WDS'11 Proceedings of Contributed Papers, Part II, 215–220, 2011.
ISBN 978-80-7378-185-9 © MATFYZPRESS
Diagnostic Lithium Beam System for COMPASS Tokamak
P. Hacek
Charles University Prague, Faculty of Mathematics and Physics, Prague, Czech Republic.
V. Weinzettl, J. Stöckel
Institute of Plasma Physics AS CR, v.v.i., Association EURATOM/IPP.CR, Prague, Czech Republic.
G. Anda, G. Veres, S. Zoletnik
KFKI-RMKI, Association EURATOM/HAS, Budapest, Hungary.
M. Berta
Szechenyi Istvan University, Association EURATOM/HAS, Gyor, Hungary.
Abstract. The COMPASS tokamak, a divertor device with ITER-relevant geometry
capable of achieving H-mode, has been re-installed in IPP Prague after its transport
from Culham in UK. A Diagnostic Lithium Beam system is being developed for
COMPASS tokamak. Its main goal is to provide edge density (Beam Emission
Spectroscopy) and edge plasma current (Atomic Beam Probe) measurements to
address the scientific programme focused on H-mode and pedestal physics. It features
several newly designed and developed parts, including improved emitter and
neutralizer. Atomic Beam Probe is an innovatory diagnostic for measurement of
poloidal magnetic field and plasma current fluctuations in the plasma edge. Currently,
the system is connected to tokamak (August 2011) and first experiments with plasma
were performed. The system still undergoes vacuum, neutralization and high voltage
testing. This article reviews the concept and current state of the Lithium Beam
diagnostic for COMPASS and provides its first test results.
Introduction
Study of high confinement regime (H-mode) is one of the most important topics of today’s
tokamak physics. H-mode is characterized by improved particle and energy confinement and is
planned for a standard ITER operation. Nowadays, it is routinely achieved on many tokamaks, usually
using divertor magnetic configuration and applying strong additional plasma heating. The transition
from lower confinement regime (L-mode) to H-mode causes formation of a steep pressure gradient at
a small fraction of plasma minor radius near the separatrix (so called pedestal region) and shift of the
core pressure profile to higher values. The pressure gradient also acts as a transport barrier for plasma
particles. However, the large pressure gradient can drive MHD instabilities that limit the pedestal
height and therefore overall plasma performance. On the other hand, these instabilities provide a
relaxation mechanism of the particle transport barrier that is necessary for stationary H-mode
operation. The most common type of such a relaxation mechanism are edge localized modes (ELMs) –
short (10–100 µs) bursts of enhanced particle and energy transport that repetitively degrade the
pedestal. ELMs cause intense heat load on the divertor plates which will be enormous in case of ITER
(≈ tens of MJ for type I ELMs with frequency 1–5 Hz [Leonard et al., 1999]), threatening to ablate a
large portion of material of the plates and significantly limit its lifetime. Several methods have been
developed to mitigate the ELMs (e.g. ELM pacing by pellet injection, resonant magnetic perturbations
etc.), the detailed ELM mechanism is however still unclear. To be able to understand pedestal physics
more deeply, it is necessary to measure important plasma parameters in the pedestal region with
sufficient spatial and temporal resolution.
One of the ways to achieve this goal is to use diagnostic neutral beams. The accelerated neutral
particle beam injected into the vacuum vessel interacts with plasma. The beam atoms are collisionally
excited and ionized. The plasma parameters (primarily plasma density and temperature) and beam
energy determine the excitation and ionization rates. The excited neutral atoms return to the ground
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state by emitting radiation with characteristic wavelengths. Dependence of the light emission on
plasma temperature is small in case of certain beam species (e.g., Li, Na), thus measured intensity of
the emitted light allows reconstruction of the electron density profile. Fast beam deflection to several
vertical positions and observation of the signal correlation allows determining the poloidal flow
velocity. Detection of spectral lines from impurities (coming from charge exchange reaction), their
Doppler broadening and Doppler shift provides information about impurity density, temperature and
flow velocity. The ionized beam particles are deflected due to the magnetic field and depending on
their Larmor radius they are either confined or get out of the plasma to the vessel wall. Their detection
provides information about magnetic field and therefore indirectly also about plasma current.
Diagnostic Lithium Beam design for COMPASS
The scheme of the lithium beam system can be seen on Figure 1. Lithium ions will be emitted
constantly during the tokamak discharge (5–10 mA) by a resistively heated solid ion emitter, then
accelerated to energies up to 100 keV and focused by ion optics. Deflection plates will be used to
vertically or horizontally deflect the beam trajectory in the plasma (<5 cm) or to target the beam
outside into a Faraday cup, which will allow a background noise measurement. Lithium ions will be
neutralized via charge exchange by passing through a chamber with sodium vapour. The light emitted
by excited lithium atoms in the vacuum chamber will be collected by CCD camera and Avalanche
Photodiodes (APDs), ionized part of the beam will be collected by Atomic Beam Probe (ABP).
Lithium emitter and beam extraction
The COMPASS diagnostic lithium beam uses thermionic lithium ion emitter with improved
properties compared to similar high-energy (30–70 keV) lithium beam experiments used at ASDEX
Upgrade [Fiedler et al., 1999], and TEXTOR [Anda et al., 2008] tokamaks. This newly developed
emitter is capable to provide significantly higher ion current (up to 8–10 mA compared to maximally
2 mA of the previous emitter design). The mechanism of the ion emission is electrolysis in the emitter
material. The emitter material is heated up to about 1250–1350 °C. At these temperatures, the lithium
ions can diffuse with relatively high velocity inside the ceramic emission layer. If an external electric
field for ion extraction is applied, the ions are emitted. The ion emitter on COMPASS comprises of
19 mm diameter flat emission surface. Emissive material is β-eucryptite (Li2O + Al2O3 + 2SiO2)
Figure 1. Scheme of the diagnostic lithium beam system for COMPASS tokamak
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Figure 2. Photo of the emitter (in the middle) and the extractor electrode.
embedded in a molybdenum cup. It is heated by a dopped SiC volume heater disc. Heating is done by
approximately AC 90 A at 3.5 V and maximum allowed surface temperature is 1380 °C. Emitter is
integrated to one unit with the focusing (Pierce) electrode and it is placed inside stainless steel
housing. Its operational lifetime (according to [Zoletnik et al., 2011]) is in range of hours of
continuous emission, of course, the value is changing according to the extracted current. ¨
Several test measurements of the emission current were made by raising the extraction and the
emitter voltage proportionally. In the test setup one power supply is connected to the emitter (this
voltage determines the ion energy) and a second power supply provides a negative extraction voltage
between the emitter and the extractor electrode. With respect to earlier experiments, the ratio of
emitter voltage to extraction voltage, which influences the beam focusing and value of the extracted
current, was set to 10. The current on the emitter power supply (which corresponds to the extracted ion
current) was measured and plotted as a function of the extraction voltage. The resulting plot can be
seen in Figure 3.
Neutralizer concept
COMPASS lithium beam has a newly designed recirculating neutralizer, which allows better
handling with sodium used for Charge Exchange (CX) reaction on the beam ions (Li– + Na0 → Li0 +
Na–). Usual solution for neutralizer at lithium beam experiments (e.g. JET [Brix et al., 2001], ASDEX
Upgrade, TEXTOR) is to keep sodium inside a reservoir closed by a plug and open it only for a short
period during shot to let the sodium vapour out. This way, the sodium content has to be replaced after
Figure 3. Test measurements—dependence of the ion current on the extraction voltage.
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a time and the vacuum components suffer from sodium deposition. The COMPASS design uses
double cone shaped neutralizer chamber with small holes for passing beam at both sides (see
Figure 4). The neutralizer chamber is attached inside the flight tube. At its bottom, there is an open
sodium container heated to approximately 250 °C (sodium has melting point 98 °C and boiling point
883 °C). The neutralizer is cooled by tubes with air and therefore the sodium vapour condenses on the
walls and flows back to the heated sodium pool—the loss of sodium at the both ends is minimized.
Thermal simulation in Ansys simulation software in Figure 5 shows thermal map of the
neutralizer in steady-state. The temperature measurements of the neutralizer (there are
4 thermocouples measuring temperature at top, bottom and both side ends of the neutralizer) showed
good agreement with the simulation (difference about 2–3 °C). The expected neutralization efficiency
values are 95%–68% for beam energy range 20–70 kV [McCormick et al., 1997].
Beam Emission Spectroscopy
There will be two optical systems equipped with interference filters for detection of lithium
spectral line at 670.8 nanometers, which corresponds to 2p–2s lithium transition. As can be seen in
Figure 4. Photo of neutralizer chamber (upside down).
Figure 5. Thermal steady-state simulation of the neutralizer chamber (Ansys).
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Figure 1, the top vertical port will be used by CCD camera for slow measurement and bottom vertical
port will be used by avalanche photodiodes for fast measurement. CCD camera with optics is already
installed on COMPASS; it has 100–200 Hz frequency range, 640×480 pixels and digital temperature
compensation. APD detector unit is still being developed, however, it will have 20–22 silicon-based,
type S8664-55 APD channels with 25 mm2 effective area, quantum efficiency of 85 % (at 650 nm) and
gain ≈ 50 at 360 V. The detector unit will also feature special low noise operation amplifier developed
in RMKI, Hungary, internal ADC with optical interface and will operate with 1 MHz bandwidth. The
whole unit will be in temperature-stabilized housing.
Atomic Beam Probe
To detect the lithium ions, a two-dimensional segmented multichannel system will be used (see
Figure 6). It will provide a direct measurement of the ion current. In front of the detector, there will be
a biased entrance slit to reduce the background noise. For Atomic Beam Probe measurements, the
neutral lithium beam with standard diameter 1–2 cm for BES measurement will pass through a
diaphragm and its diameter will be reduced to few millimeters. Also the energy of the beam will be
increased to about 100 keV with respect to BES measurements with usual beam energies around
40 keV. The size of one detector segment in toroidal direction is planned to about 0.5 mm. However,
the exact dimensions and capabilities of the detector depend strongly on the level of noise coming
from plasma (i.e. charged particles and secondary electrons generated by UV and X-ray radiation and
energetic neutrals). A test ABP detector was therefore installed on COMPASS in order to measure this
noise (Figure 6).
The ABP test detector has 25 channels (20 detector segments, 4 Langmuir probes and one channel
for grounding) and a possibility to move in a vertical direction (when maximally inserted into the
vacuum chamber, the detector plate’s front surface is 30 mm far from the wall tangent). First
measurements were done in February 2010 (more in [Hacek et al., 2010]).
Overview and plans
A Diagnostic Lithium Beam system is being developed for COMPASS tokamak. Its main goal is
to provide edge density (BES) and edge plasma current (ABP) measurements. It has several newly
designed and developed parts (e.g. lithium ionic emitter and neutralizer chamber) and includes a new
diagnostic technique (ABP) for detection of beam ions. The system was recently (August 2011).
connected to COMPASS tokamak and is being currently tested for vacuum tightness, beam extraction
(the beam energy is being consequently increased) and beam neutralization efficiency. First shots into
tokamak with hydrogen gas and also with plasma were performed. Up to now, the maximal achieved
beam energy was 40 keV and maximal beam current was 2.3 mA. The whole beam system is in place,
Figure 6. Left: Proposed design of the ABP detector. Middle and right: Photos of the test ABP
detector installed on COMPASS in order to measure the level of plasma noise.
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the only parts waiting for delivery are final flight tube, calibration rod and optics for fast BES
measurement. Final design of the ABP detector will be made after measurements with the test detector
in stable high current plasma discharges. First reconstructions of edge plasma density profiles should
be made before the end of the year.
Acknowledgments. The work was performed and supported from the grant GA CR No. 202/09/1467.
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