Electrostatic Dust Detector with Improved Sensitivity

Methods to measure the inventory of dust particles and to remove dust if it approaches safety limits will be required in next-step tokamaks such as ITER. An electrostatic dust detector, based on a fine grid of interlocking circuit traces, biased to 30 or 50 V, has been developed for the detection of dust on remote surfaces in air and vacuum environments. Gaining operational experience of dust detection on surfaces in tokamaks is important, however the level of dust generated in contemporary short-pulse tokamaks is comparatively low and high sensitivity is necessary to measure dust on a shot-by-shot basis. We report on modifications in the detection electronics that have increased the sensitivity of the electrostatic dust detector by a factor of up to 120, - a level suitable for measurements on contemporary tokamaks

Electrostatic Dust Detection and Removal for ITER

We present some recent results on two innovative applications of microelectronics technology to dust inventory measurement and dust removal in ITER. A novel device to detect the settling of dust particles on a remote surface has been developed in the laboratory. A circuit board with a grid of two interlocking conductive traces with 25 μm spacing is biased to 30 – 50 V. Carbon particles landing on the energized grid create a transient short circuit. The current flowing through the short circuit creates a voltage pulse that is recorded by standard nuclear counting electronics and the total number of counts is related to the mass of dust impinging on the grid. The particles typically vaporize in a few seconds restoring the previous voltage standoff. Experience on NSTX however, showed that in a tokamak environment it was still possible for large particles or fibers to remain on the grid causing a long term short circuit. We report on the development of a gas puff system that uses helium to clear such particles. Experiments with varying nozzle designs, backing pressures, puff durations, and exit flow orientations have given an optimal configuration that effectively removes particles from an area up to 25 cm² with a single nozzle. In a separate experiment we are developing an advanced circuit grid of three interlocking traces that can generate a miniature electrostatic traveling wave for transporting dust to a suitable exit port. We have fabricated such a 3-pole circuit board with 25 micron insulated traces that operates with voltages up to 200 V. Recent results showed motion of dust particles with the application of only 50 V bias voltage. Such a device could potentially remove dust continuously without dedicated interventions and without loss of machine availability for plasma operations

Double quantum dot turnstile as an electron spin entangler

We study the conditions for a double quantum dot system to work as a reliable electron spin entangler, and the efficiency of a beam splitter as a detector for the resulting entangled electron pairs. In particular, we focus on the relative strengths of the tunneling matrix elements, the applied bias and gate voltage, the necessity of time-dependent input/output barriers, and the consequence of considering wavepacket states for the electrons as they leave the double dot to enter the beam splitter. We show that a double quantum dot turnstile is, in principle, an efficient electron spin entangler or entanglement filter because of the exchange coupling between the dots and the tunable input/output potential barriers, provided certain conditions are satisfied in the experimental set-up.Comment: published version; minor error correcte

Evaluating the provision of flexible learning for children at risk of primary school dropout in Malawi

Communities in Malawi selected 15 children deemed "at-risk" - predominantly orphans - in Class 6 of each of 20 intervention schools to receive learning materials, support from the community and a school "buddy." An experimental evaluation found that dropout was reduced by 45% across intervention schools compared to 20 control schools. The program had spillover effects, indirectly reducing dropout among older pupils in the class not deemed at-risk. These findings imply that age, and not orphanhood, was the main indicator of dropout risk and that when targeting criteria are considered carefully, flexible learning programs can reduce dropout substantially among vulnerable children. (C) 2014 Elsevier Ltd. All rights reserved