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

Bipolar degradation monitoring of 4H-SiC MOSFET power devices by electroluminescence measurements

Expansion and contraction of Shockley type stacking faults (SF) due to the glide of basal plane dislocations are reported for the first time in a commercial 4H-SiC MOSFET bare die rated at 100A/1.2kV. Electroluminescence of the laboratory fabricated power MOSFET body diode is measured to detect the bipolar degradation which provides firm evidence of the presence of a 3C- like regions affected by SF. Accelerated bipolar degradation tests were performed on the device and it was found that the on-resistance which is a key indicator of the degradation level, increased by 15.6 % during the first stress while it decreased by 6.9 % in the second stress. Other related SiC material defects were as well detected. The electroluminescence spectra is an excellent tool for monitoring SiC power devices deterioration related to material defects without performing destructive tests such as SEM

Comparative optimization design of a modular multilevel converter tapping cells and a 2L-VSC for hybrid LV ac/dc microgrids

The paper presents the performance of the modular multilevel converter tapping cells associated with an ac filter in term of efficiency and power density in a hybrid LV ac/dc microgrid application and compares it to the performance of the conventional topology used in LV application, i.e., the Two-Level voltage source converter (2L VSC). A bi-objective optimization based on the Genetic Algorithm is hence developed, providing details on designing the components of the LCL filter, the MMC and the 2L VSC. The MMC reaches an efficiency of 99.4% when the main dcgrid is left floating. However, due to its modularity and scalability, offering multiport connections option, the MMC tapping cells has the disadvantage of low power density. Exploring the filtering capability of the equivalent arm inductance of the MMC seen from the ac grid side, optimization design results show that higher switching frequencies allow a significant volume reduction of the inductive components of the MMC/LCL filter while higher switching frequencies have little impact on the switching losses of the MMC. This has the benefit of reducing the overall footprint of the converter and encouraging the use of the MMC in LV application

Impact of nitrogen seeding on confinement and power load control of a high-triangularity JET ELMy H-mode plasma with a metal wall

This paper reports the impact on confinement and power load of the high-shape 2.5MA ELMy H-mode scenario at JET of a change from an all carbon plasma facing components to an all metal wall. In preparation to this change, systematic studies of power load reduction and impact on confinement as a result of fuelling in combination with nitrogen seeding were carried out in JET-C and are compared to their counterpart in JET with a metallic wall. An unexpected and significant change is reported on the decrease of the pedestal confinement but is partially recovered with the injection of nitrogen.Comment: 30 pages, 16 figure

Overview of JET results

Since the last IAEA conference, the scientific programme of JET has focused on the qualification of the integrated operating scenarios for ITER and on physics issues essential for the consolidation of design choices and the efficient exploitation of ITER. Particular attention has been given to the characterization of the edge plasma, pedestal energy and edge localized modes (ELMs), and their impact on plasma facing components (PFCs). Various ELM mitigation techniques have been assessed for all ITER operating scenarios using active methods such as resonant magnetic field perturbation, rapid variation of the radial field and pellet pacing. In particular, the amplitude and frequency of type I ELMs have been actively controlled over a wide parameter range (q95 = 3–4.8, βN <= 3.0) by adjusting the amplitude of the n = 1 external perturbation field induced by error field correction coils. The study of disruption induced heat loads on PFCs has taken advantage of a new wide-angle viewing infrared system and a fast bolometer to provide a detailed account of time, localization and form of the energy deposition. Specific ITER-relevant studies have used the unique JET capability of varying the toroidal field (TF) ripple from its normal low value δBT = 0.08% up to δBT = 1% to study the effect of TF ripple on high confinement-mode plasmas. The results suggest that δBT < 0.5% is required on ITER to maintain adequate confinement to allow QDT = 10 at full field. Physics issues of direct relevance to ITER include heat and toroidal momentum transport, with experiments using power modulation to decouple power input and torque to achieve first experimental evidence of inward momentum pinch in JET and determine the threshold for ion temperature gradient driven modes. Within the longer term JET programme in support of ITER, activities aiming at the modification of the JET first wall and divertor and the upgrade of the neutral beam and plasma control systems are being conducted. The procurement of all components will be completed by 2009 with the shutdown for the installation of the beryllium wall and tungsten divertor extending from summer 2009 to summer 2010