Bolgert, P.

Drevlak, M.

Gates, D.

Lazerson, S.

McMillan, M.

MPG.PuRe

Predictions of Neutral Beam Deposition and Energetic Particle Losses inW7-XS. Lazerson1, M. Drevlak2, P. Bolgert1, D. Gates1, M. McMillan3, and the W7-X Team21 Princeton Plasma Physics Labortory, Princeton, U.S.A.2 Max-Planck-Institut für Plasmaphysik, Greifswald, Germany3 Christ’s College, Cambridge, U.K.The demonstration of favorable energetic particle confinement is a key requirement for anymagnetically confined fusion device to be considered for reactor development. In deuterium-tritium reactions, Helium atoms are born with 3.5 MeV energies which must slow down throughcollisions with the bulk plasma (thereby heating it). If these particle contact the wall beforetransferring the majority of their kinetic energy to the plasma, the device may have difficultyreaching a burning state. To address such particle confinement physics, the W7-X [1] devicehas been fitted with two neutral beam injectors. These injectors accelerate neutral hydrogenspecies to 55 keV which then charge exchange to become energetic ions in the plasma. Suchenergies provide a good mimic of the Helium particles produced in a reactor (the so-called ρ∗scaling). Modeling of such neutral beam systems [2] allows optimization of the W7-X magneticstructure for improved confinement of such energetic particle species. In this work we examinepredictions of neutral beam injection for W7-X.Figure 1: Beamline geometry for the Q3, Q4, Q7 and Q8sources. Magnitude of the magnetic field is depicted for thestandard configuration.In order to accurately model con-finement of neutral beam energetic par-ticles a detailed model is required.The neutral beams in W7-X accel-erate neutral hydrogen to 55 keV(60 keV for deuterium). These par-ticles must then traverse the plasma,charge-exchanging, thus becoming en-ergetic ions (or they passing through theplasma without ionization, the so-called‘shine-through’). This process is a func-tion of the plasma densities, tempera-tures, stellarator magnetic equilibrium,and beam-line geometry. Once ionizedthe newly formed energetic ions will or-43rd EPS Conference on Plasma Physics P4.019bit in the magnetic field of the device. These trajectories are modified through collisions withthe plasma (slowing-down and pitch angle scattering) along with magnetic fluctuations in theplasma (MHD modes, RF heating, etc.). The BEAMS3D code [2] provides us with such a toolfor prediction and analysis of such a system. Interfaced to the VMEC equilibrium code [3] andusing a virtual casing principle [4], BEAMS3D can follow particle trajectories from the neutralbeam injector, traversing the plasma (where they may ionize), and all the way to the vessel wall.The neutral beam system on W7-X is composed of two neutral beam injectors with foursources each (replicas of the ASDEX-Upgrade NBI-system) [5]. The port structure throughwhich the neutral beams shine has a generally radial orientation. Each neutral beam assemblycontains four sources resulting in four distinct beam-lines each. The sources are oriented suchthat they fire across the centerline of the beam assembly. This results in two beam lines beingmore radial and two more tangential. Given the three dimensional topology of the stellarator,each neutral beam assembly is situated either 30 cm above or below the mid plane of the device.For startup two sources were identified in each neutral beam assembly as being scrapped off onthe port armor or possibly hitting an un-armored section of the first wall. Thus the decisionwas made to use only two sources in each beam line, sources Q3 (radial), Q4 (tangential), Q7(radial), and Q8 (tangential) [6]. The sources themselves are designed to inject 55 keV H0 (60keV D0) at more than 1 MW each. The ‘radial’ sources inject at 1.1 MW (1.8 MW D0) andthe tangential sources inject at 1.3 MW (2.0 MW D0). Figure 1 depicts the beam line geometryagainst the equilibrium magnetic field of the standard configuration.Figure 2: Predictions of neutral beam shine-through for the 6 magnetic configurations in W7-X. The Q3and Q4 beam lines inject in the anti-parallel direction, while Q7 and Q8 inject parallel to the magneticfield.43rd EPS Conference on Plasma Physics P4.019Figure 3: Neutral beam birth profiles for the 55 keV H+ions (Q3 Radial Source).Neutral beam shine-through will pro-vide an early validation of the physi-cal models employed in out model. Atlower plasma densities not all neutralparticles ionize, the un-ionized particlesthen pass through the plasma and con-tact armored tiles on the plasma vesselwall [6]. Figure 2 depicts estimates forshine through for the 6 baseline config-urations of W7-X. In these simulationsonly the 55 keV Hydrogen componentof the beams is considered. All config-urations share the same density (central electron density 8×1019 m−3) and temperature profiles(Ti0 = 3.2 keV and Te0 = 2.9 keV). This places focus on the geometrical aspects of the con-figurations. Such analysis neglects the effects of magnetic configuration on confinement. Allbeams with ‘radial’ configurations indicate the greatest level of particle shine-through. In ev-ery configuration the ‘tangential’ beam lines indicate approximately 5% less shine-through.Although some variation exists between magnetic configurations, it is unlikely that such dif-ferences would be measurable. Comparison with experimental measure of shine-through willconfirm if the ADAS cross sections are sufficient for capturing the bulk of the ionization physics(ion-impact, charge-exchange, electron-impact ionization).Figure 4: Velocity space distribution of the four beam linesat differing plasma energies. Colors indicate energy whilemarkers indicate beam lines.Simulations such as these allow in-vestigation of neutral beam parameterswhich are difficult to directly measure.Figure 3 depicts the birth profile for the‘radial’ Q3 source at full energy (55keV). All configurations show similarbirth profiles for the full energy com-ponent of the beam. This suggests thatconfiguration variation should not playa large role in particle birth profiles(controlling for transport changes, sim-ilar temperature and density profiles).Examination of the full, half, and third43rd EPS Conference on Plasma Physics P4.019energies indicate that the lower energy particles are ionizing at larger radii. The increased ion-ization rate in the core can be attributed to a smaller shine through for that component of thebeam. Figure 4 depicts the pitch angles at which the particles are born. As intuition would dic-tate, the tangential beam lines produce particles with smaller pitch angles that those of the radialbeams.In this work the deposition of the W7-X neutral beam system has been analyzed with theBEAMS3D neutral beam code. Shine through predictions for the neutral beam system indicatea rather weak variance on equilibrium configuration. With the the tangential sources indicat-ing a 5% increase in coupling to the plasma over the radial sources. Estimation of the birthprofiles for energetic ions show little difference between configurations. A clear separation inphase space for particle births locations is also present. Here the full, half and third energies areclearly visible. There is some overlap between the radial and tangential sources, but a distinc-tion between the populations can clearly be made. Future work will address energy depositionand current drive estimates, while a detailed wall model will allow estimation of particle losslocations.Notice: This manuscript has been authored by Princeton University under Contract Number DE-AC02-09CH11466with the U.S. Department of Energy. The publisher, by accepting the article for publication acknowledges, that theUnited States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or repro-duce the published form of this manuscript, or allow others to do so, for United States Government purposes.This work has been carried out within the framework of the EUROfusion Consortium and has received fundingfrom the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views andopinions expressed herein do not necessarily reflect those of the European Commission.References[1] Sunn Pedersen T and Andreeva T and Bosch H S and Bozhenkov S and Effenberg F and Endler M and FengY and Gates DA and Geiger J and Hartmann D and Hölbe H and Jakubowski M and König R and Laqua H Pand Lazerson S and Otte M and Preynas M and Schmitz O and Strange T and Turkin Y and the W7-X TeamNuclear Fusion 55, 126001 (2015)[2] McMillan M and Lazerson S Plasma Physics and Controlled Fusion 56, 095019 (2014)[3] Hirshman S P and Whitson J C Physics of Fluids 25, 3553 (1983)[4] Lazerson S Plasma Physics and Controlled Fusion 54, 122002 (2012)[5] Stäbler A and Feist J H and Speth E and Dunne J L and Goetz S and Heinemann B and Krauss A and Kunze RC and Lonhert H and Sielanko J and Szyszko W and Vollmer O and Wittenbencher K Fusion Tech. 620-624(1988)[6] Rust N and Heinemann B and Medelevitch B and Peacock A and Smirnow M Fusion Engineering and Design86 723 (2011)43rd EPS Conference on Plasma Physics P4.019

English