Transcript Slide 1

Introduction to PlasmaSurface Interactions
Lecture 2
Recycling
Topics covered
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Particle and energy reflection
Energy distribution of backscattered ions
Ion range distributions in solids
Hydrogen trapping in solids
Isotopic exchange
Outgassing after a discharge
Particle confinement and replacement times
• Frequently there is little need for refuelling. This leads to the
conclusion that the ions recycle ie when they reach the limiter,
divertor or wall, become neutralized and re-enter the plasma as
neutrals.
• The neutrals recycle many times in the course of a single
discharge. In most tokamaks the pulse length is at least an
order of magnitude greater than the particle replacement time.
• This is confirmed by the observation of bright hydrogen
radiation from hydrogen neutral atoms, Ha, Hb, Hg etc.
• The flux of hydrogen entering the plasma can be quite reliably
estimated by measuring the absolute amount of Ha light. We
will discuss this method in more detail when considering
impurities.
Confinement and Replacement times
• Global particle replacement time is defined as the ratio of the
total particle content within the last closed flux surface to the
total particle influx 
nV
r 

• Neutrals are ionized near the boundary and so have short time
in the plasma
• This global particle replacement time should not be confused
with central particle confinement time which is the average
time taken for an ion to be lost from the centre of the plasma
a2
p 
D
where a is the minor radius of the plasma
Surface Processes
When an ion or neutral arrives at a surface it undergoes a
series of elastic and inelastic collisions with the atoms of the
solid. As a result of these collisions it may be come buried in
the solid or it may be backscattered into the plasma
• Four possibilities
• 1.
Backscattering into the plasma
• 2.
Slow down in the solid and be trapped
• 3.
After slowing down it may thermally diffuse
back to the surface and be released.
• 4.
Collision with adsorbed or trapped atoms
leading to gas release
Recycling coefficient
• The ratio of the total flux returning to the plasma to the
incident flux is known as the recycling coefficient.
• This can actually be greater than unity because of the fourth
process.
• A general point is that an ion arriving at a surface will
normally pick up an electron from the surface and if it does
return to the plasma it returns as a neutral.
Ion Backscattering/Reflection
• This is normally the result of multiple
collisions
• It can be calculated by Monte Carlo codes
such as TRIM. Results from a series of
calculations are shown in the slides below
Particle and energy reflection coefficients
Reflection coefficients of ions backscattered from solid surfaces as a function
of reduced energy, for 3 different ratios of the target to incident mass.
(Thomas, E.W., Janev, R.K. and Smith, J., Nuclear Instruments and Methods
in Physics Research B69, 427 (1992).)
Energy distribution of backscattered H atoms
Measured energy distribution of hydrogen atoms backscattered from carbon, for
different incident ion energies, E0. The distributions are normalized at their
maximum intensity. (Aratari, R. and Eckstein, W., J Nuclear Materials 910, 162–4
(1989).).
Ion trapping in the solid
• Ion ranges depend on mass and energy
• The range and range distribution can be
calculated using Monte Carlo codes
• Some atoms will be trapped in vacancies
and interstitial sites
• Hydrogen diffuses readily in many metals
so that even when implanted it can get
back to the surface
Surface adsorption and desorption
• When an atom reaches the surface it is
energetically necessary to form a molecule in
order to desorb
• This leads to high concentrations in the solid
• The amount of gas in the walls is often many
times the amount in the plasma
• The wall thus acts as a large reservoir
• The gas is observed to come out for a long time
after the discharge
Ion range distributions
Range of D+ ions in carbon
Note the scale in angstroms
The range is short
There is a broad distribution
The range increase with energy
Ranges in other materials will
decrease with increasing mass
due to increased scattering.
They can be calculated using
Monte Carlo codes
S A Cohen and G McCracken
1979
Hydrogen transport in the solid
There are 3 clear cases after the ion slows down
1. The ion can diffuse readily and be release from the
surface, eg Ni, Mo, W, stainless steel
2. The ion can diffuse readily but cannot be released from
the surface for energetic reasons, eg Ti, Zr Nb
3. The atom cannot diffuse eg carbon, oxides
These different behaviours of course depend on
temperature. As T increases the diffusion rate tends to
increase.
The behaviour of the hydrogen in the solid is important in
understanding the recycling so I want to consider these 3
cases in some detail
Case 1. Diffusion in the solid (eg Ni)
Case 2. Diffusion,
but no release (Ti, Zr etc)
There is a potential barrier
at the surface due to the
chemical binding between
the H and the metal.
The H concentration
builds up in the solid but is
not released. Eventually
concentration of H in the
metal H/M 1

As temperature increases
the hydrogen will be
release and it is necessary
to look at the vapour
pressure curves.
Case 3. No diffusion eg C
Normallyincident D+ on C
Cohen fig3
In this case saturation occurs at the
most probable range. The H migrates
inwards and outwards due to ion
collisions until it gradually spreads to
the surface.
When the distribution reaches the
surface the H is released
It is seen that this happens gradually.
At the high surface concentrations the
H ids probably released bv collision
with incident ions
S A Cohen and G McCracken 1979
Trapping of D in carbon: Comparison of the
amount of D trapped versus the incident fluence
The amount
trapped
increases with
ion energy
because the
ions are
implanted more
deeply
Cohen McCracken
Journal of Nuclear
Materials, 84,
(1979) 157-166.
Effect on Recycling
It is clear that in all three cases the wall acts as
a large reservoir for hydrogen. The amount in
the wall is frequently an order of magnitude
larger than that in the plasma.
Whether it comes out depends on the incident
ion energy, the material, the temperature and the
fluence of incident implanted ions.
After a discharge the H is released at a rate
which depends on the diffusion coefficient and
the surface adsorption. The concentration near
the surface decrease relative to the bulk
Depletion of surface concentration
Isotope exchange
• Because of the inventory in the solid it is difficult
to change from one isotope to another.
• The amount recycled from the wall is
proportional to the amount in the wall
• Until the wall concentration of the new isotope is
built up it is not possible to get the plasma
concentration up. This is illustrated in the next
slide
Deuterium concentration after
change to D2 gas filling
A series of discharges with
D filling after many in H for
two D filling rates
,
Fabry-Perot
intensity of Dand H
Fluxes trapped in
C probe and measured after
the discharge
#
Gas release from
the wall after the discharge
McCracken, Fielding et al.
NF18 (1978) 35
Out gassing after discharge
(a) After a series of discharges in H, (b) after first seven discharges
In D (c) after the 1st discharge in Hafter b (d) after 3 discharges in H after b
DITE tokamak (McCracken, Int. Symp on PWI Julich 1976)
Outgassing after a discharge
Out-gassing from the walls after a discharge is a
difficult to study because it depends on so many
different parameters (material, fluence, temperature).
A simple difffusion model has the solution
J=Jo erf {R/(4Dt)1/2}
Empirically the rate R seems to follow a power law with
time of the form
J = A t-x
Where x varies between 0.5 and 1.0. X=0.5 is the
Particle inventories
This partially qualitative description of
hydrogen behaviour in solids is helpful in
understanding particle inventory.
It is necessary to measure:
1. What goes into the vessel
2. What comes out after the discharge
3. What is in the solid. This is the most
difficult part
4. All species separately
Where does recycling occur?
• The recycling flux density is obviously highest at
the limiters and divertors where the incident flux
is high
• Recycling also occurs at the walls due to charge
exchange neutrals.
• As the CX and ionization x-sections are roughly
equal in the range of interest the integrated
neutral recycling at the wall is comparable to the
recycling at the limiters and divertors
Modelling recycling
• Because of the inherent 3-D geometry involved
with following neutral particles it is necessary to
use Monte Carlo codes.
• Many different codes have been developed in
different labs e.g. DEGAS (PPPL), EIRENE
(Julich), NIMBUS (JET)
• To do this properly it is necessary to have a fluid
code to describe the plasma and determine the
local values of Te and n e at all positions in the
system
Recycling, Summary - 1
• Recycling occurs at surfaces where there are incident
ions or neutrals from the plasma
• It is the main contribution to the fuelling of the plasma.
The fuel recycles many times in a single discharge
• The recycled species are almost entirely neutral. Some
are backscattered as energetic atoms and some as
thermal neutral molecules (i.e. with the wall temperature)
• At typical SOL temperatures about 50% of the ions are
backscattered with an energy 5Te and a fairly broad
energy distribution
• The other 50% slow down in the solid and diffuse back to
the surface where they recombine and are released as
molecules( 0.03 eV).
Recycling, Summary - 2
• The particle inventory in the wall is very
large compared with that in the plasma
• This makes changing isotopes slow as the
wall population has to be changed
• It also makes the tritium inventory much
larger than just the plasma inventory
• Release from the surface can be diffusion
limited or recombination limited