The Quest for Fusion Energy
Daniel Jassby
Abstract and summary · read the original at the source
In one page
Daniel Jassby spent his career as a plasma physicist at Princeton, and here he audits the fusion ledger rather than the press releases. He sets the two great 2021 campaigns side by side. At the JET tokamak in England, the deuterium-tritium run yielded 59 megajoules in a shot but a peak Q — fusion energy out divided by heating energy in — of about 0.4, below the 0.67 JET itself reached in 1997; and because that record shot fired pure deuterium beams into an overwhelmingly tritium plasma, most of the fusion came from beam ions striking thermal ions, leaving a thermonuclear Q of no more than about 0.2. At the National Ignition Facility in California, one laser shot on 8 August 2021 returned 1.3 megajoules, eight times the previous record, with a burning plasma and a burn just beginning to propagate. Jassby’s verdict is that inertial confinement has taken the lead, and he sets out what an inertial power plant would still need.
Why it matters hereChapter 12 argues that a driven lattice is a route to fusion worth taking seriously, and that argument is always measured against the mainstream route — so it is worth having the mainstream route stated in its own numbers by someone who built the instruments. For chapter 1 this is calibration for the evidence ladder: what a fusion claim looks like when the energy out, the energy in and the fuel inventory are all put on one page.
What it claims
01The bookkeeping the field runs on: fusion energy gain Q is the fusion energy released in a pulse divided by the external heating energy deposited in the plasma during that pulse; scientific feasibility, or breakeven, is Q of 1 or greater; net electric power requires Q of at least 5; and in a magnetically confined plasma a burning plasma — one heated mainly by its own alpha particles — is also reached at Q of about 5, with ignition requiring twice the energy confinement time again.Opening sections, the definitions of Q, breakeven, burning plasma and ignition
Settled physics02JET’s 2021 deuterium-tritium campaign produced 13 megawatts of peak fusion power transiently, a maximum transient Q of about 0.4 and a quasi-steady Q of 0.33, against the 16 megawatts and 0.67 the same machine reached in 1997; and the celebrated 59 megajoule shot used 100 percent deuterium beams into an overwhelmingly tritium plasma, so Jassby estimates at least three quarters of the output came from beam-thermal reactions, leaving a thermonuclear Q of no more than about 0.2.The JET section, the 9 February 2022 press conference figures, citing references 8 and 11
Published and peer-reviewed03The National Ignition Facility climbed from 2.5 kilojoules per pulse in 2010 to 25 kilojoules in 2014, 55 kilojoules in 2017–18 and 170 kilojoules in 2020–21, and four shots in that winter between 98 and 171 kilojoules generated a burning plasma with alpha heating exceeding the implosion heating; the shot of 8 August 2021 then returned 1.3 megajoules with Q greater than 3, more than 1 percent of the tritium charge burned, an ion temperature near 10 kilo-electronvolts and alpha heating at least twice the implosion energy — a burn on the threshold of propagating into the surrounding frozen fuel shell.The NIF sections, citing references 13 to 16
Published and peer-reviewed04Fuel logistics separate the two approaches as sharply as the physics does: a tokamak shot uses about a thousand times more tritium than an inertial shot — JET needs at least 100 milligrams per shot where the NIF uses less than a quarter of a milligram preinstalled in the capsule — while the whole civilian tritium inventory is about 30 kilograms held from CANDU reactors at 30,000 to 100,000 US dollars per gram, and a single 2 gigawatt fusion reactor would consume roughly 100 kilograms a year.Section, Obviating External Tritium Production, citing reference 29
Published and peer-reviewed05Jassby’s own proposal is a capsule that makes its own tritium: a 50:50 deuterium-tritium core and thin inner shell ignite a burn that propagates into thick outer layers of frozen deuterium alone, with the density-radius product at least ten times the deuterium-tritium value and driver energies as large as 100 megajoules, which puts the temperature high enough — around 50 kilo-electronvolts — for deuterium-deuterium reactions to run; the surrounding one-metre flowing liquid-metal wall then absorbs the neutrons and carries the heat to a turbine, so the outer chamber is barely irradiated.Sections, Obviating External Tritium Production and Mitigation of Neutron Effects, citing references 31 to 35
Designed, not yet built06What would settle it, in Jassby’s framing: whether a large tokamak can raise Q by a factor of five on thermonuclear reactions alone, which ITER and SPARC are the only funded machines positioned to test, and whether efficient drivers delivering at least 5 megajoules at high repetition rate can be built for inertial fusion. On the record so far he judges practical fusion-based electric power a distant prospect, unlikely within the next half century.Closing sections, from ’Twenty-five years ago’ to the end
What to watch
The way in
https://doi.org/10.37282/991819.22.30A critical essay published in Inference vol. 7, no. 1, on 25 May 2022, by a retired research physicist who worked for many years at the Princeton Plasma Physics Laboratory. Inference publishes under its own terms of use rather than a Creative Commons licence, so this page carries the summary, the claims and the author’s own abstract and sends the reader to the source. The full essay, with its thirty-seven references, is free to read at inference-review.com.
How to cite it
Daniel Jassby (2022) The Quest for Fusion Energy. doi:10.37282/991819.22.30
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