This experiment aims to determine the locations of important molybdenum erosion on Alcator C-Mod that affect the plasma, understand the erosion mechanisms, and improve high-Z plasma facing component operation. Alcator C-Mod uses molybdenum and tungsten plasma facing components and boronization to deposit protective boron layers. Boronization improves plasma performance by reducing radiation and molybdenum levels in the plasma. However, boron layers erode more quickly in regions that receive high particle and heat fluxes, like the outer divertor.

1. Prepared by
Sameh Hamzawy
2013

2. Keywords
 Alcator C-Mod
 Boronization
 ICRF – 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 frequency
 PFC – plasma facing component

3. 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-Mod
 Alcator 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.
4

5.  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
6
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.

7. 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.

8. 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 ).

9. 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.

11. ANY QUESTIONS ?

12. Alcator C-Mod
12
C-Mod
ITER
http://www.iter.org/mach/heating

13.  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.

14. 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