APS2019_Sheftman

Impurity Transport in the C-2W experiment
D. Sheftman, M. Nations, D. Osin, E. Granstedt, D. Gupta and the TAE team
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
Abstract
Reduction of impurity influx from material surfaces is essential to achieve high temperatures in fusion experiments. In the beam driven field reversed configuration C-2W device, the quartz tube of the formation section and the metal surfaces in the inner divertor may both act as sources of oxygen impurities. Here we investigate the transport of oxygen with spectroscopic and fast imaging measurements. Density profiles of oxygen for different charge states in the confinement vessel and inner divertor are presented. In addition, a method for determining the particle confinement time from impurity spectral line ratios is proposed and evaluated.
Spectroscopic Study of Impurity Transport
(1) Formation
Sample spectra – O4+ emission line 278.1nm
* Emission intensities scaled for presentation purposes
Flow from Confinement toward Formation- positive velocity From Formation inwards- negative velocity
Measurement error: ±# $%/'
(2) IDIV
(3) CV
to Outer Divertor
§ High resolution Doppler spectrometer measures velocity and temperature profiles of impurities in the scrape of layer (SOL) plasma at the following regions:
I. Formation; II. Inner Divertor; III. Confinement Vessel
Emission temporal profile
§ Time traces of O4+ radiation show Impurity front, created by the Formation process, passes through the Inner Divertor before reaching the Confinement region.
§ PeakofO5+ inCVoccurslaterintime–atimelagis required for ionization of oxygen into high charge states.
Velocity profile
§ Velocity of O4+ species measured
§ Initial flow of oxygen from Formation towards Confinement
§ Steady flow of oxygen towards outer divertor electrodes later in time
§ Continuous diffusion of oxygen towards Confinement during the entire shot, slowly diminishing with time
Impurity transport meets particle confinement
§ In a closed, steady state system, charge distribution of Oxygen can be calculated for a given electron density and temperature.
§ Measured charge distribution of Oxygen on C-2W is different from that calculated in steady state conditions.
§ Difference is due to loss and recycling of impurities
Impurity confinement time can be estimated from impurity line ratios
Impurity Confinement
Impurity confinement time algorithm
Assumptions:
Source charge distribution, initial t
Measured plasma parameters: ne, Te, e
Derived charge distribution:∑) *+
Yes
t found
adjust t OD collisional-
radiative code
Calculated charge distribution:∑) *+
Example:
§ Pair of shots chosen with similar machine settings and plasma parameters:
=> 5>
*6 =1.2;10 ?@ , B6 =110CD
IDIV Source: O4+/O3+ = 1
§ For each shot, the spectrometer was set to a different oxygen charge state emission line
§ From measured O5+/O4+ density ratio, impurity confinement time derived in the core for O5+: t=0.6ms
Neutrals Source
,-.// ∝ 12 → 4 ∝ 152
Distribution IDIV
Source
Distribution SOL
Source Distribution Core
Match?
No
§ §
Source and loss fluxes are equal
Impurity Transport - Hypothesis
Confinement Inner Divertor
Formation
Hypothesis: Oxygen impurities flow from Formation into Confinement region Initial evidence: Survey spectroscopy, fast cameras
Survey spectroscopy shows Oxygen is the main impurity in C-2W
Visible + UV
Propagation of oxygen from the
Formation region is evident from
radiation profiles of different charge states
§ Top – O1+ radiation. Plasma originates in the Formation tube. Ionization front in the Inner Divertor is evident.
§ Bottom – O4+ radiation. Entire plasma column is radiating.
to Confinement
VUV
(Images not to scale)
Inner Divertor
to Formation
Summary and Future Work
§ Oxygen impurities in C-2W originate in the Formation and possibly inner Divertor regions and diffuse towards the plasma core.
§ Density of impurities is >2 orders of magnitude smaller than electron density. Radiation losses are insignificant.
§ Impurity spectral line ratios can provide measurement of impurity confinement time
§ 1D or 2D simulation will be developed to allow continuous transport model and replace 0D model. Radial profile of confinement time can be obtained
§ Soft x-ray spectroscopy will be used to measure O7+/O6+ for high Te conditions in future experiments
Confinement O5+
Impurity Density Profiles
Confinement O4+
Inner Divertor O4+
§ Oxygen density roughly constant throughout the shot, >2 orders of magnitude smaller than electron density, resulting in insignificant radiation losses
§ Profile of O5+ narrower than that of O4+ in the CV – O5+ dominates in the core, O4+ dominates in the SOL