Development of Faraday-cup-based Fast Ion Loss Detector in Wendelstein 7-X

A study on fast-ion losses due to magnetic field ripples and fast-ion-drivenmagnetohydrodynamic (MHD) modes is important in terms of view of research on fusion-born alpha losses in fusion devices. To understand fast-ion loss in Wendelstein 7-X (W7-X) plasmas, installation of fast-ion loss diagnostics for W7-X has been planned. For the Op1.2b campaign, the prototype Faraday-cup-based fast-ion loss detector (FILD) has been designed as joint cooperative project between National Institute for Fusion Science and Max Planck Institute for Plasma Physics. The Faraday-cup-based FILD is relatively cost-effective in construction compared with a scintillator-type FILD. The FILD is capable of providing the flux, pitch angle, and Larmor radius of escaping fast ions simultaneously, providing the clear understanding on fast-ion losses induced by MHD mode as well as non-axisymmetric magnetic field ripples.A Lorentz orbit code (LORBIT code and ASCOT code) has been used to find a position suitable for detection of escaping beam ions. It is found that the sufficient beam-ion flux on the head position of the multi-purpose manipulator (MPM) is expected. Therefore, we decided to install the prototype FILD head using the MPM. The detector is mainly composed of a molybdenum head having a set of two apertures restrict the orbits of fast ions that can enter the probe and eight Faraday films as a charge collector. The size and the position of thoseapertures are decided using the grid calculation program. Faraday film is a thin film of aluminum vapor deposited onto one side of the quartz substrate. The thickness of the films is approximately 0.2 μm. Electric current from each Faraday film will be carried to the low input impedance current amplifier (I-76, NF Corporation) and an isolation amplifier. The signal level of the FILD predicted by the ASCOT code is up to 0.5 μA, which is comparable with that of a FILD in the Compact Helical System (CHS)

Energy-and-pitch-angle-resolved escaping beam ion measurements by Faraday-cup-based fast-ion loss detector in Wendelstein 7-X

With the objective of understanding the energetic-particle loss mechanism in three-dimensional plasmas, a Faraday-cup-based fast-ion loss detector (FILD) was developed and installed in OP1.2b in Wendelstein 7-X (W7-X) as a collaboration between the National Institute for Fusion Science and Max-Planck-Institute for Plasma Physics. The FILD, which consists of double apertures and thin aluminum foils, was based on a magnetic spectrometer using the magnetic field of the fusion device. The double aperture limits the thermal ion, but allows the energetic ion to enter the FILD box. The thin aluminum foils serve as the ion collector. Orbit-following calculations were performed in order to find a suitable position for the FILD. The results indicated that barely co- and counter-going transit ions reached the FILD position mounted on the multi-purpose manipulator installed on W7-X. Moreover, because the injection angle of neutral beams injector installed on W7-X was relatively perpendicular, the target range of pitch angle was set to from 91 degrees to 150 degrees. An energy-and-pitch-angle map was created using a grid calculation code in order to decide on the position/size of aperture and aluminum foil. The grid calculation code indicates the position where the energetic ion would strike on the aluminum foil. Here, the map was used as the basis for designing an aluminum foil pattern with two energies and four pitch angle ranges. Also, note that the lower energy range was designed so that it accommodates a neutral beam injection energy of 55 kV. Measurements of beam ion losses were performed in neutral beam (NB) blip experiments, where concurrent increases and decreases of barely co-going transit beam ion losses due to NB injections were observed. Furthermore, this study validated that the FILD installed on a midplane manipulator probe is capable of probing a range of radii spanning 1.0–1.5 cm, over which the beam ion loss current would vary significantly

Velocity-space sensitivity of the time-of-flight neutron spectrometer at JET

The velocity-space sensitivities of fast-ion diagnostics are often described by so-called weight functions. Recently, we formulated weight functions showing the velocity-space sensitivity of the often dominant beam-target part of neutron energy spectra. These weight functions for neutron emission spectrometry (NES) are independent of the particular NES diagnostic. Here we apply these NES weight functions to the time-of-flight spectrometer TOFOR at JET. By taking the instrumental response function of TOFOR into account, we calculate time-of-flight NES weight functions that enable us to directly determine the velocity-space sensitivity of a given part of a measured time-of-flight spectrum from TOFOR

Relationship of edge localized mode burst times with divertor flux loop signal phase in JET

A phase relationship is identified between sequential edge localized modes (ELMs) occurrence times in a set of H-mode tokamak plasmas to the voltage measured in full flux azimuthal loops in the divertor region. We focus on plasmas in the Joint European Torus where a steady H-mode is sustained over several seconds, during which ELMs are observed in the Be II emission at the divertor. The ELMs analysed arise from intrinsic ELMing, in that there is no deliberate intent to control the ELMing process by external means. We use ELM timings derived from the Be II signal to perform direct time domain analysis of the full flux loop VLD2 and VLD3 signals, which provide a high cadence global measurement proportional to the voltage induced by changes in poloidal magnetic flux. Specifically, we examine how the time interval between pairs of successive ELMs is linked to the time-evolving phase of the full flux loop signals. Each ELM produces a clear early pulse in the full flux loop signals, whose peak time is used to condition our analysis. The arrival time of the following ELM, relative to this pulse, is found to fall into one of two categories: (i) prompt ELMs, which are directly paced by the initial response seen in the flux loop signals; and (ii) all other ELMs, which occur after the initial response of the full flux loop signals has decayed in amplitude. The times at which ELMs in category (ii) occur, relative to the first ELM of the pair, are clustered at times when the instantaneous phase of the full flux loop signal is close to its value at the time of the first ELM

A new disruption mitigation valve (DMV) and gas flow in guiding tubes of different diameter

A new disruption mitigation valve, the DMV-30, has been developed and tested. The orifice output area of the valve is a factor of 2.4 and 12.25 times larger than that of its predecessors, DMV-20 and DMV-10, and the gas reservoir amounts to 1.3 L while the older version used at JET had only 0.65 L. The coil which provides the magnetic field pulse for the activation of the piston by an eddy current is outside of the working gas volume such that all gas volumes are now made of stainless steel. The valve has the advantages of the previous developments: it is robust and reproducible, opens fully within 3ms and releases 50% of the gas within about 5ms (He) to 10 ms (Ar). The valve is attached subsequently to two different guiding tubes, one with an inner diameter of 38mm as used presently at JET and one with 102mm inner diameter; the aim of this paper is the analysis of the gas flows for different diameters. The front of the gas pulse propagates with a Mach number of about 2.5 through the tubes, independent of the two diameters. This high speed agrees with theoretical expectations of flow expansion of a half infinite tube in vacuum. In the quasi-stationary phase of the expansion, the gas flows with about sound speed in the 102mm tube and with about half of the sound speed in the 38mm tube