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VERSION:2.0
CALSCALE:GREGORIAN
PRODID:UW-Madison-Physics-Events
BEGIN:VEVENT
SEQUENCE:0
UID:UW-Physics-Event-9903
DTSTART:20261116T160000Z
DTEND:20261116T180000Z
DTSTAMP:20261110T221535Z
LAST-MODIFIED:20261105T164130Z
LOCATION:Chamberlin 5290
SUMMARY:Dynamics of a pressure-driven collapse during high-density ste
 llarator operation\, Thesis Defense\, R. Sassella
DESCRIPTION:High density stellarator operation is a promising scenario
  for a future fusion reactor. However\, operating scenarios may be con
 strained by pressure-driven instabilities. The core density collapse (
 CDC) is a fast\, pressure-driven loss of confinement that affects supe
 r-dense core (SDC) operation on the Large Helical Device. Previous wor
 k indicates that the CDC is triggered by a high-n ballooning mode in t
 he bad-curvature region. The ballooning mode nonlinearly couples to a 
 low-n mode in the core\, and energy is inverse-cascaded into core mode
 \, perturbing flux surfaces and causing the ejection of up to 50% of c
 ore density. To study high-resolution profile dynamics during the CDC\
 , the instability was deliberately provoked with excessive pellet fuel
 ing of the core. Over 150 CDC events were produced. The first high-res
 olution temperature and density profile measurements of a CDC were mad
 e with a 20 kHz Thomson scattering diagnostic. A hot\, narrow feature 
 in the $T_e$ profile was observed during the CDC for the first time. T
 he feature occupies about a quarter of the plasma diameter and lasts u
 pwards of 150 $\\mu$s before it collapses. This feature is hypothesize
 d to be caused by either the transfer of kinetic energy from the ballo
 oning mode to a core mode via nonlinear coupling or by an increase in 
 localized NBI deposition caused by rapid changes to the density profil
 e. The electron temperature feature is observed to collapse about 100 
 microseconds before core density\, suggesting that multiple transport 
 mechanisms may be at work. Density loss gradually propagates inward\, 
 from the transport barrier towards the magnetic axis. High resolution 
 measurements of profile dynamics broadly agree with simulation predict
 ions\, although questions about the electron temperature feature and t
 ransport mechanisms remain. CDCs demonstrate that the ballooning mode 
 places a hard limit on the accessible pressure gradient in stellarator
 s.
URL:https://www.physics.wisc.edu/events/?id=9903
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