Diagnostic Lithium Beam System for COMPASS Tokamak

Transcription

1 WDS'11 Proceedings of Contributed Papers, Part II, , ISBN 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 ( 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 215

2 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 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 (Li 2 O + Al 2 O 3 + 2SiO 2 ) Figure 1. Scheme of the diagnostic lithium beam system for COMPASS tokamak 216

3 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 + Na 0 Li 0 + 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. 217

4 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 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 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). 218

5 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 Hz frequency range, pixels and digital temperature compensation. APD detector unit is still being developed, however, it will have silicon-based, type S APD channels with 25 mm 2 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. 219

6 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. References Anda, G., D. Dunai, G. Petravich, J. Sárközi, S. Zoletnik, B. Schweer, T. Baross, I. G. Kiss and B. Mészáros, First Measurements with the re-installed accelerated Lithium beam diagnostics on TEXTOR, 35th EPS Conference on Plasma Phys. Hersonissos, 9 13 June 2008, ECA Vol.32D, P (2008) Brix, M., A. Korotkov, M. Lehnen, P. Morgan, K. McCormick, J. Schweinzer, D. Summers and J. Vince. Determination of edge density profiles in JET using a 50 kv lithium beam. Proceedings of the 28th Conference on Control. Fusion Plasma Phys.(Madeira), Europhys. Conf. Abstracts, 2001 Fiedler, S., R. Brandenburg, J. Baldzuhn, K. McCormick, F. Aumayr,, J. Schweinzer and H.P. Winter, Edge plasma diagnostics on W7-AS and ASDEX-Upgrade using fast Li beams, Journal of Nuclear Materials , 1999 Hacek, P., V. Weinzettl, J. Stockel, G. Anda, G. Veres, S. Zoletnik and M. Berta, Atomic Beam Probe Diagnostic for COMPASS Tokamak, Proceedings of the 19th Annual Conference of Doctoral Students WDS 2010, f-2, pp. 7 11, Leonard, A.W. et al., The impact of ELMs on the ITER divertor, Journal of Nuclear Materials, , McCormick, K., S. Fiedler, G. Kocsis, J. Schweinzer and S. Zoletnik, Edge density measurements with a fast Li beam probe in tokamak and stellarator experiments, Fusion Engineering and Design, , Zoletnik, S., S. Bato and G. Anda, Report on laboratory testing of enhanced Li ion emitter, not yet published,