Home Search Collections Journals About Contact us My IOPscience A new simpler way to obtain high fusion power gain in tandem mirrors This content has been downloaded from IOPscience. Please scroll down to see the full text. 2017 Nucl. Fusion 57 056014 (http://iopscience.iop.org/0029-5515/57/5/056014) View the table of contents for this issue, or go to the journal homepage for more Download details: This content was downloaded by: simonen IP Address: 104.6.66.72 This content was downloaded on 07/04/2017 at 20:32 Please note that terms and conditions apply. You may also be interested in: The magnetic mirror approach to fusion R.F. Post Gas-dynamic trap: an overview of the concept and experimental results A A Ivanov and V V Prikhodko Reduction of losses in open-ended magnetic traps C. Gormezano Plasma models for standard mirror reactors R.S. Devoto and D.J. Bender Experimental beta limit in an average minimum-B tandem mirror A.W. Molvik, T.A. Casper and A.H. Futch Energetic particle physics in fusion research in preparation for burning plasma experiments N.N. Gorelenkov, S.D. Pinches and K. Toi Plasma Heating and Confinement in Open Systems: Report on the IAEA Technical Committee Meeting, held at Tsukuba, Japan, 8–12 July 1985 S. Miyoshi Plasma physics and controlled nuclear fusion research (Summaries of the 12th IAEA International Conference, Nice, France, 12-19 October 1988) R.R. Parker, J. Sheffield, M. Wakatani et al. Trapped particle stability for the kinetic stabilizer H.L. Berk and J. Pratt IOP Nuclear Fusion International Atomic Energy Agency Nuclear Fusion Nucl. Fusion Nucl. Fusion 57 (2017) 056014 (20pp) https://doi.org/10.1088/1741-4326/aa5e54 57 A new simpler way to obtain high fusion 2017 power gain in tandem mirrors © 2017 IAEA, Vienna T.K. Fowler1, R.W. Moir2 and T.C. Simonen3 NUFUAU 1 University of California, Berkeley, CA, United States of America 2 Vallecitos Molten Salt Research, Livermore, CA, United States of America 3 Berkeley, CA, United States of America 056014 E-mail: [email protected] T.K. Fowler et al Received 26 July 2016, revised 17 January 2017 Accepted for publication 31 January 2017 Published 24 March 2017 Abstract Printed in the UK From the earliest days of fusion research, Richard F. Post and other advocates of magnetic mirror confinement recognized that mirrors favor high ion temperatures where nuclear reaction rates σv begin to peak for all fusion fuels. In this paper we review why high ion NF temperatures are favored, using Post’s axisymmetric Kinetically Stabilized Tandem Mirror as the example; and we offer a new idea that appears to greatly improve reactor prospects at high 10.1088/1741-4326/aa5e54 ion temperatures. The idea is, first, to take advantage of recent advances in superconducting magnet technology to minimize the size and cost of End Plugs; and secondly, to utilize parallel advances in gyrotrons that would enable intense electron cyclotron heating (ECH) in these Paper high field End Plugs. The yin-yang magnets and thermal barriers that complicated earlier tandem mirror designs are not required. We find that, concerning end losses, intense ECH in symmetric End Plugs could increase the fusion power gain Q, for both DT and Catalyzed DD 1741-4326 fuel cycles, to levels competitive with steady-state tokamaks burning DT fuel. Radial losses remain an issue that will ultimately determine reactor viability. Keywords: tandem mirror, advanced fuel, mirror reactor, axi-symmetric magnetic mirror, 5 fusion reactor concept (Some figures may appear in colour only in the online journal) 1. Introduction certain way to stabilize End Plugs against MHD instability, Post’s KSTM uses only circular coils giving the axisymmetric This paper draws attention of mirror specialists and the wider coil geometry sketched in the upper half of figure 1. In this fusion community to a new idea utilizing advances in magnet design, MHD stability is provided by the favorable curvature technology that potentially make tandem mirrors a viable can- of magnetic field lines in the Expander where flux diverges as didate to burn advanced fuels in the future, and also improves it emerges from the last mirror coil. performance for deuterium–tritium (DT) fuel in the nearer In the tandem mirror, the End Plugs serve only to prevent term. This is not intended to be a research paper in depth but leakage out of the ends of the straight solenoid in figure 1, instead relies on simple estimates to show why the new idea while the solenoid itself serves as the reactor [8, 9]. The is plausible, with illustrative examples of reactor parameters, solenoidal reactor is called the Center Cell. Fusion energy and concluding with subjects for further study. produced as charged particles escapes out the ends along The version of the tandem mirror considered here is a vari- expanding magnetic field lines that spread the power over a ation on the axisymmetric kinetically stabilized tandem mirror large area in the Expander, shown in more detail in figure 2. (KSTM) invented by Post [1–3], using design concepts first Fusion energy produced as radiation and neutrons is captured elucidated by Mirnov and Ryutov [4, 5], now confirmed in in the blanket shown in figure 3. Ignition in the Center Cell is the gas dynamic trap (GDT) experiment in Novosibirsk [6, 7]. achieved by mirror machines serving as End Plugs in figure 1. While the earliest tandem mirror experiments used asym- Faster end leakage of electrons compared to ions creates an metric minimum-B (yin-yang/Baseball) magnetic wells as the electrostatic potential positive in the mirrors relative to end 1741-4326/17/056014+20$33.00 1 © 2017 IAEA, Vienna Printed in the UK Nucl. Fusion 57 (2017) 056014 T.K. Fowler et al Figure 1. Axisymmetric coil geometry sketched in the upper half Figure 2. The Expander isolates hot electrons from the end walls, and electrostatic confining potentialΦ c in the lower half. stabilizes the plasma column and guides escaping plasma to the direct converter. walls and relative to the Center Cell. The resulting potential profile along the length of the machine is sketched in the performance limited only by the radial losses due to thermal lower half of figure 1. That Φc is less than Φp serves to confine gradients and trapped particles as in tokamaks, with some ions in the Center Cell. In the original tandem mirror concept, possibility of controlling these processes by end plates not confirmed in the TMX experiment [10], the peaks in potential available in toroidal devices, and feedback applied only to were created solely by neutral beams depositing fast ions in the small End Plugs. Given adequate control of radial losses, the end plugs. Here we add electron cyclotron heating (ECH), in section 7 we give examples of reactors with much less as shown. magnetic energy storage than tokamaks, even for reactors Besides the evident engineering advantages of the linear employing the Catalyzed DD fuel cycle. blanket geometry in figure 3, tandem mirrors should be able In undertaking this paper, our goal has been to use simple to achieve a plasma pressure parameter β ≈ 1, which greatly estimates to show that most issues concerning end plugging reduces required magnetic field strengths in the Center Cell. could be calculated from first principles in an actual design, in And, for an ignited Center Cell, the End Plug power required contrast with scaling laws of radial transport losses that still may to maintain ignition is fixed, giving a fusion power gain be required in the Center Cell, as in tokamaks. The remainder Q ∝ LC, the length of the Center Cell. of this paper is organized as follows. Section 2 discusses The main disadvantage has been lagging performance radial confinement in the Center Cell of tandem mirror reac- of tandem mirrors compared to tokamaks, due in part to tors. Section 3 discusses the new idea for end plugging using ion cooling by collisions with electrons at the low electron strong ECH heating and the role of the magnetic Expander in temper atures achievable in small devices; but also due to the preventing heat loss to the end walls. Section 4 discusses the appearance of radial transport when end plugging is most role of the Expander in stabilizing the End Plugs, including successful. We find that the key to reactor-level performance a discussion of trapped particle modes. Section 5 discusses in tandem mirrors lies in new technology that improves end End Plug stability at ion cyclotron frequencies and how this plugging and appears to stabilize at least some of the insta- determines the minimum plasma radius. Section 6 discusses bilities responsible for poor performance in the past. First why tandem mirrors favor reactors at high ion temper atures and foremost is the potential for complete stabilization of ion allowing consideration of advanced fuels, with examples in cyclotron modes in the End Plugs, made possible with little section 7. Section 8 suggests experiments that would give power expenditure due to the advent of high field supercon- definitive answers to issues of end plugging. Section 9 lists ductors already producing 27 tesla fields in steady state [12], some subjects for further study by theory, simulations and in coils with small dimensions (order 1 m), similar to dimen- engineering. Section 10 gives the summary including an sions of the compact, theoretically-stable End Plugs of our assessment of how well the above goals have been fulfilled. reactor model in section 7. Second is ECH enhancement of Appendix A discusses pros and cons for advanced fuels, and end plugging without resorting to so-called thermal barriers appendix B discusses direct conversion to electricity.