ASIPP
EAST tokamak employs lithium evaporation onto PFCs to achieve wall conditioning of discharges similar to the wall conditioning used on NSTX and expected for NSTX-U. The NSTX experimental database indicates that lithium coatings change discharge characteristics after 3-4 discharges after application of lithium in quantities of
150mg over ~10minutes immediately prior to the discharge. However, large evaporations during the NSTX 2010 campaign indicated longer lifetimes (~10s of discharges).
Further, NSTX discharges last for about 1s whereas NSTX-U will be capable of 5s discharges at full toroidal field and current and longer at reduced parameters. The EAST device is capable of exploring the evolution of lithium in the divertor under long-pulse conditions to inform NSTX-U operational planning.
The aim of this experiment is to study the evolution of divertor conditions during longpulse experiments on the EAST tokamak. Specifically, Langmuir Probe signals related to filter-scope signals will be used to monitor local conditions over time over a range of input powers and input currents. Any degradation of discharge performance will be monitored to determine if a strong correlation exists in EAST relating the divertor conditions and changes (if any) in the lithium-related divertor signals.
NSTX operation with evaporated coatings has been shown to improve discharge performance. In results published in 2009 (H.W. Kugel, et al., JNM, 2009) the discharge flat-top was found to increase with lithium wall conditioning. After cessation of wall conditioning, current flat-top was found to decrease until the resumption of inter-shot wall conditioning. In addition to this, on the final days of the FY2010 run campaign,
LITER was operated such that 7gm were deposited in the morning before operation and after the so-called “mega-evaporation” and also depositing particle flux on the LLD.
These discharges consistently ran about 1s in length throughout the day indicating the
LLD may have provided a reservoir effect of lithium, possibly related to the molybdenum substrate or the sheer quantity of lithium deposited. Contrary to the idea of “more Li is always better”, it was found that excessive evaporation could have detrimental effects on
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One model for recycling from a lithium coating indicates an ion fluence of ~26x the deposited amount of lithium is required to completely saturate the coating (depending on initial recycling rate). This will be tested in the EAST tokamak. Signs of saturation would be indicated by an increase in D-alpha emission, reductions in target temperature and increases in target density. On the other hand, gross erosion can be large in the vicinity of the strike-point and may reduce the amount of lithium present. Erosion of the film will be indicated by increasing carbon emission and decreasing lithium emission as the shot progresses and films eroded.
Using EAST attached divertor conditions as a guide (HY Guo, JNM 2011), peak outer target ion flux is about 1.25e23 #/m2/s. A 100mg/m2 coating results in an approximate thickness of 200nm, or 8.8e21 #/m2. If 26x fluence is required, then saturation should result for a fluence of 2.3e23 or 2s of discharge time. If, on the other hand, re-erosion dynamically refreshes the lithium surface, then evolution may occur at a much slower pace. Changes in impurity emission ratios will signal evolution of the surface. Li deposition profiles in EAST will be helpful in designing the experiment.
The NSTX-U baseline operational plan is to utilize ATJ graphite PFCs on the lower divertor. We wish to assess the optimal deposition amounts and coating lifetimes associated lithium coatings on the EAST graphite divertor target. The vertical target of
EAST may strongly alter the redeposition of eroded material and hence the coating lifetime. The EAST dataset, in addition to data from Magnum-PSI, will provide background data for NSTX-U experiments on lithium-coating lifetimes.
In all cases, full diagnostic capabilities as indicated in recent paper by H.Y. Guo (H.Y.
Guo, et al., JNM, 415 (2011) S369.) are desirable. Ideally, evaporations will be performed before each discharge as done on NSTX. Barring this, then morning evaporations will be used.
Assuming morning evaporation only is available: experiments will consist of morning evaporation of a known quantity of lithium onto the EAST PFCs.
A.
If possible – non-lithiated discharges will be taken to establish base-line machine operation.
B.
H-mode discharges will be performed for as long as sustainable. During each discharge, LP Jsat and relative spectroscopic quantities will be monitored for signs of evolution throughout the discharge. Jsat will be used to calculate net fluence.
C.
Discharges will be repeated until signs of steady-state behavior are achieved.
NOTE: the amount of lithium deposited in the morning could be reduced to make the experiment progress more quickly. In an ideal situation, the morning deposition and typical machine running parameters will be created so that evolution over several discharges will occur. This will allow cleaner comparison of overall discharge performance while PFCs evolve. Confinement time, discharge temperature and density profiles and impurity content will be monitored to assess discharge performance.
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D.
Evolution of impurity production will be compared with gross-erosion estimates and net-erosion of the coatings will be estimated from any long-term evolution.
E. If available, the fast reciprocating probe is highly desirable for this experiment.
If divertor ionization changes from net pumping to fully recycling, the pressure balance between target and upstream locations will change. This can result in flow reversal on certain flux-tubes that the upstream mach probe could diagnose.
F.
If available, the Material and Plasma Evaluation System (MAPES) diagnostic is highly desirable for this experiment. Material migration during a discharge is thought to play a role in NSTX performance with lithium wall conditioning and this could enable quantification of the amount of migration occurring during discharges in EAST. On the other hand, net erosion from graphite walls can coat divertor surfaces covering up deposited lithium layers. The state of the wall source or sink of lithium would help identify the processes controlling lithium-coating lifetime in the EAST tokamak.
Plasma current
Toroidal field
Shape
Beams
Cryopumps
Essential diagnostics:
ECH:
Fast wave rf:
C-coil:
0.6–1.2 MA
2 T, normal field
Double Null or Lower Single Null
Whatever required for long-pulse operation
None.
Langmuir probes
Divertor spectroscopy
Core and tangential Thomson
Magnetics
Fast reciprocating probe (if available)
MAPES (if available)
Whatever required for long-pulse operation
Whatever required for long-pulse operation
Whatever required for long-pulse operation
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1.
erform lithium evaporation into the machine using ½ the maximum available
P lithium charge (e.g. if 15gm/evaporator is the maximum, use 7.5gm/evaporator this day). Obtain RGA spectrum during evaporation as well as total pressure in the vessel. Expose MAPES during evaporation, retract and evaluate coating composition with LIBS or other methods. (0 shots, may be performed any day)
2.
roduce long-pulse discharge immediately following lithium evaporation. Monitor
P probe and spectroscopy for indications of evolving PFC surfaces and core performance. Monitor mach signals from FRP (if available) throughout discharge.
During single discharge, expose fresh MAPES sample to plasma (but not to evaporation), retract and evaluate coating composition with LIBS or other methods. Continue discharges until equilibrium divertor and core conditions obtained. (1-6 shots, depends on shot length and experimental conditions)
3.
n a second day, depending on results of the first day, increase or decrease the
O evaporated lithium quantity by 50% ensuring saturation of the PFCs before the end of the run-day. This accomplishes a second data point in a lithium quantity scan. Repeat long-pulse discharges as in step 2 until saturation reached.
(1-6 shots, depending on results of first day of experiments and conditions)
4.
robe net fluence will be examined against evolution of spectroscopic signals to
P test and refine the saturation model previously developed on NSTX.
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