measurement for the optical alignment and an absolute calibration, we successfully measured Te = 72.2 eV and ne = 0.43 × 1016 m−3 for the coil-supported case and Te = 79.2 eV and ne = 1.28 × 1016 m−3 for the coil-levitated case near the inner edge in the magnetospheric plasmas

A new multi-tracer pellet injection for a simultaneous study of low- and mid/high-Z impurities in high-temperature plasmas

A new multi-tracer technique in the Tracer-Encapsulated Solid Pellet (TESPEL) method has been developed in order to acquire simultaneously the information about the behaviors of various impurities, i.e., to study concurrently the behaviors of low- and mid/high-Z impurities in magnetically confined high-temperature plasmas. In this new technique, an inorganic compound (for example, lithium titanate, Li2TiO3) is proposed to be used as a tracer embedded in the core of the TESPEL, instead of pure elements. The results of the proof-of-principle experiment clearly demonstrate the applicability of the new multi-tracer technique in the TESPEL method for the simultaneous study of behaviors of low- and mid/high-Z impurities in high-temperature plasmas

Vacuum ultraviolet spectroscopy in detached plasmas with impurity gas seeding in LHD

We have carried out vacuum ultraviolet (VUV) spectroscopy of impurity ions in detached plasmas with impurity gas seeding in the Large Helical Device (LHD). In neon (Ne) gas seeding experiments, temporal evolutions of VUV spectral lines from Ne IV–VIII were recorded by a grazing incidence spectrometer. In addition, spatial profiles of fully ionized Ne density were measured by charge exchange spectroscopy. An electron temperature range where each ion emits is inferred based on the comparisons of the measured line intensity ratios with the calculations using collisional-radiative models

Improvement of electron temperature and density accuracy in Thomson scattering diagnostics by an accumulation of 100 laser pulses within 5 milliseconds

In order to measure the electron temperature and density in high-electron-temperature and low-density plasmas, improvement of the signal-to-noise ratio of the Thomson scattering measurement signal in the Large Helical Device has been tried. One hundred laser pulses of about 1 J in 5 ms were injected at 50 μs intervals into plasmas in an almost steady state. By averaging the scattered signals obtained from the same plasma and the same location, the noise in the signals and the error in the background level were also reduced. When the number of signals to be averaged was large, the scattering of the electron temperature profile was small and the magnitude of the error also became small.journal articl