Parallel currents in the scrape-off layer of JET diverted discharges

Imbalances between the inboard and outboard legs of the single null divertor in tokamak Alcator C-Mod are observed to reverse when the direction of the toroidal field is reversed. These imbalances are measured by embedded probes in the target plates, tomographic reconstructions of bolometry and line radiation, and visible imaging. Density imbalances of about a factor of ten at the targets are observed at moderate density, decreasing as the density is raised until they are almost balanced. The data indicate that the electron pressure is not imbalanced, thus arguing against momentum imbalance as the cause of these drift-induced effects. Instead, power flux imbalance caused by E, A B convection, and enhanced by radiation, is suggested as the underlying cause.

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“…However, our observed pressure balance, which is in contradiction with the JET interpretation [24], is evidence against this mechanism.…”

Section: Mechanism Of Divertor Imbalancecontrasting

confidence: 98%

The effects of field reversal on the Alcator C-Mod divertor

Hutchinson

LaBombard

Goetz

et al. 1995

Plasma Phys. Control. Fusion

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“…Good agreement is observed between experimental data and the thermoelectric current model, which is consistent with observations made on JT-60U [68], ASDEX-Upgrade [69], COMPASS [70], JET [71] and others. For the case shown here, the dominant driving term for thermoelectric current is the temperature asymmetry at the divertor targets.…”

Section: Thermoelectric Currentssupporting

confidence: 90%

“…Experimentally determined currents at the divertor target plates measured by target Langmuir probes are also shown (red). [66] as observations on JET [71] showed that the pressure gradient term only plays a significant role in the event of small target temperature asymmetries. In the discharge and time modelled in this case, the ratio of the electron temperatures at the hot and cold plates ranged from 1.0 to 5.0.…”

Section: Thermoelectric Currentsmentioning

confidence: 91%

Characterisation of detached plasmas on the MAST tokamak

Harrison

2011

Journal of Nuclear Materials

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The mitigation of the steady-state heat loading to plasma-facing components in the divertor structures is an essential prerequisite for the operation of next-generation conventional and tight aspect-ratio Tokamaks. Currently, the most attractive means of reducing this heat loading is the so-called "detached" regime, which is the primary focus of this study.This thesis describes work that was carried out on the MAST tokamak to accurately characterise the plasma conditions within divertor plasmas, and to test the validity of those measurements. To assist in this study, an interpretive code, OSM [1], has been upgraded with additional numerical schemes to ascertain the dominant mechanisms governing parallel scrape-off layer transport including flux expansion, dynamic viscosity and cross-field drift motion in attached conditions. It has been found that the invocation of flux expansion, parallel viscosity and diamagnetic drift motion assist the OSM numerical scheme to converge in the presence of (static+dynamic) parallel pressure gradients often observed between the outboard midplane and lower outer divertor leg.It has also been found that the ion pressure, which is currently unknown, could resolve the observed pressure discrepancy.Line-of-sight spectroscopy of high-n Balmer emission lines has been used to ascertain line averaged T e and line integrated n e during the detached phase of MAST discharges with high spatial resolution (≈ 7mm) to determine electron static pressure profiles along flux contours. Furthermore, narrow-bandwidth imaging spectroscopy of D α , D γ , CII and CIII emission during the onset and sustainment of detachment has been carried out with high temporal (5kHz) and spatial (≈ 3mm) resolution. These data have been combined with ion flux measurements to the divertor target plates using embedded Langmuir probes (6-9mm radial resolution) and upstream Thomson scattering measurements of n e , T e . The data was input into the OSM code, coupled with the kinetic neutral transport code EIRENE [2], to check the experimental data set for internal consistency. The experimental data collected has been used to reconstruct the plasma conditions within the detached divertor leg, heavily constrained by experimental data. EIRENE calculations with re-constructed plasma conditions are able to reproduce experimentally observed D γ /D α line emission ratios to within a factor iii iv of 2 within the recombining region. AcknowledgmentsThe work presented in this thesis is the culmination of the work of many people, who I gratefully acknowledge here. Foremost, I thank Dr. Steven Lisgo for his guidance, enthusiasm and assistance over the last 4 years. It is only on reflection that I appreciate how he kept this work on track, whilst encouraging me to explore my own ideas and how doing so has enhanced the quality of the work presented here. Drs. Andrew Kirk and Geoff Fishpool also deserve special thanks for their guidance, and above all, their understanding in the final months of this work. I also owe great thanks to...

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“…Thermo-current is driven by a temperature difference between the inboard and outboard PFC target plates which causes a current flow to compensate the difference in the sheath potentials [3,4,5]. In DEMO this current is estimated to have a steady state value of approximately 150 A per divertor when plasma attachment begins [6].…”

Section: Introductionmentioning

confidence: 99%