Asdex Upgrade Team

B Kurzan

H W Müller

J Adamek

J Horacek

M Aftanas

P Bilkova

P Böhm

P Vondracek

R Panek

R Schrittwieser

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Radial profiles of the electron temperature on COMPASS and ASDEX 
Upgrade from ball-pen probe and Thomson scattering diagnostic 
J. Adamek
1
, H.W. Müller
2
, J. Horacek
1
, R. Schrittwieser
3
, P. Vondracek
1
, B. Kurzan
2
,  
P. Bilkova
1
, P. Böhm
1
, M. Aftanas
1
, R. Panek
1
, COMPASS Team
1
 and ASDEX Upgrade 
Team
2
 
1
Institute of Plasma Physics, Prague, Czech Republic 
2
Max-Planck-Institut für Plasmaphysik, Garching, Germany 
3
Institute for Ion Physics and Applied Physics, University of Innsbruck, Austria  
1 Introduction 
Investigations of plasma potential, electric field, electron temperature and its fluctuations 
remain important tasks in the context of anomalous edge plasma transport [1]. The ball-pen 
probe (BPP) [2–5] for direct measurements of the plasma potential Φpl was developed by 
Adamek et al. [6, 7] in the Institute of Plasma Physics in Prague. A combination of BPP and 
common Langmuir probe (LP) provides the value of the electron temperature Te by the dif-
ference between the plasma and floating potential Te =(Φpl – Vfl )/2.2 [4]. The BPP/LP head 
was mounted on the reciprocating midplane manipulator. Comparative measurements using 
probe diagnostics and edge/core Thomson scattering on COMPASS and ASDEX Upgrade 
are reported in the paper. 
2. Probe construction and experimental set-up 
The fast reciprocating probe shaft on the COMPASS and ASDEX Upgrade midplane ma-
nipulator was used to insert a probe head containing BPPs and LPs as seen on Fig. 1a and Fig. 
1b, respectively. Measurements of the plasma and floating potential are performed by means 
of the central BPP2 with the retraction depth (h = –0.5 mm) and one of the Langmuir probes 
(LP1 or LP2) on the tokamak COMPASS. The diameter of the probe head is about 3 cm on 
the top (Fig. 1a). The ball-pen probes are made of stainless steel collectors with diameters of 
2 mm and alumina shielding with an inner diameter of 5 mm. The Langmuir probes have the 
same dimensions as on ASDEX Upgrade. They are made of graphite pins with diameters of 
0.9 mm and protruding 1.5 mm into the plasma. The poloidal distances between the LPs and 
BPPs are 4 mm. The data acquisition system is working with a sampling frequency f = 5 
MHz. 
41st EPS Conference on Plasma Physics P2.011
Similar measurements on ASDEX Upgrade were performed by means of BPP1 and two 
neighbouring Langmuir probes LP2 and LP4 (Fig. 1b). The ball-pen probes consist of 
stainless steel collectors with diameters of 4 mm, fixed at h = –0.5 mm and of boron nitride 
shielding tubes with inner diameters of 6 mm. The whole probe head has a diameter of 5 cm. 
The temporal resolution of the potential measurements is limited by the sampling frequency  
f = 2 MHz of the data acquisition system. The Thomson scattering system on ASDEX Up-
grade is described in [8, 9]. The measurements on COMPASS were performed in circular and 
diverted plasma with Ohmic heating and typical values of toroidal magnetic field BT = 1.15 T, 
plasma current IP = 100 kA, density ne = 310
19 
m
–3
 in the circular plasma and with IP = 160 
kA, density ne = 310
19 
m
–3
 in the diverted plasma. The high resolution multi-point Thomson 
scattering diagnostic provides the electron temperature and density profiles on the 
COMPASS covering a region from 16 mm below the mid-plane to 284 mm above the mid-
plane in a vertical direction [10]. The diagnostic consists of two sub-systems, observing core 
core and edge plasma region, with a small overlap. The two sub-systems share the laser 
beams (two 1.5 J, 3 Hz Nd-YAG lasers, passing vertically through the plasma). Spatial res-
olution after mapping on midplane is up to 5 mm for the core and up to 3 mm for the edge 
regions [11]. The measurements on AUG were performed in L-mode discharges with NBI 
heating 1 MW (#28277) or ECR heating of 0.7 MW (#28292), a toroidal magnetic field BT = 
2.5 T, a plasma current IP = 1 MA and a line averaged density ne =4-510
19
m
–3
.  
 
      (a)            (b) 
Fig. 1: (a) - Picture of the ball-pen probe (BPP) head used in COMPASS midplane manipulator. It consists of 
three ball-pen probes made of stainless steel collector and alumina shielding tube and two graphite Langmuir 
probes. The front side of the probe head has a diameter of 3 cm. (b) – Picture of the ball-pen probe head used in 
AUG. The probe head has four BPPs made of stainless steel collector and boron nitride shielding tube. In ad-
dition, there are four graphite Langmuir probes. The front side of the probe head has a diameter of 5 cm. 
41st EPS Conference on Plasma Physics P2.011
(a)               (b) 
Fig. 3: Radial profiles of the electron temperature measured by BPP/LP (red line) and Thomson scattering 
(green squares) during L-mode discharge with NBI (a) or ECR (b) heating on ASDEX Upgrade. 
3 Radial profiles of the electron temperature  
Radial profiles of the electron temperature measured by the BPP/LP technique and by 
Thomson scattering on COMPASS are plotted in Fig. 2. The electron temperature provided 
by Thomson scattering diagnostic is plotted only with error bars less than 50%. The com-
parative measurements have been performed for circular and diverted plasma during dis-
charges #4797 and #7044. The position is plotted with respect to the separatrix provided by 
EFIT reconstruction. The agreement of both techniques is well seen for both types of dis-
charges. However, the radial profiles measured by Thomson scattering mapped on midplane 
and probe technique are systematically shifted during the diverted configuration. The dif-
ference between the positions of the separatrix from EFIT and maximum of the plasma po-
tential of BPP, which represents the radial electric field equal zero Er = 0 V/m, was used as a 
correction of the R coordinate of the Thomson scattering data in Fig.2b.  
A comparison of the radial profiles of the electron temperature measured by BPP/LP and 
Thomson scattering techniques on ASDEX Upgrade is plotted in Fig. 3. The measurements 
were performed during L-mode discharges #28277 and #28292 with NBI and ECR heating, 
      (a)            (b) 
Fig. 2: Radial profiles of the electron temperature measured by BPP/LP (red line) and Thomson scattering 
(green squares) during circular (a) and diverted (b) plasma on tokamak COMPASS. 
41st EPS Conference on Plasma Physics P2.011
respectively. The electron temperature provided by edge Thomson scattering diagnostic is 
plotted only with error bars less than 50%. It is seen in the figure that both techniques are 
providing profiles in good agreement for both heating mechanisms.  
4 Conclusions 
The radial profiles of the electron temperature provided by ball-pen/Langmuir probe and 
Thomson scattering techniques are in a good agreement for circular and diverted plasma and 
different heating mechanisms (ohmic, NBI, ECRH). The comparative measurements also 
confirm that the formula Te=(Φpl - Vfl )/2.2 is valid for both tokamaks and for different posi-
tions of the probe head, outside and inside the separatrix. This gives us a strong tool to pro-
vide the electron temperature using only two floating probes, which doesn’t require any ex-
ternal power supplies or sweeping technique. Therefore, the electron temperature is meas-
ured with high time and space resolution. The probe technique is limited by the transition of 
Langmuir probe to the self-emitting regime during the deep reciprocation [7]. 
Acknowledgements 
This work was carried out within the EURATOM and was supported by projects GA CR 
P205/12/2327 and P203/14/35260S of the Grant Agency of Czech Republic and MSMT 
Project #LM2011021 and project P19901-N20 of the Austrian Science Fund (FWF). 
References 
[1] Silva C. et al., European Physical Society, Finland (2013), P5.103 
[2] J. Horacek et al., Nucl. Fusion 50, 105001 (2010). 
[3] H. W. Müller et al., Nucl. Fusion 51, 073023 (2011). 
[4] J. Adamek et al., Contrib. Plasma Phys. 50, 854 (2010). 
[5] J. Adamek et al., Contrib. Plasma Phys. 54, No. 3, 279 – 284 (2014) 
[6] J. Adamek et al., Czech. J. Phys. 54, C95-C99 (2004). 
[7] J. Adamek et al., Czech. J. Phys. 55, 235 (2005). 
[8] H. Murmann, S. Götsch, H. Röhr, H. Salzmann, and K.H. Steuer, Rev. Sci. Instrum. 63, 4941 (1992) 
[9] B. Kurzan, M. Jakobi , H. Murmann, Plasma Phys. Control. Fusion 46, 299 (2004). 
[10] P. Bilkova et al., Nucl. Instrum. Meth. A 623 (2010) 656.   
[11] M. Aftanas et al., Journal of Instrumentation 7 (1) (2012) C01074-C01074. 
41st EPS Conference on Plasma Physics P2.011


Radial profiles of the electron temperature on COMPASS and ASDEX Upgrade from ball-pen probe and Thomson scattering diagnostic

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Radial profiles of the electron temperature on COMPASS and ASDEX Upgrade from ball-pen probe and Thomson scattering diagnostic

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