# Presentation: 143774

## Measurements of Transient Heat Fluxes During Global MHD Events
S.P. Gerhardt

Complements “Effect of ELMs on SOL widths” Proposed by Joon-Wook Ahn in BP TSG

- S.P. Gerhardt
*Complements “Effect of ELMs on SOL widths” Proposed by Joon-Wook Ahn in BP TSG*

*These Measurements, Along with Those of Halo Currents, are Necessary In Order to Assess the Success of Any Proposed Mitigation Strategy*

## References

- [1] T.C. Hender, et al,  ITER Physics Basis, NF **47**, S128 (2007).
- [2] A. Loarte, et al, IT/P3-34, FEC 2004
- [3] R. Arnoux, et al, EX/7-2 Ra, FEC 2008
- [4] Y.K.M. Peng, et al, FT/P3-14, FEC 2008_x0013_
- [5] G. Federici, Phys. Scr. **T124**, 1 (2006)
- [6] V. Riccardo, et al, Nuclear Fusion **45**, 1427 (2005)
- [7] G. Pautausso, et al, 31st EPS Conference

## Background

- Impulsive heat loads can cause melting/ablation of PFC surfaces.
- Carbon Ablation: 35 MJ/m2s1/2, Tungsten Melting: 40 MJ/m2s1/2  [2]
- From ITER [1]:
- Adiv,dis =Adiv=73.5 m2=25m2 (=7 from multi-machine dataset)
- div=Wth=1-10m msec             ( is a “stretching factor”, ~10)
- Wdiv=Wth=350 MJ                   (=1 is a pre-disruption loss factor)
- Result: Wdiv/Adiv,disdiv1/2=300MJ/m2s1/2
- For ORNL CTF Concept [4]
- Adiv,dis =Adiv=71.4 m2=25m2 (=7 as in ITER)
- div=0.2m msec
- Wdiv=23 MJ
- Result: Wdiv/Adiv,disdiv1/2=170MJ/m2s1/2
- For these disruptions, divertor lifetimes are measured in hundred discharges. [5]
- How to remedy this apparent problem?
- Re-assess the assumptions above (the present XP):
**Is the full thermal energy present at disruption? (refine ST estimates of ********)**
**What is the broadening of the SOL? (refine ST estimates of ********).**
**Is all the power conducted to PFC, or is some radiated?**
**How is the duration of the divertor heat pulse related to the plasma energy loss duration? (refine ST estimates of ********)**
- Apply soft-landing and fast-shutdown techniques.

## MHD-Induced Loading Can Occur in Multiple Stages

- First (large) stored energy loss when RWM grows

- Second (small) stored energy loss when plasma impacts LOBD

- 128858

## Large Pre-Disruption Stored Energy Loss is the Norm in NSTX

* **Similar to observations in JET. [6]*
* **The exceptions to the large-loss trends in JET were from ITB disruptions and VDEs.  *
*Large Fraction of the Stored Thermal Energy Is Typically Lost Before the Actual Disruption*
* **Early loss is maybe slower, but may be larger in magnitude and more focused on the divertor structures. *
* **Might it lead to loading as bad as the TQ?*

## Two More Prameters to Assess: This XP

***SOL Broadening Parameter *********
- Values near 7 are typical

***Time Stretching Parameter*********
- Values of 5-10 are typical

***Both Figures From Ref. [2]***

## XP Designed For (New) Diagnostic Capacity

- Fast IR Camera Viewing Lower Divertor
*New capability*
*Timescale and spatial distribution of divertor heat pulse during thermal quench.*
- USXR chords from VTOP array
*Timescale for thermal energy loss from core plasma.*
*Use of different filters allows T**e**(t) measurement?*
- Divertor Bolometry
*Radiated power from lower divertor, the region of the disruption.*
- Fast Diamagnetism
*Timescale of thermal energy loss*
- Lower Divertor Halo Current Measurements
- BT sensor array and new shunt-tile array add to last years HC XP.

## Experimental Plan, Assuming (Enough) Diagnostics Work

- Part 1: Deliberate Disruptions (25 shots)
- Step 1: Reference Shot: XXXXXX, high- LSN, 4MW, gap-control algorithm, IP=1000kA, BT=0.45 kG, No SPAs, VTOP with 10m filters, try to get centered Ip rampdown.      (2 shots)
- Step 2: At t=0.4, freeze PF3 voltage, then apply pushing voltage to force VDE, make necessary diagnostic adjustments. (3 shots)
- Step 3: Repeat Step 2 with VTOP filters at 100 & 5m (3 shots)
- Step 4: Repeat Steps 2 & 3 at IP=800 kA (5 shots)
- Step 5: Reload Pressure-Limit Disruption 128858, but add FB to stabilize shot, try to get centered IP rampdown.        (3 shots)
- Step 6: Turn off feedback, allow disruption (add n=3 braking if necessary). Do with Vtop filters at 100, 10, & 5m (4 shots)
- Step 7: Add enormous amounts of LFS gas  density limit disruption (4 shots)

- Part 2: Piggyback
- Utilize full diagnostic set on days likely to produce large disruptions.
- (Ex.: RWM & TM XPs, High NI IP XPs)