# Presentation: 147522

## Scrape-Off-Layer Current (SOLC) Study in NSTX

**Hiro Takahashi**** NSTX Forum 2009**
**Boundary Physics Break-out Session**
**Princeton Plasma Physics Laboratory**
**December 8 - 10, 2008**

**Thanks: **
**Eric Fredrickson**
**Stefan Gerhardt**
**Robert Kaita**

**Related Papers:**
**PRL100(2008)205001**
**EPS(2005)4.018**
**NF44(2004)1075-1096**
**NF42(2002)448-485**

- Tak NSTX Forum '09

- Dec. 8-10, 2008

## Scrape-Off-Layer Current (SOLC) Study in NSTX

- Tak NSTX Forum '09

- Dec. 8-10, 2008

## SOLC-Lithium Synergy in ELM Study

**SOLC-ELM Causality Hypothesis.**** **** **
**Sheath-breakdown ELM Trigger Model.**
**Thermo-electric potential impressed across sheath exceeds breakdown limit.**
**Use lithium to alter sheath conditions for systematic study.**
**ELMs in NSTX Suggest Need for New Model?**** **
**Lithium can create ELM-free discharges.**
**Edge gradients not unequivocally more stable under ELM-free conditions.**
**RMPs can generate, not suppress, ELMs.**** **
**Lithium Effect in NSTX in Need of Physical Explanations?**** **
**Lithium raising ion saturation current density or lowering sheath Te ratio****?**
**Lithium raising sheath breakdown limit (perhaps through work function)?**** **
**Sheath Density and Temperature Control by Lithium Aids *****Systematic***** Test of SOLC-ELM Causality Hypothesis.**

- Tak NSTX Forum '09

- Dec. 8-10, 2008

## Sheath-Breakdown ELM Trigger Model

- Tak NSTX Forum '09

- Dec. 8-10, 2008

## Forward-Problem Solving Successful at ELM Peak

- Tak NSTX Forum '09

- Dec. 8-10, 2008

## Technical Approaches to SOLC Study in NSTX

**Utilize New and Existing Diagnostics.**** **
**Magnetic**
**New tri-axial magnetic probe toroidal and poloidal arrays**
**Existing Mirnov coils (only those not shielded)**
**Tile Current**
**New array(s) in top divertor **
**Existing 4-sensor array (Gerhardt) in bottom divertor **
**Langmuir Probe**
**New closely-spaced radial 1D array(s) in top divertor**
**Existing probes around machine**
**New 2D array (UI) in bottom divertor**** **
**Develop Analysis Methods.**** **
***Inverse-*****problem (************-fit optimization) for SOLC distribution**
**Langmuir I-V curve analysis for *****magnetized***** plasma**

**coming run period**

- Tak NSTX Forum '09

- Dec. 8-10, 2008

## Experimental Proposal for Coming Run Period

**Description**** **Measure properties of plasma sheath at divertor tile surface using the existing set of Langmuir probes in ELMing discharges in several magnetic configurations. Knowledge gained will help optimize new and upgraded diagnostics - magnetic, "tile-current," and Langmuir probe - to be used in the proposed study.
**Experimental Plan**** **Establish an ELMing discharge in an LSN configuration with separatrix strike point in the inboard divertor in one case and outboard divertor in another, and in a near-DN configuration with strike point in the outboard divertor. Make Langmuir probe measurements, once in swept-bias-voltage mode and another in ion saturation current mode. Slowly sweep strike point location over a distance comparable to inter-probe separation to obtain a "radial" profile of sheath parameters. Conduct experiment with and without lithium injection.

- Tak NSTX Forum '09

- Dec. 8-10, 2008