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Ion energy measurements in the Scrape-Off-Layer of MAST using a
Retarding Field Analyzer
P. Tamainab*, M. Kočanbc, A. Kirka, J. Gunnb, J-Y. Pascalb, M. Pricea
EURATOM/CCFE Fusion Association, Culham Science Centre, Abingdon, Oxon,
a
OX14 3DB, UK
b
c
CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France.
EURATOM Association, Max-Planck-Institut fürPlasmaphysik, 85748 Garching, Germany
Abstract
We present first ion energy measurements obtained in the Scrape-Off Layer (SOL) of the
MAST tokamak using a Retarding Field Analyzer (RFA). Results from two sets of
experiments are reported. First, ion temperature (Ti) profiles were measured in L-mode
discharges. Consistent with what was observed in other machines, Ti is larger than the
electron temperature Te with a ratio in the range 1-2.5, which is on the low side of the multimachine database. This is consistent with a relatively high degree of ion-electron thermal
coupling. Second, the RFA was used to estimate the ion energy in ELMs. Two different
scenarios were studied and gave very different results. In the first one, a type-I ELMy Hmode, ions with energies exceeding 500eV were found as far as 20cm away from the
separatrix. In the second one, featuring type-III ELMs, no ion with energies larger than 200eV
was detected 10cm outside the separatrix.
______________________________________________
JNM keywords: P0600 Plasma Properties
PSI-19 keywords: Cross-Field Transport, Edge Plasma, ELM, MAST, Probes
PACS: 52.55.Fa, 52.55.Rk, 52.25.-b, 52.35.Py
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*Corresponding author address: Association Euratom-CEA, CEA Cadarache, F-13108 St.
Paul-lez-Durance, France.
*Corresponding author E-mail: patrick.tamain@cea.fr
Presenting author: Dr Patrick Tamain
Presenting author e-mail: patrick.tamain@cea.fr
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1. Introduction
The characterization and understanding of energy fluxes to the divertor and to plasma facing
components is one of the main targets of tokamaks edge physics research. Extensive work has
been done in that field in the last decades, using flush mounted and reciprocating Langmuir
probes. However, if a lot of data has been gathered concerning the electron temperature (Te),
much less is available for the ion energy. This is of particular concern for predictions for
future machines like ITER since it is the energy of the ions that determines the damage by
physical sputtering on plasma facing materials and hence the plasma contamination by
impurities. Furthermore, in the absence of an indication of the ion temperature (Ti), Ti  Te is
usually assumed to infer the electron density or power fluxes from Langmuir probes data.
Such an assumption is not true for most plasmas since the existing measurements show that
Ti  Te in the Scrape-Off Layer (SOL) by a factor which ranges from 1 to 10, dependent on
the level of electron-ion thermal coupling ([1] and references therein).
In this paper, we present first ion energy measurements obtained in the SOL of the MAST
spherical tokamak using a Retarding Field Analyzer (RFA) [2]. In section 2, we present
briefly the experimental setup. Results from two sets of experiments are then reported:
measurements of the ion temperature in the edge of L-mode discharges are presented in
section 3 and the ion energy in ELM filaments is investigated in section 4.
2. Experimental setup
Ion energy measurements in the edge plasma of MAST were performed using the RFA probe
successfully used in the Tore-Supra tokamak [3][4] (similar RFAs have been used in a
number of other machines [1]). The probe was mounted on the MAST fast reciprocating
manipulator, located in the outboard midplane. Given the design of this RFA probe, the
relatively large Larmor radius of the ions in the edge of MAST (  3mm in typical SOL
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conditions) is not expected to have any significant effect on the measured ion parallel energy
distribution. The bidirectional RFA is equipped with two identical analysers, one facing the
lower-outer divertor (the ion-side, i-side, referring to the ion B  B drift direction pointing
downwards), the other (the electron-side, e-side) connected either to the upper-outer or to the
lower-inner divertor plate according to whether the plasma is run in connected double-null
(CDN) or lower single-null (LSN) configuration . With all the voltages referred to the
machine ground, each analyser is composed of a negatively biased front electrode (the slitplate), a discriminator grid biased to VGr1  0 (grid 1 in Fig. 1 of [3]), a negatively biased
electron repelling grid (grid 2 in Fig. 1 of [3]) and a grounded collector. Only ions with
incident energies larger than Z i  e  VGr1 can reach the collector, building up a current I c .
Thus, the I c  VGr1 characteristic can be related to the energy distribution of the ions [5]. In all
the discharges analyzed in this paper, grid 2 is biased to -200V.
3. Ion temperature measurements in L-mode plasmas
Ti profile measurements were performed in various L-mode scenarios (Table 1). VGr1 was
swept from 0 to 180V while the slit plate was held at VSP  165V to measure the ion
saturation current density J sat . In some discharges, SOL Te was measured simultaneously
with Ti by sweeping VSP between the subsequent I c  VGr1 characteristics. Figure 1 shows
typical time traces of the RFA signals for the e-side analyser. The collector current I ce  side
exhibits strong intermittency similar to that observed on the ion saturation current but
smoother probably due to the limited bandwidth (~15kHz) of the collector current
measurements. I ce  side scales inversely with VGr1 , giving I c  VGr1 characteristics that can be
fitted using standard RFA theory [5]. Data from the i-side analyser (not shown here) is much
more difficult to interprete in the same frame. Although the expected characteristics were
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observed in the outer SOL, the collector current shows little dependence on VGr1 deeper in the
plasma for voltages up to 120V. This could be explained by large sheath potentials V sh (RFA
theory predicts that I c starts decreasing with VGr1 only for VGr1  Vsh ), but Te in the MAST
edge is typically below 30eV so that, assuming Vsh  3Te , V sh should be lower than 90V. The
data measured by the i-side analyzer are therefore not included in the analysis.
Resulting temperature profiles for four discharges are shown in Figure 2. Since the density
and the plasma current are varied simultaneously, it is impossible to decouple their effect on
SOL temperatures. In both pair of scenarios (#22734/22735 and #22772/22810),
measurements are well reproducible and give Ti e  side  20eV
in the SOL, with
Ti e  side / Te  1  2 . Because of the perturbing effect of the probe and in the absence of reliable
measurements from the i-side, these values need to be corrected for the mean parallel flow
contribution [6]. The e-side analyser facing the direction of the mean flow with velocities
ranging from 0.2 to 0.7 times the local acoustic velocity [7], the unpurturbed ion temperature
Ti can be different by up to a factor 1.3, so that Ti / Te  1  2.6 . It is worth noticing that
these results are quantitatively consistent with previous power balance studies in MAST [8]
which showed that, if it is assumed that Ti  Te at the divertor, then the total power calculated
from Langmuir probes measurements at the targets is equal to the power convected out of the
confined plasma. Using the OSM2 transport model, this ratio can then be extrapolated to
Ti / Te  2  2.5 in the midplane [9], in reasonable agreement with our measurements.
Finally, although the limited bandwidth of the collectors’ electronics prevents looking
precisely into the question, it is interesting to look at the intermittent behaviour of the
collector current in Figure 1. One can notice that at high grid 1 voltage (VGr1>110V) some
events visible on the J sat signal are correlated with a peak on the collector current while
others of the same amplitude are not (dash rectangles on Figure 1). This suggests that there is
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a spread in the ion energy in filaments. Very little data is available in the literature on Ti
fluctuations but, provided electronics with more adapted bandwidth for the collector current is
used, such studies could be carried out with the RFA looking at the correlation between J sat
and I c fluctuations.
4. Ion energy measurements in ELMs
Another issue related to the ions energy in the edge is that of ELMs. In ASDEX, it was
demonstrated that 25% of the plasma energy released during an ELM does not reach the
divertor and is deposited on the other plasma facing components [10]. It is therefore crucial to
get some information about the energy of ELM ions in the SOL. First and -up to recentlyonly direct measurements of ion energies in ELMs were performed in JET [11] and gave
results compatible with the predictions of models [12]. Recent results obtained in ASDEX
[1][13] gave Ti ELM ≈ 50-100 eV 6cm out of the separatrix, also in-line with models [12][14].
Tore-Supra’s RFA was used to address this issue in MAST. The duration of an ELM being
shorter than the time period necessary to sweep VGr1 , the strategy was to hold VSP and VGr1
constant and run several times the same shot for different values of VGr1 . In all the considered
shots VSP  165V . Two different scenarios were studied: a LSN type-I ELMy H-mode and a
CDN type-III ELMy H-mode.
Figure 3 shows the time traces of the collectors currents during ELMs for four different values
of VGr1 with the RFA located 19cm out of the separatrix in the LSN scenario. Each ELM is
characterized by a sharp signal on the collectors. In spite of the marginal bandwidth, most of
them exhibit several peaks reminiscent of the filamentary structure evidenced with Langmuir
probes measurements [15]. Once again, the two energy analyzers behave differently. The eside collector measured almost exclusively positive peaks (ion currents) while most ELMs
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appear on the i-side collector as a combination of a positive and a negative peak. As for Lmode measurements, it is difficult to understand such a different behaviour of both sides. The
existence of negative bursts on the i-side collector would suggest that electrons with energies
larger than 200eV reach the RFA (Grid 2 is biased at -200V) but this is surprising so far out in
the SOL and it is hard to explain the asymmetry. However, in both cases a clear impact of
VGr1 can be seen on the average amplitude of the positive bursts (Figure 4). A standard RFA
fit on these data leads to ion temperatures of 87eV for the i-side and 512eV for the e-side. The
former is in the ballpark of what is expected from models and the latter is far too large to be
trusted ( Ti  250eV at the top of the pedestal), even though the ELMs in the considered
scenario are sawtooth triggered. This discrepancy between the two values may be explained
by a lack of statistics. Indeed, Figure 3 shows that for a given VGr1 , there is large spread in the
peak amplitude of the collector signal during an ELM.
Other measurements were performed in a CDN scenario featuring type-III ELMs. This time,
no signal was detected on the collectors 10cm away from the separatrix, with VGr1  200V .
Thus the radial decay of energetic ELM ions is extremely dependent on the considered ELMy
scenario.
5. Conclusions
First ion temperature measurements in the Scrape-Off Layer (SOL) of MAST have been
performed using a Retarding Field Analyzer (RFA). Ti was found smaller than 20eV in the
SOL, with Ti / Te  1  2.5 , in different plasma conditions. Hence, MAST seats on the low
side of the values found in other machines. A rough analysis of the signal fluctuations suggest
that filaments do not all have the same ion temperature. Measurements were also performed in
the far SOL during ELMs. In an H-mode with sawtooth-triggered type-I ELMs, large bursts
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of ion current are detected 20cm away from the separatrix. In type-III ELMy discharges, no
ion was detected with energies larger than 200eV 10cm in the SOL. The limited size of the
database does not allow one to calculate a reliable Ti in ELM filaments, so that there is a clear
need for more measurements. This will soon be made possible by a new RFA probe head that
is currently being designed and tested in MAST.
Acknowledgements
This work was funded by the United Kingdom Engineering and Physical Sciences Research
Council under grant EP/G003955 and the European Communities under the contract of
Association between EURATOM and CCFE. The views and opinions expressed herein do not
necessarily reflect those of the European Commission.
References
[1]
M. Kočan et al., 19th PSI Conference, San Diego (2010).
[2]
R. Pitts et al., Rev. Sci. Instrum. 74, 4644 (2003).
[3]
M. Kočan et al., Rev. Sci. Instrum. 79, 073502 (2008).
[4]
M. Kočan et al., Plasma Phys. Control. Fusion 50, 125009 (2008).
[5]
H. Kimura et al., Jpn. J. Appl. Phys. 18, 2275 (1979).
[6]
F. Valsaque et al., Phys. Plasmas 9, 1806 (2002).
[7]
P.A. Molchanov et al., Plasma Phys. Control. Fusion 50, 115010 (2008).
[8]
A. Kirk et al., Plasma Phys. Control. Fusion 46, 551 (2004).
[9]
A. Kirk et al., Plasma Phys. Control. Fusion 46, 1591 (2004).
[10]
A Herrmann et al., Plasma Phys. Control. Fusion 46, 971 (2004).
[11]
R. Pitts et al., Nucl. Fusion 46, 82 (2006).
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[12]
W. Fundamenski et al., Plasma Phys. Control. Fusion 48, 109 (2006).
[13]
M. Kočan et al., to be submitted.
[14]
A. Kirk et al., J. Nucl. Mater. 390-391, 727 (2009).
[15]
A. Kirk et al., Plasma Phys. Control. Fusion 47, 315–333 (2005).
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Figures and tables captions
Table 1: macroscopic plasma parameters of the L-mode database discharges. The discharge
type is either Connected Double Null (CND) or Lower Single Null (LSN). Density ranges
indicate discharges in which the density increased continuously during the RFA plunge. Also
indicated is whether the voltage sweeps made measurements of Ti and Te available.
Figure 1: Time traces of RFA signals 2.6cm inside the separatrix (#22734). Top: voltage
applied to the discriminator grid VGr1; middle: current collected on the e-side collector I ce  side
(black) and smoothed over a 200μs sliding window and rescaled (x3) (red); bottom: ion
e
saturation current collected on the slit plate I SP
. The dash lines help to isolate individual
events.
TS
Figure 2: Temperature profiles measured by the RFA ( TeRFA
/ i ) and Thomson Scattering ( Te )
in 4 shots of the L-mode database.
Figure 3: Time traces measured during ELMs for 4 different bias voltages VGr1 (from left to
right: 200V, 350V, 425V, 505V) with the RFA located 19cm out of the separatrix in the SOL.
Top: Dα trace in the lower divertor; middle: current on the e-side collector; bottom: current on
the i-side collector. Time is plotted relative to the time tELM of the beginning of the ELM rise
on the Dα signal. Note that the time scale of the top row is different from those of the two
others.
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Figure 4: I c  VGr1 characteristics of the ELM signals for both collectors as measured 19cm
away from the separatrix in an SND ELMy H-mode. The empty symbols stand for individual
ELMs, and the full symbols are the average values for a given Grid 1 voltage.
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Figures and tables
Shot #
22734
22735
22772
22810
22829
22830
Type Ip [kA]
CDN
420
CDN
420
CDN
630
CDN
630
LSN
630
LSN
630
ne [1020 m-2]
~1.1
~1.1
~1.5
~1.5
2.4 → 2.8
1.4 → 3.3
PNBI [MW]
0
0
0
0
2
2
Ti RFA
Y
Y
N
Y
Y
Y
TeRFA
N
N
Y
Y
Y
Y
Table 1: macroscopic plasma parameters of the L-mode database discharges. The discharge
type is either Connected Double Null (CND) or Lower Single Null (LSN). Density ranges
indicate discharges in which the density increased continuously during the RFA plunge. Also
indicated is whether the voltage sweeps made measurements of Ti and Te available.
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Figure 1: Time traces of RFA signals 2.6cm inside the separatrix (#22734). Top: voltage
applied to the discriminator grid VGr1; middle: current collected on the e-side collector I ce  side
(black) and smoothed over a 200μs sliding window and rescaled (x3) (red); bottom: ion
e
saturation current collected on the slit plate I SP
. The dash lines help to isolate individual
events.
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TS
Figure 2: Temperature profiles measured by the RFA ( TeRFA
/ i ) and Thomson Scattering ( Te )
in 4 shots of the L-mode database.
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Figure 3: Time traces measured during ELMs for 4 different bias voltages VGr1 (from left to
right: 200V, 350V, 425V, 505V) with the RFA located 19cm out of the separatrix in the SOL.
Top: Dα trace in the lower divertor; middle: current on the e-side collector; bottom: current on
the i-side collector. Time is plotted relative to the time tELM of the beginning of the ELM rise
on the Dα signal. Note that the time scale of the top row is different from those of the two
others.
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Figure 4: I c  VGr1 characteristics of the ELM signals for both collectors as measured 19cm
away from the separatrix in an SND ELMy H-mode. The empty symbols stand for individual
ELMs, and the full symbols are the average values for a given Grid 1 voltage.
16