J. Plasma Fusion Res. SERIES, Vol.4 (2001) 153-160

ls Dust Formation a Goncern for Fusion Devices?
WINTER Jiirg*
Institute of Experimental Physics II, Ruhr-University, D-44780 Bochum, Gerrnany(Received: 5 December 2000 / Accepted: 3 October 2001)

Abstract
Plasma-surface-interactions are responsible for the formation of small particles (dust) in fusion
devices. This is particularly pronounced in devices with carbon wall materials. Though no major problem
in present fusion devices, the large dust quantities generated in future machines are expected to be

critical. The chemical reactivity and the retention of Tritium in carbonaceous dust makes it a safety
hazard. Radioactivity leads to electrical charging ofdust and to its interactions with plasmas and electricfields. This may have adverse effects for machine operations but can be used to advantage for particle
removal. The paper will discuss mechanism for dust formation, dust characterization and address its
expected behavior in fusion devices. New options for particle removal based on recent developments industy plasma research will be presented.

Keywords:
dust, dust removal, plasma-surface-interaction, erosion, redeposition, radioactivity

1. Introduction

are present in the start-up phase of the plasma they leadto increased values of Zat l2l with the disadvantage ofhigh plasma resistivity and inherent instability. This canbe compensated by an increased rate of current rise or
by injection of additional heating power during start-up.
The ionized impurities are lost within a few particle
confinement times and stick to the wall.
Particles with dimensions in the range between a
few nm and a few l/10 mm (dust) are found on thebottom of many fusion devices. The particles collectedfrom fusion devices with carbon wall components,
TFTR, DIII-D, Alcator C-mod and from the Sirens

When materials are exposed to high plasma loads,
the emission of particles is often observed. In magnetic

confinement fusion devices this leads to the so called

UFO's, glowing objects moving through the plasma,heated up by the absorbed power. They can be readily[].

In some cases the trajectories
are influenced by an interaction with the magnetic fieldobserved by cameras

indicating that the objects are charged. Thus theobservation of particle-plasma-interaction

is notsurprising to the physicist working at a fusion machine.
The major concern has been so far the triggering of

disruptions, i.e. the uncontrolled loss of the stored
plasma energy when solid particles get into the fusion

simulator [3], have a size distribution centering around I

pm. In the case of the DIII-D tokamak, the massconcentration was found to be in the range from 0.1-l

plasma. Cooling of the plasma by dissociation andionization losses and the excessive radiation from theionized impurity atoms are driving mechanisms for[LEcm-2 on vertical surfaces, 10-100 ltgcm-2 on the floor

and lower horizontal surfaces [4]. The total amount isestimated to be between 30_l2O grams [4] for several

disruptions. Narihara and coworkers [2] have shown thatabout 106 particles with a diameter <2 ltmfalling into ahundred discharges, corresponding to an integrated

well developed discharge do not influence the plasma

plasma exposure time of a few hours. This amount issignificantly. The discharge is rather robust. If particles
aCorresponding author's@20O1 by The Japan Society of Plasmae-mail: jw@ plas.ep2.ruhr-uni-bochum.de
153

Science and Nuclear Fusion ResearchWinter J., Is Dust Formation a Concern for Fusion Devices?

still small and not of major concern. Particle formation

away from the confined plasma suffers a less intense

and interaction with the plasma does not appear to be a
critical problem in present devices.

heat and particle exposure and is deposition dominated.

In the framework of the ITER project it has become

erosion and deposition dominated areas are often withina few mm of each other [2].
In the TEXTOR -94 tokamak for example, theaverage H (D) flux at the leading edge of the poloidal
graphite limiter is 1023 m-2 s-r. The averaged erosion
yield (including both sputtering and chemical erosion) is
2 x l0-2 resulting in a primary loss rate of 2 x 1021 m-2s
C atoms. This corresponds to a gross erosion of lm per

Depending on the gradients of the plasma fluxes,

clear however, that additional aspects of dust formation
are important for future machines [5]. They run longpulses and will have large plasma fluences. As will be

discussed below in detail, some dust production
mechanisms are proportional to the plasma fluence andare thus giving rise to dust inventories very much larger
than those we find in present machines. Incorporation ofhydrogen isotopes into dust will lead to large dustbound Tritium inventories [6,7]. This is a major safetyconcern. Reducing the dust bound Tritium inventory inyear at the limiter tip by regular plasma exposure. A
large fraction of the eroded material is redeposited

locally reducing gross erosion significantly (typically bya factor of 10) [13]. Some part of the eroded material
however is gradually transported in multiple erosiondeposition processes from the erosion dominated to thedeposition dominated areas [13]. Figure I shows aphotograph of a poloidal limiter segment with the shiny
erosion dominated top part and flaking redeposited
carbonaceous material located on the curved sides of thelimiter, just a few mm radially outward from the top.
Deposition of previously eroded carbon occurs inpresence of fluxes of hydrogen isotopes. Hydrogen isincorporated (co-deposited) into the growing carbon
layer. The properties of the redeposited layers varystrongly with the composition of the plasmas fluxes (FV
C ratio), the ion energy at impact and the substratetemperature. Bombardment by energetic ions during
deposition and high substrate temperatures tend to lower

future machines has thus become a key issue.

This paper addresses some important open
questions concerning the mechanisms, by which dust isformed, the characterization of the dust properties andthe understanding of the behavior of the particles in themachine. Consequences arising from the radioactivity ofdust, in particular electric charging and dust levitation in

weak plasmas [8,9] will be discussed. Experiments haveshown that plasma-levitated particles may be readily
removed by appropriately shaped electric fields [0,11].
This may offer a novel solution for the removal of dust
from fusion devices.

2. Transport of Wall Material
Graphite or carbon fiber composites are preferredmaterials for high heatflux components because of their
good thermomechanical properties and their low atomic
number Z. They are also considered for ITER. Carbon
however suffers from large erosion rates due to a highphysical sputtering yield and additional chemical
erosion. Hydrocarbons are formed as a consequence of
atomic and ionic hydrogen exposure from the fusion
plasma and they are released from the surfaces into theplasma edge where they are dissociated and ionized andtransported. As will be discussed in detail in a later
section, the deep scrape - off layers (SOL) of a fusion
plasma, i.e. the plasma regions close to the wall, may
resemble closely hydrocarbon process plasmas.

A magnetic confinement fusion device is a closed
system for condensable materials. Only volatile gases
leave through the vacuum pumps at the end of a plasma
pulse. Material eroded somewhere by plasma-surfaceinteractions is deposited at another place in the machine.
The plasma strike zone on limiters and divertors whichhandle most of the heat- and particle outflow are usually

Fig. 1 Segment of the poloidal graphite limiter of

TEXTOR-94 with the shiny erosion dominatedzone (top) and redeposited flaking layers on thesurface curved away from the plasma strike zone
lcourtesy J. v. Seggern [20], TEXTOR-94 teaml.

dominated by erosion. The wall area which is further
154

Winter J., Is Dust Formation a Concem for Fusion Devices?

the hydrogen isotope concentrations. If the depositioncolumnar growth structure and a porous, blister-likesurface texture. The thicker ones show cracks acrosstheir surface. Failed redeposited layers are the largest
fraction in the dust collected from TEXTOR [8]. An
example is shown in figure 2. A fraction of about l5Voof the flakes is ferromagnetic due to incorporated metal
atoms [8]. Although the material for the metallic
structures inside TEXTOR is amagnetic stainless steel
and inconel, the incorporation of sputtered metallic
atoms and the loss of the phase structure leads toferromagnetic carbonaceous dust particles. Spheres withdiameters between 0.01 and 0.1 mm and with a large
iron concentration are also found [22]. Some of them
exhibit a texture, indicating different crystallographic
phases. It is very likely that these particles were

occurs on cold surfaces via neutral radicals withinsignificant energy the concentration may be as high as
2 H/C atoms. Modeling with the ERO-TEXTOR MC code [13,14] of the erosion-redeposition processes atlimiters in TEXTOR has shown, that importantparameters for the amount and location of redeposition

are the surface loss probabilities of C atoms andhydrocarbon ions or radicals. They can vary over several

orders of magnitude for different species and flux
scenarios [15]. Whereas ionic species dominate at thelimiter or in the shallow SOL, the fraction of neutrals
increases strongly in the deep scrape off-layer.

Deposition from neutral radicals is becoming
increasingly important here. Neutrals are not influenced
by magnetic fields. Deposition may occur at positions,which are not reached by the magnetically guided ioniccompletely molten and that the phase separation

redeposition have been demonstrated in the magnetizedtoroidal plasma experiment TOMAS in which carbon

occurred during resolidification. They may be due tosplashed metal which was melted by excessive power
loading during exposure to fast electrons or unipolararcing. They may also be due to agglomerated metal
atoms on hot carbon limiters. Metal atoms are firstreleased by sputtering or evaporation, then transported

film deposition from ECR plasmas in methane andthrough the plasma and finally deposited on the carbon

acethylene was investigated [6]. Deposition fromneutral radicals is also believed to be responsible forthick deposited layers on the cooled louvres of the JET
divertor, quite remote from the plasma interaction zone

limiters. Carbon is usually not wetted by metals. If the
carbon limiter surface becomes sufficiently hot, thefluxes. Different sticking for reactive radicals, theimportance of neutrals for the layer deposition and thetransport of material due to repeated erosion andsurface mobility of metal atoms becomes large enoughto allow surface agglomeration [23]. The agglomerated
spheres are finally relased by plasma-surface-

tr7l.
Dust formation from carbon wall materials isparticularly pronounced because of its large erosion
rates. The estimates for the ITER dust inventory areseveral kg up to several 10 kg, depending on theoperation scenario. The amount of Tritium trapped ininteractions.

The ejection of whole graphite grains withdiameters of a few Arm may occur, when the material isweakened by repeated heating (thermal fatigue). Cracks

the dust appears to be prohibitive. Dust formation rates
and the specific tritium retention is expected to be muchsmaller when high heat flux components from the high-Z element tungsten are used. Whether the very low
plasma impurity concentrations of high-Z material
required for ignition of a fusion plasma can be realized
is still unclear, however.

3. Dust Properties and Dust Formation
Mechanisms
Redeposited carbonaceous layers have large
stresses and a very poor mechanical strength, in general.

They are usually brittle, delaminate from the substrateand fall off the surface as flakes [18-20]. Even if thelayers do not fail immediately, this often occurs after alonger time, in particular after venting of the machinefor maintenance or repair [21]. Many flakes have aO,rn rl

Fig. 2 Dust particle collected from TEXTOR-94
155

Winter J.. Is Dust Formation a Concern for Fusion Devices?

develop and propagate due to the thermally induced
stresses.

For very high power fluxes from giant Edge
Localized Modes (ELMs) or disruptions the ejection ofdebris and in particular evaporation or sublimation are
dominant 1241. The vapor has a high density close to thesurface and the vapor particles have a short mean freepath. Coagulation processes from supersaturated vapor
leads to the formation of small particles with average
diameters of several l0 to 100 nm. This mechanism wasverified in a laboratory experiment in which different
carbon materials were repeatedly exposed to powerloads of 0.5 - 1.5 GW m-2 using a high power electron
gun 1241. Transmission electron microscopy of thereleased material shows small globular clusters, see
figure 3, and also evidence for the formation of fullerene
like materials. A similar type of particles was identified

in the TEXTOR dust (figure 4) consisting of

30 nm

agglomerates of individual globular particles of about

100 nm diameter. They may have been formed bycoagulation from C vapor during arcing, ELMs or
disruptions. In Tore Supra fullerenes and nanotubes
were identified in the collected dust [25]. In Extrap

Fig.3 Globular particle from agglomerationofsupersaturated C-vapor [24].

rounded particles are found which were partially ablated

while moving through the plasma [26].
Another mechanism for the formation of the verysmall particles in fusion devices with carbon wallmaterials is their growth in the cold plasma regions
close to the wall or in a detached divertor. These
plasmas have electron temperatures as low as 1-2 eV
and a high concentration of neutrals with a large fractionof hydrocarbons from chemical erosion. The total gas
pressures go up to a few Pa. The measured local
hydrocarbon concentrations can be in excess of ll%o[27]. These conditions are close to those of a process
plasma where dust formation has been observed bymultiple ion-molecule reactions and agglomeration
processes t28,291.

Figure 5 shows the mass spectra of positive ions,
neutrals and negative ions which are formed in the bulkof a glow discharge process plasma in methane which isoperated at pressures of about 1 Pa. Details of the2(F rrnexperiment are reported elsewhere [30]. The equilibrium
gas composition is about equal parts of H2 and CI{n. The
mass resolution was set to 12 atomic mass units suchthat going from one maximum to the next corresponds
to the addition of another carbon atom to the previous
molecule. It is evident from figure 5 that very large
species are growing in the plasma. The mechanism is
via repetitive reactions between an hydrocarbon ion and

Fig. 4 Agglomerate of globular nanoparticles from the

TEXTOR- dust.

a neutral hydrocarbon molecule. The formation of large

negative ions is particularly pronounced. Negative
particles are well confined in the potential well of theglow discharge and thus have a large residence time.
156

Winter J., Is Dust Formation a Concern for Fusion Devices?

4. Radioactive Dust Particles and Dust107

p=0'57 Pa

RemovalFor the following considerations we assume acarbonaceous particle of 1 pm thickness with a diameter

I=4O Hz10"

I=100 mA
F=42.5 sccm

Iof 5 pm with a total concentration of hydrogen isotopesof 0.4 per C atom of which 50Vo is Tritium. The meandensity of such type of redeposited carbon film isassumed to be 1.5 gcm-3, a value found for redeposited
a-C:H layers in the deep scrape off-layer of TEXTOR[34]. The total number of T decays in the carbonaceous
particle is then about 5 x 102 s-r. If all electrons leave
the particle into the vacuum, the steady state positive
charge on the particle will depend on its effective
neutralization rate which is unknown a priori. Sincehydrogen saturated carbon films are often good
insulators, a significant charge may accumulate. 500
elementary charges suffice to levitate our particle in anelectric field E = 38 Vcm-r. Electrostatic repulsion ofparticles due to radioactive charging and jump-up
against gravity has been observed during the Tritium
clean down of TFTR, for instance [35].
Another effect associated with the radioactivity ofparticles is the formation of a "nuclear induced plasma"in which levitation of particles has been observed [8-7l rnox

anions

a

o tv

a'

cloH*-

c2oHr-

l03

150 200 250
Mass/amu

Fig. 5 Mass spectra with low resolution of positive andnegative plasma ions and of neutrals from a DC
glow discharge in methane.

The growth process is so rapid, that even the limited
residence time of the positive ions is long enough toallow breeding of cations with mass 150 in thisexperiment. Detailed analysis of the negative ion spectra
reveals the growth of large linear chains (up to about n= l0 C atoms) in the initial stage, then the collapse to

ring structures (for l0 < n< 20) and the three
dimensional collapse and isomerization towards

101. A nuclear induced plasma is formed by products ofnuclear reactions traveling through a gas and producing
electron-ion pairs in their tracks as well as excited atoms
and molecules. Dust particles placed into a nuclear
induced plasma, are affected by flows of electrons andions. Due to the different electron and ion velocities thefullerenes for n > 20 t3I), in full agreement withmolecular stability calculations [32,331. Big ions can be
identified already 50 ms after the start ofthe discharge.

As has been discussed elsewhere [18], once
negative ions are formed by dissociative electron
attachment, they are well confined in the edge of afusion plasma. Since Cr, Cz, ..Cn clusters and several
C,It radicals exhibit a high electronegativity (CH2 =dust particle has a negative average charge inequilibrium. Since several mechanisms are responsible
for the charging of a radioactive particle, the sign andquantity of the charge may vary. Nuclear induced
plasma were produced near atmospheric pressures byalpha-particles and fission fragments from a thin layerof 2s2cf and by neutron activated radioactive CeO2
particles undergoing B-decay [0]. Plasmas were
induced in neon and argon at subatmospheric pressure in
the range from 0.25 x 105 Pa up to 105 Pa between twoparallel metal electrodes in a cylindrical glass chamber.
Melamine- formaldehyde (MF) particles and cerium
oxide (CeO2) particles were introduced into the plasmas.
Conical structures of levitated CeO2- and of MF-

3.39 eV, C2H = 2.94 eV) these species are rather stableagainst electron detachment. Direct experimental
evidence for such dust growth processes within the edge
plasmas of fusion devices is not available yet. Becauseof their potential importance, they deserve increased
attention, however.
Dust formation by flaking of redeposited layers andthe growth of particles in the plasma edge are bothproportional to the plasma fluence. They are thus ofimportance for future long pulse devices. Dust formation
mechanisms based on evaporation or particle ejection
due to thermal overloading and off- normal operation
can in principle be avoided by good engineering andoperation scenarios with wide safety margins.

particles near a hole in the upper electrode wereobserved when this electrode had a positive polarity.
Figure 6 presents a video image of the conical structure.
Charges of the levitating dust grains were estimatedfrom a balance ofthe gravitational force and the electricforce to be in the range from 200 e up to 400 e. In thet57

Winter J., Is Dust Formation a Concern for Fusion Devices?

Iotzdonchrnherwdl- ror.?t..'

if..i3 i:.t
a-prddcrrnd

ftdofr4neutr

Fig. 6 Conical structure of a plasma suspended particle

cloud.
Fig. 7 Schematics of the electrode arrangement forparticle removal from a dusty plasma.

case of the reactor-activated CeO2 particles, broad
regions with levitating dust in the central part of theinterelectrode space were observed when the electricfield was less than 30 V/cm. These experiments on
dusty plasmas in a radioactive environment demonstratethat radioactive particles may be levitated in weakplasmas and that it is possible to control their position

by means of electrodes.
To remove charged dust grains from the volume ofthe experimental chamber an electrostatic probe wasdeveloped, the so-called "plasma vacuum cleaner"(fig.7) t101. It is a probe consisting of two electrodes:
the main electrode on positive potential and thescreening electrode on negative potential. Bothelectrodes generate the electric field which induces aflow of negatively charged dust grains from the plasma

into the probe. Figure 8 shows a video frame
demonstrating the removal of dust grains from theFig. 8 Video image of dust particle removal by theelectrode arrangement shown in fig.7.

plasma volume.

5. Some Effects of Dust on Fusion Devices
vessel volume when the toroidal magnetic field is
switched on. The charging of dust due to the radioactive
decay of Tritium may lead to additional complex effects.
During break down and in the initial current ramp phaseof a fusion plasma rapidly varying eddy currents are
flowing in the vessel. They cause electric fields whose

Most dust particles fall to the bottom of the device.
After some operation period a significant reservoir will
have accumulated. Since the dust is mobile it may well
fill gaps of the complex in-vessel structures which were
designed for functional reasons and thus lead to failure
and malfunction. The heat transfer to cooling structures

strength and direction is difficult to predict. The electric

is strongly impeded by a dust layer on top.
Small particles may be re-injected into the fusion
plasma by magnetic and also by electric forces when
dust flakes are charged by plasma contact. They may
then be levitated close to the wall. Magnetic particles
experience a VB-force and may be sucked into the main

field strength may be large enough to levitate thecharged dust particles. If these particles get into themain volume of the vessel before a significant current isestablished, burnthrough of the plasma becomes difficult
because of high radiation losses. Compensating for this158

Winter J., Is Dust Formation a Concem for Fusion Devices?

in future devices by more ohmic heating power isresearch, low temperature plasma physics in reactive
gases and dusty plasma physics may help to successfully
tackle such an important and demanding task.

difficult, because particularly low loop volrages (of theorder of I Volt) are required for the sake of thesuperconducting coils.
Dust particles are a sink for electrons. The charge

Acknowledgement
The author is grateful to J. von Seggern for thepermission to use fig.l. He benefitted a lot fromon a dust grain is about 103 elementary charges for a I/rm diameter particle per eV electron temperature,
increasing with particle size and electron temperature.
The heavy negative particles may change the dynamic
response of the edge plasma e.g. to low frequency
acoustic waves. There are a number of forces acting on
particles in a plasma: electrostatic forces due to their

discussions with A.P. Nefedov. M. Rubel. C.H. Skinnerand P. Wienhold.

This work was in part supported by Deutsche
Forschungsgemeinschaft (SFB 191) and by Ministerium
fi.ir Wissenschaft und Forschung, Nordrhein-Westfalen.

charge and the local electric field, drag forces by thestreaming ions, thermophoretic forces, gravitation etc.
Thus fine particles can be transported far away from

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