WDS'11 Proceedings of Contributed Papers, Part II, 227–232, 2011.
ISBN 978-80-7378-185-9 © MATFYZPRESS
U-probe for the COMPASS Tokamak
K. Kovařík,1,2 I. Ďuran,1 J. Stöckel,1 J. Seidl,1,2 D. Šesták,1 J. Brotánková,3 M. Spolaore,4
E. Martines,4 N. Vianello,4 C. Hidalgo,5 M. A. Pedrosa5
1
Institute of Plasma Physics AS CR v.v.i., Association EURATOM/IPP.CR, Prague, Czech Republic.
Faculty of Mathematics and Physics, Charles University in Prague, Czech Republic.
3
Institute for Plasma Research, Bhat, Gandhinagar, Gujarat, India-382428.
4
Consorzio RFX, Associazione EURATOM-ENEA sulla fusione, Padova, Italy.
5
Laboratorio National de Fusión, Asociación EURATOM-Ciemat, Madrid, Spain.
2
Abstract. A complex electrostatic-magnetic probe diagnostics, named as ‘U-probe’, is
being prepared for measurement of properties of the filamentary structures in the edge
plasmas of COMPASS tokamak. Probe will be composed of two identical towers.
Each tower will house 3 sets of 3D coils one balanced triple probe and six single
Langmuir probes. The design of the probe is based on U-probe used on reversed field
pinch RFX-mod [1] and takes advantage of our experience accumulated during design,
manufacturing and exploitation phase of our combined electromagnetic probe at TJ-II
stellarator. This new COMPASS diagnostic will measure vorticity and longitudial
electric current of the plasma filaments. Analysis of the properties and, particularly,
the correlation between filament vorticity and its current is expected to shed light into
physics of filamentary structures in the tokamak plasma edge. The simulation of the
probe surface temperature evolution as a function of expected incident heat loads
within the COMPASS edge plasmas is presented.
Introduction
Due to their characteristic shape, plasma structures elongated along the magnetic field lines are
called plasma filaments. Filaments have increased density and temperature in comparison to the rest of
the edge plasma [1]. They are conducting electric current along them as was recently observed on
RFX-mod [2, 3]. Therefore, the filaments carry out energy from plasma and deposit it unequally on
the walls of the vacuum vessel. Particularly, large groups of filaments are representing severe danger
for the first wall and divertor components as well as inserted diagnostics and instrumentation.
This work describes design of the new probe diagnostics, the so-called U-probe, see Fig. 1, for the
COMPASS tokamak. It will be part of complex diagnostics system for studies of the particle and
energy transport as well as appearance, sustaining and disintegration of transport barrier in the edge
plasma and scrape-off layer. Acquired knowledge will be used on analysis and mitigation of large and
potentially dangerous structures mentioned above.
Plasma filaments
Important parameters of the plasma filament are transversal profiles of electron density ne,
electron and ion temperature Te and Ti, plasma potential Φ, vorticity ω, and parallel electric current
density jpar.
We are interested in measurement of vorticity and parallel electric current density. Neither
vorticity nor parallel electric current density can be measured directly. Vorticity is derived from
plasma potential profile, see equation 1, measured by radial array of Langmuir probes. Locally, we can
neglect toroidal shaping of the whole plasma.
1
1  d 2Φ d 2Φ d 2Φ 
ΔΦ =  2 + 2 + 2 
B
B  dr
dy
dz 


( jr , j y , j z ) = j = 1 rot B = 1  ∂B y − ∂Bz , ∂Bz − ∂Br , ∂Br − ∂B y 
μ0
μ 0  ∂z
∂y ∂r
∂z ∂y
∂r 
ω=
227
(1)
(2)
KOVAŘÍK ET AL.: U-PROBE FOR THE COMPASS TOKAMAK
Figure 1. Schematic drawing of the U-probe for COMPASS tokamak.
Component of plasma potential second derivative parallel to the magnetic field (z-direction) is
assumed to be zero because the properties of the filaments along the magnetic field lines, i. e. along
the filament, are assumed to be constant. Radial component (r-direction) of the Laplace operator will
be calculated directly from radial array of Langmuir probes and poloidal component (y-direction) will
be calculated from temporal evolution of the signal of the Langmuir probes using poloidal velocity of
the filament movement. The poloidal velocity will be measured from time delay of the signal between
two radial arrays with fixed distance.
Similarly, the parallel electric current density will be calculated from magnetic field according to
equation 2. We neglect the toroidal components of the derivatives as well as in vorticity computation.
Magnetic field will be measured by a set of 3D coils distributed around the filament. Electric current
density calculation needs magnetic field vector measured in at minimum 3 different positions placed
within a plane perpendicular to the magnetic field lines but not placed on one axis.
We want to measure the electron density and electron temperature at the top of the probe head to
get global information about the general plasma parameters during the probe operation.
Requirements for dislocation of measuring elements mentioned above imply necessity of two
identical parts of the probe head – towers – fixed in parallel to each other with a given distance
between them. Each tower has to house 3 sets of 3-axial coils (3D coils), radial array of Langmuir
probes at the poloidal side and a triple probe at the top of the probe (radially oriented). Usage of two
identical towers will also give us information about disturbing of the filament during passage over the
first tower and the poloidal component of the filaments velocity. Shape of the two towers connected at
the bottom with support structure is similar to the letter U, therefore, the whole probe was baptized as
“U-probe”.
U-probe design
As previously mentioned, the U-probe consists of two identical towers. The probe will be inserted
in the tokamak scrape-off layer near separatrix. Therefore, it has to sustain relatively high thermal
loads in the order of a few hundreds of kW/m2. Exposure to the plasma will be usually for about 0.5 s
each 12–15 minutes. As a result, we need material that will allow heating up to several hundreds of
degrees Celsius with a good thermal conductivity and high specific heat. Moreover, the material has to
be an electrical insulator not to short-circuit the electric fields and currents in the filaments. Boron
nitride ceramics with specifications given in Table 1 has been chosen as the best material according to
these criteria.
Material for Langmuir probes was chosen according to the similar requirements as the material for
the probe head. The only difference is the request on high electrical conductivity. We also require easy
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KOVAŘÍK ET AL.: U-PROBE FOR THE COMPASS TOKAMAK
replaceability of those tips damaged due to the heat or mechanical loads, over the U-probe lifetime.
Two materials fulfilled these requirements – graphite and tungsten. We have chosen the graphite,
because there is no risk of welding with other metals at high temperatures and tungsten is very hard to
be machined by usual methods and too brittle when turned (particularly, as we want to use 2 mm thick
tips).
As the typical dimensions of the filaments are in order of a few cm and velocities in the order of
km/s, see Table 2 [4], the temporal and spatial resolution of the U-probe diagnostic has to be in the
order of microseconds and millimeters respectively.
Spatial resolution is given by the distance and the thickness of the tips. The compromise between
the heat load resistance of the tips and the spatial resolution was found in the tips diameter of 2 mm.
Axis distance of the tips will be 4 mm, i. e. free distance between walls of the tips remain 2 mm. This
is also the minimum distance allowing mechanical works with safe electric separation of the tips. As
the best solution of the tip fixation we found a system of a female screw tip simply screwed on a male
screw that is fixed to heat conducting substrate hidden inside the probe head. Signal wires are fixed to
the screws at the opposite side of the substrate, see Fig. 3.
The 3D coil system is fixed in a rift with the same dimensions as the 3D coil blocks. As the coils
are wound from a copper wire of diameter of 0.1 mm insulated with enamel, the signals will be
reconnected directly in the probe head to more massive wires that will better sustain any mechanical
manipulation with no risk of breaking.
Each tower will consist of a boron nitride coffin and a covering lid with wall thickness of 5 mm.
Space inside each tower is divided into two parts. One houses radial array of the Langmuir probes and
the second covers 3D coil sets, see Fig. 4. Position of the array of the Langmuir probes is as near as
Table 1. Main parameters of used boron nitride.
density ρ
thermal conductivity λ
specific heat c
electric resistivity
2.0 g/cm3
30.13 W/m/K
1600 J/kg/K
1014 Ω cm
Table 2. Typical dimensions and velocities of filaments in tokamaks [4].
Direction
dimension
velocity
Toroidal
a few meters
30 km/s
poloidal
< 5 cm
2-3 km/s
radial
< 5 cm
< 1 km/s
Figure 2. Schematic drawing of mutual position of poloidaly drifting current filament and the two
radial arrays of Langmuir probes (filled circles) at both towers in the four consecutive time instances.
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KOVAŘÍK ET AL.: U-PROBE FOR THE COMPASS TOKAMAK
Figure 3. Schematic view of Langmuir probe fixation (1 – signal outlead, 2 – screw, 3 – support plate
of alumina, 4 – female screw for fixation of the screw in the support plate, 5 – boron nitride wall of the
probe, 6 – graphite tip of Langmuir probe).
Figure 4. Design drawing of one tower of the U-probe.
possible to the toroidaly oriented wall to minimize shadow effect of the second tower or another
disturbance of the filament approaching because they will have the key role for search and observation
of the filaments. Coil systems are located at the “rear” side because the electric current is more stable
than electrostatic structures.
We have performed simple 1D simulation of the U-probe surface temperature as a function of
plasma parameters i.e. density and temperature. We have used the simple equation of temperature
diffusion for material with density ρ, thermal conductivity λ and specific heat c, equation 3. Boundary
condition was the energy flow q0 given by the flow of the particles, equations 4 and 5. Assumption of
losing of all energy by impact was used.
dT
λ
=
ΔT
dt
ρc
q 0 = ΓE kin
(3)
(4)
1
2k T
ne
(5)
4
m
According to this simulation, the surface temperature of the U-probe will not exceed 850 °C for
plasmas up to ne = 6·1018 m-3 and Te = 35 eV that is assumed in the scrape-off layer, see Fig. 5. This
temperature is well below the temperature limit for melting of the boron nitride of 3000 °C.
Γ=
Probe manipulator
Probe will be located at the outer wall of the COMPASS tokamak above the midplane. Diameter
of the access port is 95 mm, see Fig. 6. Probe manipulator doesn’t have to only fix the probe in a given
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KOVAŘÍK ET AL.: U-PROBE FOR THE COMPASS TOKAMAK
position but also it has to allow in-situ radial movement of the probe from separatrix to „fully-hidden“
position in the access port. The manipulator has to allow change of inclination of the probe head and
its rotation around axis to align the U-probe according to the magnetic field, i. e. axis of the towers
perpendicularly to the separatrix (about 15 degrees) and the U-probe plane perpendicularly to the
magnetic field lines (about 10 degrees). This rotation and inclination will be set off-site in the
mechanical workshop for the whole experimental campaign. Required precision of the alignment of
the U-probe is less than 3 degrees.
Figure 5. Final surface temperature of the boron nitride after 0.5s discharge with given constant
plasma parameters.
Figure 6. Schematic drawing of the U-probe positioning inside the COMPASS tokamak.
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KOVAŘÍK ET AL.: U-PROBE FOR THE COMPASS TOKAMAK
Summary
The U-probe has been designed for scrape-off layer observations of the electrostatic and magnetic
properties of the plasma filaments in COMPASS. Optimal location and orientation of the measuring
elements were analyzed in detail. The way of plasma filament properties evaluation from measured
data was explained in detail, particularly, for the electrostatic part that is important for the filament
search and analysis. Heat loads on the boron nitride U-probe surface due to plasma flow were
calculated to establish maximal surface temperature after each shot. It was found that the surface
temperature is well below the acceptable limit. Results obtained with the U-probe will significantly
improve our knowledge about processes observed in the scrape-off layer and anomalous transport and
transport barriers in the edge plasma. Construction and commissioning of the U-probe will be finished
before end of 2011.
Acknowledgment. The work was supported by project of MPO 2A-TP1/101, CSF 202/08/H057, AS CR
#AV0Z20430508, MSMT #7G09042 and #7G10072 and Euratom. The views and opinions expressed herein do
not necessarily reflect those of the European Commission.
References
[1] M. Spolaore et al., Direct measurements of current filament structures in magnetic-confinement fusion
device, Physical Review Letters 102, 165001 (2009).
[2] M. Spolaore et al., Magnetic and electrostatic structures measured in the edge region of the RFX-mod
experiment, Journal of Nuclear Materials 390-391 (2009) 448–451.
[3] E. Martines et al., Current filaments in turbulent magnetized plasmas, Plasma Phys. Control. Fusion 51
(2009) 124053.
[4] A. Schmid et al., Experimental observation of the radial propagation of ELM induced filaments on ASDEX
Upgrade, Plasma Phys. Control. Fusion 50 (2008) 045007.
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