Neutral beam heating on the TCV tokamak

EUROFUSION WPMST1-CP(16) 16479 A.N. Karpushov et al. Neutral beam heating on the TCV tokamak Preprint of Paper to be submitted for publication in Proceedings of 29th Symposium on Fusion Technology (SOFT 2016) This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. This document is intended for publication in the open literature. It is made available on the clear understanding that it may not be further circulated and extracts or references may not be published prior to publication of the original when applicable, or without the consent of the Publications Officer, EUROfusion Programme Management Unit, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK or e-mail Publications.Officer@euro-fusion.org Enquiries about Copyright and reproduction should be addressed to the Publications Officer, EUROfusion Programme Management Unit, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK or e-mail Publications.Officer@euro-fusion.org The contents of this preprint and all other EUROfusion Preprints, Reports and Conference Papers are available to view online free at http://www.euro-fusionscipub.org. This site has full search facilities and e-mail alert options. In the JET specific papers the diagrams contained within the PDFs on this site are hyperlinked Neutral beam heating on the TCV tokamak Alexander N. Karpushova, René Chavana, Stefano Codaa, Vladimir I. Davydenkob, Frédéric Dolizya, Aleksandr N. Dranitchnikovb, Basil P. Duvala, Alexander A. Ivanovb, Damien Fasela, Ambrogio Fasolia, Vyacheslav V. Kolmogorovb, Pierre Lavanchya, Xavier Llobeta, Blaise Marlétaza, Philippe Marmilloda, Yves Martina, Antoine Merlea, Albert Pereza, Olivier Sautera, Ugo Siravoa, Igor V. Shikhovtsevb, Aleksey V. Sorokinb, Matthieu Toussainta a Ecole Polytechnique Fédérale de Lausanne (EPFL), Swiss Plasma Center (SPC),CH-1015 Lausanne, Switzerland b Budker Institute of Nuclear Physics SB RAS, 630090 Novosibirsk, Russia The TCV tokamak contributes to physics understanding in fusion reactor research by harnessing a wide experimental tool set: in particular flexible shaping and high power electron cyclotron heating. Plasma regimes with high plasma pressure, a wider range of temperature ratios and significant fast-ion population are now attainable with a TCV heating system upgrade. In a first stage, a 1 MW neutral beam was installed (2015) and is reported in this paper. The installation of the NB injector required modifications of the vacuum vessel and considerable work on the machine infrastructure, resulting in a shutdown from late 2013 to mid-2015. TCV is now operating partly as a European Medium-Size Tokamak (MST) facility under the auspices of the EUROfusion consortium. The NBI was intensively operated in the February – July 2016 phase of the MST campaign. Record ion temperatures of 2.02.5 keV and toroidal rotation velocities up to 160 km/s were promptly attained in the first few L-mode discharges with NB injection. Ion temperatures up to 3.5 keV were subsequently achieved in ELMy H-mode. The injector produces a focused deuterium neutral beam with 25 keV energy, 1 MW neutral power and 2 s duration. Keywords: TCV tokamak; neutral beam heating; upgrades 1. Introduction The Tokamak à Configuration Variable (TCV, RO≅0.88 m, a≤0.25 m, BT≤1.54 T) [1] is characterised by the most extreme plasma shaping capability worldwide (plasmas elongation κ up to 2.8, positive and negative triangularity −0.7≤δ≤1), the highest microwave Electron Cyclotron (EC) power density in plasma, and a high degree of flexibility in its heating and control schemes. Main TCV missions [2] are to contribute to the physics basis for more efficient ITER exploitation, and optimisation of the tokamak concept, plasma scenarios, heating and control techniques for DEMO and beyond. This requires access to plasma regimes and configurations with high normalised plasma pressure and a wide range of electron/ion temperature ratios, covering Te/Ti~1. Implementation of preferential ion heating at the MW power level allows the extension of Ti/Te to beyond unity and fills the gap between predominantly electron heated experiments and fusion reactor conditions. A phased upgrade program [3] is underway on TCV, mainly consisting of adding ion heating (NB injectors), increasing the available electron heating power (X2 and X3 gyrotrons) and installing a divertor structure with variable closure, equipped with gas valves, pumping units and magnetic field coils. A neutral beam injector (NBI), delivering 1 MW power along a tangential (double-pass) line of sight, at energies in the 15-30 keV range, was installed and commissioned, and provided research results in 2016. Two 750 kW, gyrotrons were also commissioned and integrated with three remaining first-generation 500 kW gyrotrons, providing a projected total of 3 MW X2 ECRH power. Two additional 1 MW dual-frequency (X2 and X3) gyrotrons and a second, 1 MW, 50-60 keV neutral beam are planned. The ASTRA code was used to simulate the plasma response to combined neutral beam and EC heating in TCV geometry [4]. With the upgraded (1.5→3.5 MW) X3 EC system, NBH (1-2 MW) TCV could bring the plasma close to the β-limit in H-mode (βN~2.8, an important regime for ITER and DEMO), provide direct momentum input to the plasma, and generate a high fast ion fraction for studying wave-particle interaction phenomena of interest for burning plasmas. The Te/Ti~1 condition is already met with ~1 MW of NB power with 1 MW of X3 ECH. The Te/Ti is expected to vary between 0.5 and 3.0 in TCV’s high density (H-mode) confinement regimes. 2. Neutral beam injector TCV’s NBI installation was based on considerations of beam access, shine through and orbit losses [5]. A specific geometric arrangement of the NB injection with the beam line at mid plane oriented tangentially relative to the plasma axis was chosen to maximise heating efficiency whilst satisfying machine access limits. The basic characteristics of TCV’s NB system [5] are listed in Table 1. The 15-30 keV beam energy is safe with respect to orbit losses for IP≥250 kA. The 0.25...1.05 MW power, tuneable during TCV discharges, enables studies of the plasma reaction to NBH power variation. The neutral beam injector design is based on a development of the NBI for plasma heating at Budker _______________________________________________________________________________ author’s email: alexander.karpushov@epfl.ch INP [6]. The injector incorporates a standard positive ion source and elements shown in Figure 1. An average nominal current density of 0.3 A/cm2 was chosen for the ion source [5]. Table 1: NBI characteristics. NB injector reference scenario: NB power injected in TCV 1 MW Nominal beam energy 25 keV Max. NB pulse duration 2 sec Beam full energy fraction in power ≥70 % NB operation domain: Beam power range 0.25…1.05 MW Beam energy range 15…25 keV Beam main species DO & HO Power sweep response (P/(dP/dt)) ≤5 ms Full power modulation on-time 5 ms…2 sec Minimal modulation off-time 5 ms Modulation rise/fall time ≤0.5 ms 100% power modulation up to 200 Hz emission surface of the plasma grid is 224 cm2, corresponding to a transparency of 46%. The ion source is connected to the vacuum tank through a DN 400 gate valve and a 700 mm length neutraliser. Two cryo-pumps with total pumping speed of 3×105 l/s in molecular flow regime for deuterium gas, are used during beam formation. Each cryopump consists of 1.6 m2 surface copper cryopanel cooled by two coldheads (cooling capacity 2×1.5 W at 4.2 K) and a chevron 0.83 m2 radiation shield with a transparency of 25%, cooled with liquid nitrogen. Figure 2: NBI-TCV plasma box (A) and plasma grid (B). Figure 1: Neutral beam injector: 1 – RF plasma source, 2 – magnetic screen, 3 – ion-optical system, 4 – neutral beam; 5 – adjusting device; 6 – ions source gate-valve; 7 – vacuum tank; 8 – cryopump cold head; 9 – liquid nitrogen volume; 10 – cryo-panels, 11 – neutralizer, 12 – bending magnet, 13 – diaphragm, 14 – ion dump for positive ions, 15 – calorimeter. The plasma emitter is formed with up to 40 kW of inductively coupled RF power at ~4 MHz in the plasma box [6] (ceramic aluminium oxide chamber, 346 mm inner diameter 120 mm long, Figure 2-A). A species mix with full, half and 1/3 of acceleration energy of 76:17:7 % (power ratios) was measured during the beam commissioning (see Figure 3). A high power focused neutral beam with small divergence was developed for the TCV device featuring narrow access ports where only small size, high power density beams can pass. The ballistic beam focusing is provided by spherically shaped multi-aperture electrodes in the ion optical system [7]. Slit apertures in the ion optical system reduce the focused beam width in the direction along the slits which is determined by the ion temperature of plasma emitter. 47 mm long slits with a step of 6 mm perpendicular to the slits are placed inside the 250 mm diameter area (Figure 2-B). The total Figure 3: Neutral beam energy components in power fractions Detectors for the beam alignment (aiming device) and the movable calorimeter are located at the exit of the vacuum tank. A retractable calorimeter can absorb the full duration (2 s) beam pulse at full power (1 MW). Neutral beam operation is overseen by an instrumental computer (LCS) and electronic control modules integrated into the TCV plant control system. This system can handle a large variety of low-voltage analog and digital input/output signals. Protection and interlocks are implemented at the hardware level together with several status monitors and controls. All functions necessary for safe NBI operation are included in the LCS that is designed to protect itself from potentially dangerous external situations and commands. A beam dump protection system is implemented on TCV to protect against overheat of beam facing elements in the area of beam-wall interaction. The combined RT processing beam inhibit signal generated by plasma disruption detector, a plasma density interlock and direct pyrometric measurements of beam dump surface temperatures are available to the NBI control system. 3. NBI optimisation and power control NBI power control through the plasma discharge is a powerful tool in fusion plasma studies as gradual power ramps up/down permit the investigation of power thresholds for particular processes; e.g. transition between low and high confinement. The 100% ON-OFF pulse–width modulation is successful on JET [8] as the time taken to slowdown NB fast ions in the plasma is relatively long (∼100 ms) compared to the beam ON/OFF time (40 ms) and the plasma is therefore relatively insensitive to the modulation process. JET employs 16 independently controlled ion sources (PINIs) to provide time averaged power with a resolution smaller than an individual PINI increment (a similar technique is also used on ASDEX Upgrade). In smaller machines, with a small number of beam sources, a faster fast ion slowdown and lower plasma confinement time (TCV, MAST), the plasma would respond strongly to beam modulation, so an alternative power modulation approach is required. As beam divergence is dependent on both beam current and acceleration voltage (through the perveance), ramping the ion beam current will affect the beam cross section, and beamline transmission. The real time control of an arc current of a high perveance MAST PINI allows variations of the neutral beam power by ~20% with only minimal effect on the beam footprint [9]. digital and analog control waveforms are calculated, transmitted to the LabView LCS program, and uploaded in the FPGA memory of PCIe LCS cards. Following trigger reception, the beam pulse control sequence is executed, and analog and digital control waveforms are transmitted to NBI power supplies. Examples of the TCV-NBI pulses with power variation and modulation are shown in Figures 5&6. Figure 5: “Slow” power sweep (0.23…1.05 MW) at minimal divergence. Figure 6: NBI power steps and modulation during NBI commissioning on the TCV. 4. First shots with NB heating on the TCV Figure 4: Example of the perveance scan at 24.8 keV A neutral power variation in the range of 0.25…1.0 MW has been implemented on TCV by simultaneous variation of RF power (plasma density in the RF box) and extraction voltage keeping a minimal beam divergence (optimal perveance). The optimisation procedure for the TCV NBI was performed at several (46) extraction energies; the optimal beam currents (RF power levels) were experimentally adjusted to minimise the beam divergence; here, minimal divergence (perveance scans) corresponds a minimum of the beam width on the calorimeter (Figure 4). The voltage on the suppression (2nd) grid and the bending magnet current were also optimised at each power/energy level. The desired neutral beam power vs time waveform (P0(t)) is designed in Matlab. The binary beam ON/OFF, beam energy, neutral and ion currents time traces are calculated accounting for their dependencies on P0(t) in order to retain a minimal footprint beam width. The First experiments with NBH (Figures 7&8) demonstrate a core plasma ion temperature in L-mode increasing from about 600-800 eV (typical in TCV at high density in Ohmic plasmas) to ~2 keV, in agreement with ASTRA predictions. The plasma rotation with 1 MW NB injection reaches 150-180 km/s (CO-NBI direction), while the typical values for spontaneous intrinsic rotation without NBI are less than 30 km/s. The heating neutral beam was intensively used in TCV experiments during the period of February-July 2016, mostly in the MST1 (European Medium-Size Tokamak) experimental program of the EUROfusion consortium. More of 60% (20 of 33) MST1 experiments used NB heating. ~25% of TCV discharges (579 shots) used NB injection into plasma during this period. Beam availability was 85-90%, with most (7-10%) faults in NB injection in TCV related to problems with NBI control electronics and power supplies. The total energy delivered by the NBI into TCV was limited to ¼ (0.5 MJ) of the design value (1 MW, 2 s) due to nonoptimal angular characteristics (divergence or/and focal length) of the beam compared to the beam duct, and the subsequent overheating that this provoked. Resolution of this problem is ongoing and will include a modification of the beam duct and improved ion optics (grids). Culham Science Centre) for useful discussions and help with NBI operation during the EUROfusion MST1 campaign, to members of SPC-EPFL and Budker INP teams involved in the work on manufacturing, installation, commissioning and operation of heating beam. Figure 9: Ion and electron temperature profiles in the high ELMy H-mode discharge. Figure 7: Global parameters of the TCV plasma with NB heating in L-mode, TCV shot 51458, 5th NBI shot in plasma. This work was supported in part by the Swiss National Science Foundation. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. References [1] [2] [3] [4] Figure 8: Plasma radial profiles with and without NBI in Lmode; TCV shot 51458. [5] The TCV record ion temperature of 3.5 keV (Figure 9) was achieved in the MST1 high confinement ELMy H-mode experiments. NB injection on TCV facilitates H-mode access, changes sawtooth and ELM frequencies, and provides a significant (up to 70 kA) plasma current drive. With the installation of the first 1 MW neutral beam TCV has greatly extended the range of accessible plasma parameters that are highly relevant to tokamak fundamental physics and machine operation studies and will strongly contribute to the ITER and DEMO projects. Acknowledgments The authors are very grateful to Manfred Sauer (ex. Inst. fur Plasmaphys., Forschungszentrum Julich GmbH) for participation in the NBI installation and commissioning on TCV, to Dr. David L. Keeling and Tim Robinson (EURATOM/CCFE Fusion Association, [6] [7] [8] [9] A. Fasoli for the TCV Team; Overview of physics research on the TCV tokamak; Nucl. Fusion 49 104005 (2009) S. Coda for the TCV Team; The science program of the TCV tokamak: exploring fusion reactor and power plant concepts; Nucl. Fusion 55 104004 (2015) A. Fasoli for the TCV Team; TCV heating and in-vessel upgrades for addressing DEMO physics issues; Nucl. 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