INFLUENCE OF BORONIZATION ON OPERATION WITH

HIGH-Z PLASMA FACING COMPONENTS IN ALCATOR C-MOD

Prepared by Sameh Hamzawy

2013

KeywordsAlcator C-ModBoronizationICRF – ion cyclotron resonance

frequency, its primary role power on ITER and future fusion reactors is to provide bulk plasma heating where high Z metallic plasma facing components (PFCs) are being envisioned .

RF – radio frequencyPFC – plasma facing component

What is the aim from this experiment ?

This experiment aims to determine where are the most important Mo erosion locations affecting the plasma, their size, and the erosion mechanism.

In addition to understanding what leads to the rapid loss of boronization layers and what can be done to improve solely high-Z PFC operation.

4

Introduction to Alcator C-ModAlcator C-Mod :

The name is an acronym of the Italian “Alto Campo Toro”, which means "high-field torus"

Alcator C-Mod Alcator C-Mod tokamak

is a fusion experiment that uses only high-Z refractory metal, molybdenum (Mo) and tungsten (W), as plasma facing components (PFCs) . Although. It is the world’s highest magnetic field tokamak plasma confinement experiment.

What is the meaning of the Boronization ?

It is a thermo-chemical diffusion procedure in which hard and wear resistant boron layers are obtained by diffusing boron into the material’s surface. 

Why we use Mo and W as plasma facing components (PFCs)?

Due to their low tritium (T) solubility.Capability to handle high heat fluxes with low

erosion, Carry away heat efficiently Have high melting point Robustness to nuclear damage and activation

6

Alcator C-Mod Fig.Alcator C-Mod vessel.

(A) refer to the upper gusset protection tiles. (B) refer to the outer limiter ,and (C) refer to the top of the outer divertor. The vertical line corresponds to R = 70 cm. The poloidal location of tiles removed for surface analysis prior to the 2005 are darkened.

Plasma performance with and without boronized PFC surfaces

Fig. 3. A set of characteristic traces from pre- (- - -) and post-boronization (—) discharges

Without boron-coated PFCs:

Rapid increase in the density and radiation. An equilibrium was then reached with marginally improved particle confinement, poor energy confinement due to high core molybdenum and radiation.

With boron-coated PFCs: We observe lower

radiated power and core Mo density accompanying much better energy confinement.

Localized erosion of B coatings

The boron layers have not been eroded from the majority of PFC surfaces and typically contain 1% Mo and lesser amounts of other impurities (O, C, Ar) in the near surface (<0.1 lm).

But The outer divertor surfaces showed the highest Mo surface concentrations(10–50%), which is presumably due to net erosion of the boron layers by the high particle and heat fluxes to those surfaces. These regions of ‘plasma-cleaned’ surfaces extend from the bottom of the outer divertor vertical section to R 0.67 m (see the Fig ).

Effect of boronization on plasma startup

Boronization of PFC surfaces has important effects on a number of plasma characteristics during the C-mod discharge startup phase

Prior to the first boronization: when PFC surfaces are un-coated and the RF power increased, the radiated power and Zeff (effective state of the impact ion )both rose over time..

after the first (and second) boronization: both the radiated power and Zeff

drop and stay low.

Fig. 8. Discharge sequence showing the variation of the radiated power (a) and Zeff (b) during the 2005 run campaign. Both pre and post-boronization periods are shown.

ANY QUESTIONS ?

12

Alcator C-ModC -M o d

IT E R

http://www.iter.org/mach/heating

Fig. 2. (a) Poloidal map of analyzed Mo tiles, (b) effective boron layer thickness on Mo tiles shown as the depth from surface of Mo. Multiple data points at a single poloidal location correspond to multiple measurement locations on the same tile.

General experiment description

Fig. 2. (a) Poloidal map of analyzed Mo tiles; (b) effective boron layer thickness on Mo tiles

shown as the depth from surface at which the B:Mo ratio is 1:1 as determined by proton (2500 keV) Rutherford backscattering spectroscopy. Multiple data points at a single poloidal location correspond to multiple measurement locations on the same tile.

The overnight boronization period is 12 h resulting in a coating thickness 150–200 nm assuming uniform deposition over a 10 m2 area. The thickness of the overnight boronization layer appears to be fairly uniform poloidally as well based on the tile surface analysis performed on a set of tiles removed from the tokamak after 7 years of operation, Fig. 2.

The layer thickness is typically 5 lm at most poloidal locations except at regions of the outer divertor that receive large power and particle fluxes. Thinner layers are found at the inner divertor because those tiles were installed for a shorter period