Nuclear fusion with polarized fuel

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Springer Proceedings in Physics 187 Giuseppe Ciullo Ralf Engels Markus Büscher Alexander Vasilyev Editors Nuclear Fusion with Polarized Fuel Springer Proceedings in Physics Volume 187 The series Springer Proceedings in Physics, founded in 1984, is devoted to timely reports of state-of-the-art developments in physics and related sciences Typically based on material presented at conferences, workshops and similar scientific meetings, volumes published in this series will constitute a comprehensive up-to-date source of reference on a field or subfield of relevance in contemporary physics Proposals must include the following: – – – – – name, place and date of the scientific meeting a link to the committees (local organization, international advisors etc.) scientific description of the meeting list of invited/plenary speakers an estimate of the planned proceedings book parameters (number of pages/articles, requested number of bulk copies, submission deadline) More information about this series at Giuseppe Ciullo Ralf Engels Markus Büscher Alexander Vasilyev • • Editors Nuclear Fusion with Polarized Fuel 123 Editors Giuseppe Ciullo Dipartimento di Fisica e Scienze della Terra Polo Scientifico e Tecnologico Ferrara Italy Markus Büscher Forschungszentrum Jülich Peter Grünberg Institute Jülich Germany Ralf Engels Forschungszentrum Jülich Institute for Nuclear Physics Jülich Germany Alexander Vasilyev National Research Centre “Kurchatov Institute” Gatchina Russia ISSN 0930-8989 Springer Proceedings in Physics ISBN 978-3-319-39470-1 DOI 10.1007/978-3-319-39471-8 ISSN 1867-4941 (electronic) ISBN 978-3-319-39471-8 (eBook) Library of Congress Control Number: 2016942029 © Springer International Publishing Switzerland 2016 This work is subject to copyright All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed The use of general descriptive names, registered names, trademarks, service marks, etc in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made Printed on acid-free paper This Springer imprint is published by Springer Nature The registered company is Springer International Publishing AG Switzerland Foreword Energy in a wider sense is what drives human efforts to ensure not only survival of a growing world population, but survival under human conditions Especially, as long as the population growth cannot be contained, this inevitably means a growing energy demand for a long time on a world scale On the long run limited energy resources such as coal or hydrocarbons and even fuel for nuclear fission energy production will be exhausted Only “sustainable” energies such as from solar and wind resources, as well as from nuclear fusion with its large amounts of available fuel, will remain viable options A host of problems connected with energy production and living under human conditions has not yet been addressed: storage of energy to maintain its continuous flow, agriculture and production of sufficient food for all, increase of the amount of partly poisonous waste, climate changes by rising world temperatures, etc To all solutions of these interconnected problems, fusion energy could contribute substantially if realized An example could be desalination of seawater to fertilize arid African areas thus preventing large-scale population migrations The radioactive waste from fusion processes will not cause unmanageable problems Despite the admittedly slow progress of approaching the energetic break-even, there is a hope that the different paths towards this goal, magnetic (“tokamak” or “stellarator”) or inertial (e.g., “laser”) confinement fusion, will be successful An old and somewhat forgotten or postponed idea is that with the use of spin-polarized fuel particles (D, T, and 3He) the yield of the nuclear fusion reactions could be enhanced, in the cases of the T(d, n)4He and 3He(d, p)4He reactions even by up to 50 %, thus suggesting, for e.g., a lower break-even threshold and/or lower required input power A number of other parameters of the fusion plasma, such as the emission directions of reaction products, could also be controlled by preparing the spin states of the fuel particles accordingly The technologies and the v vi Foreword understanding of the production of spin-polarized beams or targets have reached a stage from which promising developments of spin physics towards fusion energy applications can start This, however, would require new efforts and resources in the field The status of the field and the new ideas have been summarized in this volume Cologne Prof Dr Hans Paetz gen Schieck Contents Polarized Fusion: An Idea More Than Thirty Years Old! What Are We Waiting For? Giuseppe Ciullo Spin Physics and Polarized Fusion: Where We Stand H Paetz gen Schieck The PolFusion Experiment: Measurement of the dd-Fusion Spin-Dependence Alexander Vasilyev, L Kochenda, P Kravtsov, V Trofimov, M Vznudaev, Giuseppe Ciullo, P Lenisa, Ralf Engels and H Paetz gen Schieck Hyper-Polarized Deuterium Molecules: An Option to Produce and Store Polarized Fuel for Nuclear Fusion? Ralf Engels, G Farren, K Grigoryev, M Mikirtychiants, F Rathmann, H Seyfarth, H Ströher, L Kochenda, P Kravtsov, V Trofimov, Alexander Vasilyev, M Vznudaev and H Paetz gen Schieck A Polarized He Target for the Exploration of Spin Effects in Laser-Induced Plasmas I Engin, Markus Büscher, P Burgmer, K Dahlhoff, Ralf Engels, P Fedorets, H Feilbach, U Giesen, H Glückler, F Klehr, G Kukhalashvili, A Lehrach, T Leipold, W Lesmeister, S Maier, B Nauschütt, J Pfennings, M Schmitt, H Soltner, K Strathmann, E Wiebe and S Wolf 15 35 45 55 Relevant Spatial and Time Scales in Tokamaks F Bombarda, A Cardinali and C Castaldo 69 Depolarization of Magnetically Confined Plasmas R Gatto 79 vii viii Contents Ion Polarization in Magnetic Fields 107 S Bartalucci Prospects for Direct In Situ Tests of Polarization Survival in a Tokamak 115 A.M Sandorfi and A D’Angelo 10 DD Fusion from Laser Interaction with Polarized HD Targets 131 J.P Didelez and C Deutsch 11 Polarization of Molecules: What We Can Learn from the Nuclear Physics Efforts? 139 D.K Toporkov, D.M Nikolenko, I.A Rachek and Yu.V Shestakov 12 RF Negative Ion Sources and Polarized Ion Sources 145 N Ippolito, F Taccogna, P Minelli, V Variale and N Colonna Index 153 Contributors S Bartalucci INFN - Laboratori Nazionali di Frascati, Frascati, Rome, Italy F Bombarda ENEA, Frascati (Rome), Italy P Burgmer Peter Grünberg Institut, Jülich, Germany Markus Büscher Peter Grünberg Institut, Jülich, Germany; Institut für Laser- und Plasmaphysik, Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany A Cardinali ENEA, Frascati (Rome), Italy C Castaldo ENEA, Frascati (Rome), Italy Giuseppe Ciullo Istituto Nazionale di Fisica Nucleare (INFN) of Ferrara and Physics and Earth Science Department, University of Ferrara, Ferrara, Italy N Colonna INFN, Bari, Italy A D’Angelo Università di Roma Tor Vergata, Roma, Italy; INFN Sezione di Roma Tor Vergata, Roma, Italy K Dahlhoff Zentralinstitut für Engineering, Elektronik und Analytik, Jülich, Germany C Deutsch LPGP, Université Paris-Sud (UMR-CNRS 8578), Orsay, France J.P Didelez IPN, CNRS/IN2P3 & Université Paris-Sud (UMR-CNRS 8608), Orsay, France Ralf Engels Institut für Kernphysik, Forschungszentrum Jülich, Julich, Germany I Engin Institut für Kernphysik, Jülich, Germany G Farren Institut für Kernphysik, Forschungszentrum Jülich, Jülich, Germany P Fedorets Institut für Kernphysik, Jülich, Germany ix 140 D.K Toporkov et al thanks to the method of laser pumping and the polarization of helium atoms in the range of 80 % can be obtained at a production rate of about 1019 atoms s−1 [2] 11.2 Sources of Polarized Hydrogen Isotopes A brief history of the development of polarized atomic hydrogen sources is given in [3] The modern sources of polarized atoms with a high degree of polarization of the atoms in the range of ≈90 % may provide the maximum intensity up to 1017 atoms s−1 , which is certainly insufficient to feed a tokamak reactor, that requires at least 1021 atoms s−1 The parameters of the modern atomic beam sources (ABS) and the achieved intensities are given in [3] The basis of the ABS is the classical Stern–Gerlach method—a spatial separation of atoms in an inhomogeneous magnetic field In an inhomogeneous B-field, atoms are driven towards or away from high-field regions, depending on the sign of projection of the magnetic moment of the atoms It should be noted that the magnetic moment of an atom is determined by the magnetic moment of an electron as it is much larger than the nuclear moment Usually sextupole magnets are used, where the magnitude of the B-field is proportional to r (r is the distance from the axis of the magnet) In this case, atoms, emitted from a point-like source, placed on the axis at the entrance of a cylindrical bore magnet of radius r m , will oscillate around the magnet axis They pass through the magnet without hitting the pole pieces, if their velocity lies within the solid angle ΔΩmag = π μ B Bm , kT (11.1) where ΔΩmag is the magnet acceptance solid angle averaged over the velocity distribution of the beam, μ B the Bohr magneton, Bm the field strength at the location of the pole-tip r m , k the Boltzmann constant, T is the temperature of the beam-shaping nozzle In practice the nozzle of atoms is located at some distance from the magnet entrance and the acceptance solid angle of such a system is a convolution of the magnet acceptance solid angle and the geometrical one For Bm = T and T = 100 K ΔΩmag results as 0.02 sr The pole-tip field is limited to 1.7 T [4] for the segmented sextupole permanent magnets and to 4.8 T for superconducting magnets [5] In order to obtain the highest flux of polarized atoms from an ABS, one should use magnets with the highest magnetic pole-tip field and lower the temperature of the gas Unfortunately at low nozzle temperatures the recombination of atoms in the dissociator and the scattering processes in the beam increase Figure 11.1 presents some results on the intensity measurement at different distances and different nozzle temperature These measurements were performed with a free molecular beam in order to avoid complications from changes in the degree of dissociation One can see that at low nozzle temperatures or at large distance from the nozzle, the intensity of the beam is saturated earlier and no longer depends on the gas flow through the nozzle Such 11 Polarization of Molecules: What We Can Learn from the Nuclear Physics Efforts? 141 Fig 11.1 Intensity of the free molecular beam measured as a function of the gas flow through the nozzle Left—for different distances from the nozzle—measured at FILTEX (D.Toporkov 1991) Right—at a fixed distance for different nozzle temperatures [6] a behavior is explained by the process of intra-beam scattering, i.e scattering that occurs between the faster particles of the beam overtaking slower particles within the beam Some estimation of this effect is given in [3] The ABS intensity seems to saturate at a flux of polarized atoms of about 1017 atoms s−1 11.3 Source of Polarized Molecules As it was shown in the previous section, the main process limiting the intensity of the atomic beam in ABS is the intra-beam scattering inside the dense supersonic beam The polarized hydrogen or deuterium atoms from an ABS cannot be stored, because they are radicals that react with most materials or recombine in a very short time During these reactions the nuclear polarization is lost at least partially A successful attempt has been performed to produce polarized hydrogen (deuterium) molecules, when polarized hydrogen (deuterium) atoms from an ABS recombine into molecules without loss of initial polarization [7] However, the production rate of polarized molecules by this method can not exceed the flow rate of atoms from an ABS The polarized deuterium molecules could be frozen to ice and used for laser-induced inertial fusion or as a fuel for tokamak reactor To avoid the chain: molecules—polarized atoms—polarized molecules, a suggestion of direct production of the polarized molecules from a D2 gas through a spatial separation of the different nuclear-spin states has been proposed [8] In a molecule the electron spins are paired and, therefore, the magnetic moments of the electrons cancel Due to the much smaller nuclear magnetic moment, as compared to the electron, the focusing power of the sextupole magnets of a standard ABS is not sufficient to induce a clear separation of the molecules in different spin states The only way to overcome this 142 D.K Toporkov et al problem is to apply much stronger magnetic field gradients and to increase the interaction time of the molecules in the magnetic field area To fulfill these conditions long superconducting magnets of higher pole numbers are a possible option The cold surface of these magnets can be used as a cryopump for the molecules with nuclear spins parallel to the external magnetic field, because they will be defocused with respect to the magnets symmetry axis Molecules with anti-parallel nuclear spins are not influenced by magnetic field and can be stopped by aperture limiters or inside the magnets when the diameter of the cylindrical magnets decreases along the beam line Some possible configuration of such a source were discussed in [9] We are going to use our ABS with superconducting magnets to prove the idea of the production of nuclear polarized molecules The details of the proposed research setup are presented in Fig 11.2 Two downstream superconducting magnets of the ABS with a magnetic pole-tip field up to 4.8 T are planned to be used for the focusing of the molecules with negative total momentum projection into the compression tube placed at the distance of 1500 mm from the magnet exit The diameter of the compression tube is 30 mm and a length of 200 mm A quadrupole mass spectrometer will be installed for measuring the pressure in the chamber of the compression tube The nozzle of the hydrogen molecules is a copper block having a circular slit with a diameter of 41 mm and a width of 0.1 mm located 36 cm from the entrance of the first magnet The source is mounted on the vessel with liquid helium and the temperature of the source can be varied in wide range down to 15 K A circular aperture limiter will be installed at the entrance of the first magnet forming a ring diaphragm with a width of mm and central diameter of 41 mm In such a geometry the direct ballistic flux of molecules cannot reach the compression tube Hydrogen molecules in the ortho-state (S = 1, m S = −1 both proton spins are anti-parallel to the magnetic field axis) will be slightly focused on the beam axis Molecules in the state (S = 1, m S = 1) will be defocused in opposite direction and hit the cold surface of the magnets where they will be frozen out Molecules in the state S = 1, m S = and the para-state S = 0, m S = will not be influenced by the magnetic field and, therefore, they will follow their ballistic trajectories and will miss the entrance of the compression tube Only molecules in the state with S = 1, m S = −1, which Fig 11.2 The scheme of the proposed arrangement, to obtain polarized hydrogen molecules using existing ABS with superconducting sextupole magnets For detail description see text 11 Polarization of Molecules: What We Can Learn from the Nuclear Physics Efforts? 143 are slightly focused by the strong gradient of the magnetic field near the pole tip of the superconducting magnets can enter this compression tube These molecules can be detected via an increased pressure in the compression tube, when the magnet is switched on Without the possibility to switch the sextupole magnets on/off the experiment would not be feasible To reach the necessary sensitivity of the pressure gauge or quadrupole mass spectrometer, a powerful differential pumping system for the non focused molecules is essential to decrease the background pressure around the compression tube to values in the order of 10−9 mbar and below All other molecules leaving the source should be pumped by the surface of the helium vessel, which temperature can be lowered down to 2.2 K The pumping speed of this pumping system has to be high enough to realize a free molecular flow between the source of molecules and the separating magnet This is a key point, which may limit the total flux of hydrogen molecules from the source A molecular beam source should be capable to produce an intense, well collimated beam of molecules with a minimum of background gas density in the chamber At present the nozzle for the molecules is a ring slit and in future it will be replaced by a glass capillary array which will provide a more intense beam in forward direction at the same total throughput Preservation of the nuclear polarization in a molecule during the transportation remains an open question Only limited information on nuclear polarization of hydrogen molecules from recombination of polarized atoms is available [7] 11.4 Conclusions We are going to produce polarized hydrogen molecules using the existing ABS with superconducting sextupole magnets and to prove the ability of this method To produce a high flux of polarized molecules one has to employ a longer superconducting magnet with a big aperture and a large number of poles In such a magnet the gradient of the magnetic field will be concentrated near the pole tips and will provide good spatial separation of the molecules Some details of the geometry of the proposed magnet are given in [9] References Ch Leemann et al., Helv Phys Acta 44, 141 (1971) S Karpuk et al., Phys Part Nucl 44, 904 (2013) D Toporkov, in XVth International Workshop on Polarized Sources, Targets, and Polarimetry September 9–13, 2013 Charlottesville, Virginia, USA (PoS, PSTP 2013), p 064 A Vassiliev et al., Rev Sci Instrum 71, 3331 (2000) L Isaeva et al., Nucl Instrum Methods A 411, 201 (1998) T Wise, A.D Roberts, W Haeberli, Nucl Instrum Methods A 336, 410 (1993) R Engels et al., Rev Sci Instrum 85, 103505 (2014) 144 D.K Toporkov et al Yu Shestakov et al., in Proceedings of 13th International Symposium on High Energy Spin Physics September 8–12 (Protvino, Russia, 1998), p 415 D.M Nikolenko et al., in Proceedings 14th International Workshop on Polarized Sources, Targets and Polarimetry, PSTP 2011 (St Petersburg, 2011), p 73 Chapter 12 RF Negative Ion Sources and Polarized Ion Sources N Ippolito, F Taccogna, P Minelli, V Variale and N Colonna Abstract The requirement of a neutral beam injection system with hydrogen or deuterium beam energy up to MeV for the ITER project has recently triggered new research on negative ion sources, from production to acceleration and neutralization before the injection in the tokamak New and more reliable negative ion sources are being developed, both within the ITER project and in the perspective of other research tokamaks Furthermore a renewed interest in polarized fusion triggered new studies also in polarized ion sources A review of our last results in modelling a typical radiofrequency hybrid negative ion source is here reported, together with a brief introduction to the atomic-beam polarized ion sources A potential future integration between the two research fields is discussed 12.1 Introduction: Negative Ion Sources Neutral beam injection heating has been a reliable and powerful method to heat and drive the current of fusion plasmas At energies above 100 keV/nucleon, the neutralization efficiency for positive ions decreases drastically while staying at around 60 % for negative ions Therefore, a negative-ion-based Neutral Beam Injection (NBI) system is inevitable for a large-scaled and current-driven fusion experiment such as ITER (International Thermonuclear Experimental Reactor), where an injection energy of MeV is needed [1] It is generally accepted [2, 3] that H− ions are produced in two different ways: • in the volume by dissociative attachment of slow electrons (Te < eV) to highly vibrationally excited levels of hydrogen molecules H2 ; N Ippolito (B) · V Variale · N Colonna INFN, via Orabona, 4, 70125 Bari, Italy e-mail: F Taccogna · P Minelli CNR-NANOTECH, via Amendola, 122, 70126 Bari, Italy © Springer International Publishing Switzerland 2016 G Ciullo et al (eds.), Nuclear Fusion with Polarized Fuel, Springer Proceedings in Physics 187, DOI 10.1007/978-3-319-39471-8_12 145 146 N Ippolito et al • on caesiated surfaces by atomic conversion, because caesium lowers the work function of the metal surface and increases therefore the electron transfer probability to the particle hitting the surface In both cases the production of negative ions requires the formation of precursors: vibrationally excited hydrogen molecules for volume production and hot atoms for surface production Therefore, it is clear that the negative ion source requires a region where molecular vibrational excitation and dissociation processes are largely promoted These two mechanisms are optimized in the Radio-Frequency Inductively Coupled Plasma (RF-ICP) discharge selected by the ITER board as the official source of the tokamak and shown in Fig 12.1 [4] This hybrid (volume and surface negative ion production) source consists of three parts: firstly, the cylindrical driver, where RF coils are used to generate a H2 plasma Secondly, the rectangular expansion region, where the plasma expands into the actual source body, and finally the extraction region The latter two are separated by a nonhomogeneous magnetic field of the order of 10 mT, called magnetic filter field (see Fig 12.1) The extraction system is a three-grid system consisting of the Plasma Grid (PG), the Extraction Grid (EG) and the Grounded Grid (GG) The extraction grid is equipped with permanent magnets, in order to separate the co-extracted electrons from the negative ion beam by bending them differently The coverage of surfaces with a thin layer of caesium is achieved by evaporating caesium from an oven mounted on the back flange of the source body By introducing caesium, ion current densities have been measured, which were an order of magnitude higher, with a simultaneous reduction of co-extracted electrons by a factor of ten Fig 12.1 Sketch of the RF-ICP hybrid negative ion source 12 RF Negative Ion Sources and Polarized Ion Sources 147 It is fundamental to study and model the gas kinetic coupled with the plasma dynamics inside the RF-ICP negative ion source In the next two sections the expansion region models of neutral and plasma behaviour will be described and results will be presented We always will deal in the next sections with hydrogen, but similar analysis will be done in the next future for deuterium 12.2 Gas Kinetics and Dynamics in the Expansion Region In order to understand the formation of negative ion precursors, a Direct Simulation Monte Carlo (DSMC) model (from the exit plane of the driver to the entrance of the extraction region, cm from the PG) has been developed In the expansion region, the plasma is continuously replenished from the driver region Therefore, the plasma subsystem is considered as a fixed background An axial decay of the electron and ion density and temperatures is used according to the results of the plasma model presented in the next section Into this plasma background, neutral pseudo-particles of H and H2 are launched from the source line (at the driver exit plane at z = 0) with a half-maxwellian velocity distribution and a Boltzmann vibrational population For the atoms a translational temperature of TH = 12000 K has been considered, while for molecules a translational temperature TH2 = 1200 K equal to the vibrational temperature Tν was chosen [5] The ratio of H/H2 = 0.2 [5] between atomic and molecular density is fixed at the source location An open boundary condition is implemented on the right side of the simulation domain All the parameters were chosen to represent the experimental conditions as close as possible The first electronic state of hydrogen atoms H (n = 1S) is considered In fact, due to the typical neutral time scale (Δt ≈ 10−8 s), the spontaneous emission process is considered fast and all the electronic excited states of H decay into the ground state For the molecules, fifteen vibrational levels of the fundamental electronic state of H2 (X1 g+ , v = 0, , 14) are taken into account Owing to the τ RT ∼ τT T (respectively vibrational-translational, scaling of relaxation times τV T rotational-translational and translational-translational relaxation times for energy exchange), one can usually treat the problem of vibrational relaxation by assuming that both the rotational and translational degrees of freedom have already attained equilibrium For the plasma conditions studied here, direct electron-impact excitation [6] appears to be the dominant vibrational heating mechanism Electron excitation processes are distinguished in direct vibrational excitation (eV processes) and vibrational excitation caused by the radiative decay of higher singlet electronic states (EV processes) The formers involve excitations only with jumps |ν f − νi | < 5, while the latters are not limited in the vibrational level jumps Proton induced excitation (pV), charge exchange (p-CX) and dissociation (pV-diss) are also included Concerning the destruction processes of vibrational states, electron-induced electronic excitations, ionization, dissociation, dissociative ionization and dissociative attachment [6] of 148 N Ippolito et al Fig 12.2 Axial profiles for gas density and H atom temperature H2 molecules are taken into account in the model Due to the low-pressure regime, neutral-neutral relaxation processes have been neglected To quantify surface produced H− by neutral conversion on the PG, neutral atom density and translational temperature have been computed and their axial profiles are shown in Fig 12.2 The effect of H pumping by molecular dissociation and by wall H+ neutralization gives a relatively high atomic temperature, that reaches a value of 0.75 eV in front of PG as shown in Fig 12.2 12.3 Plasma Kinetics and Dynamics in the Expansion Region The plasma model consists of a Particle-in-Cell/Monte Carlo Collision (PIC-MCC) technique [7] simulating the region going from the driver exit plane till the extraction grid EG plane In this preliminary version of the model the driver is not yet simulated and a prescribed flux of maxwellian plasma is injected from the driver (injection conditions are reported in Table 12.1) [8] + Electrons, volume-produced and surface-produced negative ions, H+ , H+ and H3 are simulated, while a fixed atom and molecular density are set with a prescribed vibrational distribution taken from the gas model described in the previous section The z (perpendicular to PG) and y coordinates are simulated while uniformity is considered along x direction (magnetic filter direction) This choice is justified by the interest in studying the top-bottom dishomogeneity driven by E × B and diamagnetic ∇ P × B (where ∇ P is the pressure gradient) drifts related to the magnetic filter field The 2.5 dimensionality is due to the fact that even if the self-consistent electricfield 12 RF Negative Ion Sources and Polarized Ion Sources 149 Table 12.1 Injection conditions of the simulation Physical parameters Value H− density current from PG (A m−2 ) Plasma density from driver n e/H+ /H+ (m−3 ) 660 6/2.4/3.6 1017 Plasma temperature from driver Te/H+ (eV) 12/1/1 (m−3 ) Gas density n H/H2 Gas temperature TH/H2 (eV) PG-plate bias (V) EG bias (kV) Aperture diameter (mm) PG thickness (mm) j 1/4 1019 0.8/0.1 10 is not solved in the x-direction, the particles are tracked along this direction and particle-wall interaction with a thin sheath is considered at x-boundaries as follows: if electrons reach the lateral walls with an energy larger than the local electric potential a secondary electron emission is considered with a coefficient equal to 0.2, otherwise a sheath mirror reflection is applied; negative ions reaching the wall are lost if their energy is larger than the local potential, otherwise they are reflected back; finally, all positive ions are lost Based on this approximation the circular openings become slits In the present model, ten flat apertures are considered The diameter of each is Dh = 10 mm, while the gap between each aperture (from one center to the other center) is G h = 20 mm The filter field assumes a bell-shaped z-profiles with a peak cm from PG with Bx,max = mT (no y-dependance is included and the possible mirroring effects due to the filter field curvature are neglected), while the electron deflection field (y and z components) in each aperture has an alternating y-versus from one aperture to the other A fixed negative ion flux emitted from the PG surface by neutral conversion is set at JH− ,0 = 660 A m−2 The bias plate and PG bias are set to a potential φ B P = φ P G = 0, while the EG potential is φ E G = kV Different bulk collisions are implemented using MCC technique The list of relevant collisions are reported in [9] Finally, in order to make the simulation possible, a larger vacuum permittivity is used with = 25 This value still allows a detailed resolution of the single aperture, which results composed by 25 cells The cell size is Δz = 10−4 m (the larger vacuum permittivity allows to use a five times larger cell) with a total number of grid points N g = N y Nz = (1450) (589) The macroparticle weight used is w = 108 The electron current extracted is not uniformly distributed along the different apertures but it is strongly dis-homogeneous Such a picture is confirmed by the 2D map of the electron density reported in Fig 12.3 (left) It is evident the plasma asymmetry generated by a Hall effect in the filter already predicted by expansion region models [10] The electrons are drained towards the bottom wall due to the Hall drifts (the electric field and the pressure gradient are directed in the z-direction 150 N Ippolito et al Fig 12.3 Maps of (left) electron density and (right) plasma potential in the expansion region of the RF-ICP negative ion source while the filter field is along x-direction) Here the interaction with the wall and the formation of a fluctuation in the electric potential along y-direction, see Fig 12.3 (right), induce an extra (anomalous) transport across the filter field lines and an oblique electron flux structure in the filter region 12.4 Polarized Ion Atomic-Beam Sources Polarized fusion, meaning the use of polarized fuel for magnetically or inertially confined fusion reactions, is an old idea that is now encountering a renewed interest [11] This is basically due to the possibility of lowering the ignition point (increasing the reaction rate) and giving preferential directions to the fusion products [12], so mainly reducing the wall damage and allowing a better control on the whole machine A further possibility, especially in view of DEMO reactor (DEMOnstration Power Plant: it is the nuclear fusion power plant that should be built after ITER to demonstrate the feasibility of production of electrical energy from nuclear fusion) or so-called “second generation” tokamaks, is to reduce or suppress neutron-producing reactions 12 RF Negative Ion Sources and Polarized Ion Sources 151 in favor of aneutronic reactions, with obvious advantages in terms of wall damage and activation of the containing structures A fundamental step in the road to polarized fusion is the preparation and injection of polarized fuel into the reactor The generation and extraction of a polarized atomic beam (hydrogen or deuterium at first) is a key feature in that sense, just like in the usual not-polarized fusion we described in Sect 12.1 In this section we briefly review the operation scheme of a particular type of polarized ion source [13], the Atomic Beam Source (ABS), being of major interest for possible future applications of the RF-ICP source we described above We leave out in this paper the equally important Optically-Pumped Polarized Ion Sources (OPPIS) and Lamb-Shift ion Sources (LSS) A typical Atomic-Beam ion source starts with a dissociator, a device where molecules (H2 or D2 ) are dissociated by an RF field, giving an output beam of thermal H or D atoms Usually a low velocity beam is asked for at the exit of the dissociator, mainly to avoid an excessive heating of the vessel and to achieve higher beam densities to increase the efficiency of the subsequent ionization (see below), so the exit nozzle of the dissociation may be cooled under 100 K The resulting atomic beam is then passed through an arrangement of sextupole separating magnets (usually several separated sextupoles are used), acting as a focusing lens for the electron magnetic moments aligned with the field gradient (Stern-Gerlach effect) The electrons with magnetic moments aligned in the opposite direction are defocused, so the atomic beam comes out with (in principle) a high electron-spin polarization This electronpolarized beam is then passed through appropriate radio-frequency fields in order to induce hyperfine transitions between different states, selecting the hyperfine states with similar nuclear polarization The electron-spin polarized beam is so converted into a nuclear-spin polarized beam This beam is then ionized for ion acceleration A number of different ionizers are currently used with ABS sources The first issue to face with is whether negative o positive ions are required for acceleration For energies above 100 keV/nucleon only negative ions are suitable for successive efficient neutralization, but for compact tokamaks or different machines even positive ions are eligible To form positively charged ions, electron impact ionizers are usually used (hot-filament or Electron-Cyclotron-Resonance (ECR) plasma produced), while negative ions can be obtained by different charge exchange schemes: one possibility is to accelerate the positive ions and then direct the beam through an alkali vapor target to obtain negative ions by double charge exchange; a second method requires the (slow) positive ion beam to cross a counter-flowing Cs atomic beam, which gives larger currents and lower emittance, but also troublesome Cs sputtering at the apertures A third method [13] is very promising and could furthermore take advantage of the RF-ICP negative ion sources we are currently studying and modelling, as described in previous sections A deuterium plasma injector is indeed used as a ionizer to produce polarized negative hydrogen ions, exploiting the large charge exchange cross section of the reaction: H0↑ + D− =⇒ H− ↓ +D 152 N Ippolito et al A polarized ion source of this kind has been developed at the Institute of Nuclear Research (INR) of Moscow [13] with an arc-discharge plasma source With this ionizer, currents of several mA of polarized H− ions have been reached, with ionization percentage up to 90 % In our opinion this type of ionizer is particularly interesting for future polarized ion sources, since the use of RF-ICP could not only improve the results already obtained by providing higher densities of D− ions, but it also allows a different scheme where the fast D− ions bombard polarized H atoms in order to more efficiently ionize them 12.5 Conclusions Fusion-related ion sources are now experiencing an enormous development, aiming at optimizing them for the extremely demanding requirements of fusion devices Within this environment, we are currently working to a PIC-MCC method modelling of a typical RF hybrid negative ion source to be used in ITER, DEMO or more compact tokamaks On the other hand, if polarized fusion is considered for future fusion reactors, polarized ion sources will have to be developed In this sense, an integration of state-of-the-art expertise in the fields of unpolarized and polarized ion sources could be particularly fruitful In this respect, we believe that R&D activity on polarized ion sources for fusion application should be pursued in the near future References 10 11 12 13 R.M Hemsworth, J.H Feist, M Hanada et al., Rev Sci Instrum 67, 1120 (1996) M Bacal, Nucl Fus 46, S250 (2006) M Capitelli, M Cacciatore, R Celiberto et al., Nucl Fus 46, S260 (2006) U Fantz, P Franzen, W Kraus et al., Plasma Phys Control Fus 49, B563 (2006) U Fantz, H.D Falter, P Franzen, E Speth, R Hemsworth, D Boilson, A Krylov, Rev Sci Instrum 77, 03A516 (2006) R Celiberto, R.K Janev, A Laricchiuta, M Capitelli, J.M Wadehra, D.E Atems, At Data Nucl Data Tables 77, 161 (2001) D Tskhakaya, K Matyash, R Schneider, F Taccogna, Contribu Plasm Phys 47, 563 (2007) F Taccogna, P Minelli, N Ippolito, Rev Sci Instrum 87, 02B306 (2016) F Taccogna, R Schneider, S Longo, M Capitelli, Phys Plasm 15, 103502 (2008) G Fubiani, J.-P Boeuf, Phys Plasm 21, 073512 (2014) H Paetz gen Schieck, Eur Phys J A 44, 321 (2010) Ch Leemann et al., Helv Phys Acta 44, 141 (1971) A Zelenski, Rev Sci Instrum 81, 02B308 (2010) Index A Astrophysical S–factor, 17, 44 Average global temperature, C CO2 emission, Cross beam experiment, 27 Cross section analyzing power, 19 spin–correlation, 19, 36 D degrees scenario, E Electron screening, 18, 28, 44 Energy resources, I Ion acceleration by radiation, 135 N Neutron suppression, 8, 22, 39, 72, 82, 108 Nuclear fusion cross–section, 17, 71, 118, 132 DD reaction measurements, 43 emission direction control, 7, 37, 39, 81, 119, 125 five-nucleon reactions, 18 four–nucleon reactions, 21, 37 fuel, 74 reaction gain increase, 7, 20, 37, 39, 81, 119, 126 time scales, 73 with polarized fuel, 5, 15, 47 Nuclear polarization He, 56 atomic, 47, 51, 140 collective modes in plasmas, 84 depolarization in plasmas, 111 depolarization mechanisms, 83, 122 deuterium, 52 instabilities in plasmas, 92 molecular, 30, 47, 51, 140 phenomena, 36 relaxation, 57 survival, 31, 74, 80, 82, 108, 123, 136 P Polarimeter Breit-Rabi Polarimeter, 10 Lamb-shift, 40, 49 nuclear, 40 Polarized targets solid HD, 124, 133 PolFusion experiment, 8, 35 Q Quintet Suppression Factor, 8, 24, 38 S Sources gas–jet, 61 neutralized ions, 140 polarized atomic beams, 10, 40, 47, 140 © Springer International Publishing Switzerland 2016 G Ciullo et al (eds.), Nuclear Fusion with Polarized Fuel, Springer Proceedings in Physics 187, DOI 10.1007/978-3-319-39471-8 153 154 T Tokamak DIII-D, 123 FTU, 71, 129 Index Ignitor, 74, 80, 108 ITER, 74, 80 JET, 69 TFTR, 69 ... this field 1.2 Towards Nuclear Fusion with Polarized Fuel The starting point for research on nuclear fusion with polarized fuel is the development of polarized targets for nuclear fundamental physics... come along with the possibilities of polarized fuel for nuclear fusion: i Is it possible to inject the polarized fuel without loss of polarization? ii Can polarization survive in fusion environments?... development of polarized targets at different accelerators for fundamental nuclear physics might give new impulses to the idea of polarized fusion, as they propose a way to produce polarized fuel Fusion
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