Papers on simplified, high-field stellarators

V. Queral, F.A. Volpe, D. Spong, S. Cabrera, F. Tabarés
Initial Exploration of High-Field Pulsed Stellarator Approach to Ignition Experiments
J. Fusion Energy 37, 275 (2018) (open access)

Possibly the first conceptual design of a high field stellarator (10 T in the plasma). To clarify, it was meant to study burning plasmas. This can be done in brief discharges using copper coils, similar to the IGNITOR tokamak. By contrast, Renaissance Fusion aims at strong d.c. fields to take full advantage of the steady-state nature of stellarators and build a continuously operating power-plant. This is why we adopt High Temperature Superconductors. Despite the different goal, the paper has many merits for stellarator reactor design: it included demountable sectors and liquid walls, now recognized as one of few possible solutions to fast particle losses in stellarators [Boozer]. It also shows the merits of high fields in making stellarator reactors more compact: under aggressive assumptions (emulating the stellarator record for “normalized pressure”, β ~ 5%, but at high field), 30 m³ of plasma would suffice. This is the size of Wendelstein 7-X! With the more conservative assumptions made at Renaissance Fusion, a stellarator slightly larger than W7-X, and with a 4x higher field, could be a reactor.

J.L. Li, J. Austin, K.C. Hammond, B.Y. Israeli and F.A. Volpe
Large vacuum flux surfaces generated by tilted planar coils
Plasma Phys. Control. Fusion 61 (2019)

Numerically shows that the simplest possible coils (planar) can generate helical equilibria (stellarators, torsatrons etc.) of low aspect ratio (“fat doughnuts”, as desired in a compact power-plant): the simple trick is to properly tilt the planar coils. As a corollary, some of the coil non-planarity (thus, complexity) in advanced optimized stellarators can be traded for a simple increase in coil-tilt.

D.A. Gates, D. Anderson, S. Anderson, M. Zarnstorff and 45 coauthors, including F. Volpe
Stellarator Research Opportunities, A report of the National Stellarator Coordinating Committee
J. Fusion Energy 37, 51 (2018)
Free preprint at

Paper of the US stellarator community, nice overview of “Important topics for advanced stellarators” (Sec.2) and research needs (Sec.3). Among others, it advocates simpler, possibly demountable coils.

Papers on liquid metal walls

S.M.H. Mirhoseini, R.R. Diaz-Pacheco, F.A. Volpe
Passive and active electromagnetic stabilization of free-surface liquid metal flows
Magnetohydrodyn. 53, 45 (2017)

Fusion requires flowing liquid metal walls of constant, uniform thickness, despite the instabilities, turbulence and other effects. With this motivation, here we show that a free-surface liquid metal flow becomes flatter in the presence of a sufficiently strong magnetic field, either alone (passive stabilization) or in combination with an electrical current passing through the liquid metal (active stabilization). The results are interpreted in terms of an effective viscosity and effective gravity.

S.H.M. Mirhoseini, F.A. Volpe
Resistive sensor and electromagnetic actuator for feedback control of liquid metal walls in fusion reactors
Plasma Phys. Controll. Fusion 58, 124005 (2016)

Our 2017 paper (see above) demonstrated passive and open-loop active stabilization of flowing liquid walls. Even better, the ultimate form of control will be closed-loop active stabilization in feedback with measurements of liquid metal thickness. To that end, in this 2016 paper we demonstrated resistive sensors of liquid metal thickness and jxB actuators, to locally control it.

S.M.H. Mirhoseini, F.A. Volpe
Space- and time-resolved resistive measurements of liquid metal wall thickness
Rev. Sci. Instrum. 87, 11D427 (2016)

The electrical conductance between electrodes immersed in the liquid metal can be used as a simple proxy for the local thickness. Here a matrix of electrodes is shown to provide spatially and temporally resolved measurements of liquid metal thickness in the absence of plasma. First a theory is developed for m x n electrodes, and then it is experimentally demonstrated for 3×1 electrodes.

Papers on High Temperature Superconductors (HTS)

G.Majkic, R. Pratap, A. Xu and 7 coauthors including V. Selvamanickam
Engineering current density over 5 kA mm−2 at 4.2K, 14T in thick film REBCO tapes
Supercond. Sci. Technol. 31, 10LT01 (2018)

Paper reporting record-high engineering current density, Je = 5.2 kA/mm2 at 4.2 K, 15 T. This is seven times higher than Je of industry-standard REBCO tape, implying that the same amount of HTS material can generate 7x higher magnetic fields, all the rest remaining equal -very important for our high-field stellarator (and tokamaks alike).

V.Selvamanickam, Y. Chen, X. Xiong, Y.Y. Xie et al.
High Performance 2G Wires: From R&D to Pilot-Scale Manufacturing
IEEE Trans. Appl. Supercond. 19, 3225 (2009)

Seminal paper written by our collaborator and advisor V. Selvamanickam (now a Professor at Univ. Houston) when he was the CTO of SuperPower: in 2008, kilometer lengths of 2G HTS wire which had been sought after for 20 years were demonstrated for the first time with excellent critical current performance. Zr-doping was shown for the first time in Metal Organic Chemical Vapor Deposition (MOCVD) to significantly enhance critical current performance in magnetic fields. Finally, a device based on 2G HTS wires was energized in the electric power grid for the first time, demonstrating the transition of the technology from the laboratory, to manufacturing, to in-grid operation.

Business papers on commercial fusion

Article on the tremendous progress made by public fusion research and how, with the scientific feasibility now in sight (ITER will start operation in 2025), a handful of start-ups started tackling the next big questions: economic attractiveness, industrialization, commercialization. The paper makes no mystery of the challenges ahead. Nevertheless, the upsides are immense, urgent and yet so close: in decades of research, a key fusion performance indicator called triple product increased by a factor 1,000,000; yet another small increase by a factor 2x, and the reactor will produce net energy. For these reasons, now is the time to gather private investments, talents, innovative ideas and business partnerships to make fusion smaller, cheaper, possibly faster, and put fusion electricity on the grid.

M. Kupp
It’s Time to Place a Macro Bet on Nuclear Fusion
Forbes (2020, under consideration)

There are many approaches to fusion and, to date, it is unclear which concept or set of concepts will win the race and be commercialized. Starting from this simple fact, the paper argues for diversified fusion investments: fusion should be treated as a macro bet in itself, rather than part of a broader energy bet. A fusion “megafund” amalgamating many fusion ventures in a single financial entity would be a possible mechanism to infuse more capital in private fusion while reducing the risk for the investors to such a point that funding could be sourced through debt securities, such as bonds. The megafund structure allows for a high degree of asset diversification, the ability to use debt (which has a lower cost of capital than equity), increased risk optionality for investors through the creation of different tranches, and a greater pool of available capital.