The role of particle, energy and momentum losses in 1D simulations of divertor detachment

TY - JOUR

T1 - The role of particle, energy and momentum losses in 1D simulations of divertor detachment

AU - Dudson, Benjamin Daniel

AU - Allen, Joe Oliver

AU - Body, T

AU - Chapman, Ben

AU - Lau, Cheryl Si Ching

AU - Townley, Luke

AU - Moulton, D.

AU - Harrison, J.

AU - Lipschultz, Bruce

N1 - © 2019 IOP Publishing. This is an author-produced version of the published paper. Uploaded in accordance with the publisher’s self-archiving policy.

PY - 2019/5/3

Y1 - 2019/5/3

N2 - A new 1D divertor plasma code, SD1D, has been used to examine the role of recombination, radiation, and momentum exchange in detachment. Neither momentum or power losses by themselves are found to be sufficient to produce a reduction in target ion flux in detachment (flux rollover); radiative power losses are required to a) limit and reduce the ionization source and b) access low-targettemperature, T_target, conditions for volumetric momentum losses. Recombination is found to play little role at flux rollover, but as T_target drops to temperatures around 1eV, it becomes a strong ion sink. In the case where radiative losses are dominated by hydrogen, the detachment threshold is identified as a minimum gradient of the energy cost per ionisation with respect to T_target. This is also linked to thresholds in T_target and in the ratio of upstream pressure to power flux. A system of determining the detached condition is developed such that the divertor solution at a given T_target (or lack of one) is determined by the simultaneous solution of two equations for target ion current - one dependent on power losses and the other on momentum. Depending on the detailed momentum and power loss dependence on temperature there are regions of T_target where there is no solution and the plasma 'jumps' from high to low T_target states. The novel analysis methods developed here provide an intuitive way to understand complex detachment phenomena, and can potentially be used to predict how changes in the seeding impurity used or recycling aspects of the divertor can be utilised to modify the development of detachment.

AB - A new 1D divertor plasma code, SD1D, has been used to examine the role of recombination, radiation, and momentum exchange in detachment. Neither momentum or power losses by themselves are found to be sufficient to produce a reduction in target ion flux in detachment (flux rollover); radiative power losses are required to a) limit and reduce the ionization source and b) access low-targettemperature, T_target, conditions for volumetric momentum losses. Recombination is found to play little role at flux rollover, but as T_target drops to temperatures around 1eV, it becomes a strong ion sink. In the case where radiative losses are dominated by hydrogen, the detachment threshold is identified as a minimum gradient of the energy cost per ionisation with respect to T_target. This is also linked to thresholds in T_target and in the ratio of upstream pressure to power flux. A system of determining the detached condition is developed such that the divertor solution at a given T_target (or lack of one) is determined by the simultaneous solution of two equations for target ion current - one dependent on power losses and the other on momentum. Depending on the detailed momentum and power loss dependence on temperature there are regions of T_target where there is no solution and the plasma 'jumps' from high to low T_target states. The novel analysis methods developed here provide an intuitive way to understand complex detachment phenomena, and can potentially be used to predict how changes in the seeding impurity used or recycling aspects of the divertor can be utilised to modify the development of detachment.

KW - detachment

KW - simulation

KW - tokamak

UR - http://www.scopus.com/inward/record.url?scp=85069484772&partnerID=8YFLogxK

U2 - 10.1088/1361-6587/ab1321

DO - 10.1088/1361-6587/ab1321

M3 - Article

SN - 0741-3335

VL - 61

JO - Plasma Physics and Controlled Fusion

JF - Plasma Physics and Controlled Fusion

IS - 6

M1 - 065008

ER -