What General Fusion’s LM26 paper reveals about compressional heating at scale

LM26’s 36-turn compression coil assembly surrounds the vacuum vessel where each lithium liner shot is conducted and characterised

(Image courtesy of General Fusion)

General Fusion has posted to arXiv the first full scientific account of plasma compression experiments on Lawson Machine 26, its large-scale magnetised target fusion device, submitted simultaneously for peer review. Across eleven compression shots completed since LM26 began operating in 2025, the highest-performing results show electron temperature rising more than threefold to approximately 0.72 keV (around 8.4 million degrees Celsius), with plasma density and poloidal magnetic field each increasing tenfold. The authors conclude that integrated physics modelling indicates compressional heating accounts for the majority of that temperature rise – a distinction the paper takes care to establish through convergent independent models rather than assertion.

What the integrated physics model actually establishes

The paper’s central analytical contribution, and the element that carries most weight for a specialist audience, is the methodology used to isolate the cause of the temperature rise. Two independent computational frameworks were applied: a Bayesian Plasma Reconstruction model treating each shot as sequential independent time slices, and an Integrated System Model fitting time-dependent simulations of the full compression sequence. Both frameworks were fitted to diagnostic data, and the authors report they converge on the same interpretation, with compressional heating accounting for the majority of the observed temperature increase and the residual attributed to Ohmic heating and boundary losses.

The diagnostics underpinning that reconstruction span magnetic field probes, Thomson scattering, Absolute Extreme Ultraviolet detectors, multi-chord interferometers, and fast-camera imaging capturing plasma-wall interaction during compression. The agreement across that diagnostic suite, and across both modelling frameworks, is what Tony Donné, Chair of General Fusion’s Science and Technology Advisory Committee and former Chief Executive of EUROfusion, described as “particularly encouraging” – adding that it provides confidence that planned machine upgrades will perform as modelled.

Shot-to-shot progression across eleven compressions

Table I of the paper gives compression parameters for LMC-2 through LMC-11 in full. Liner compression ratios at first touchdown rose from 1.60 in LMC-2 to 3.20 in LMC-11, with peak liner velocities increasing from 170 m/s to 255 m/s and peak kinetic energy growing from 3.41 MJ to 4.70 MJ across that sequence. Normalised poloidal field increases tracked compression ratio closely, reaching 10.8 times starting values in LMC-11. The paper reports that plasma stability was maintained until deep into compression and that no significant plasma contamination by the lithium liner was observed during the stable compression phase.

General Fusion describes LM26 as the first MTF demonstration machine built at what it characterises as commercially relevant scale, operating at 50% of the planned commercial-scale diameter. The solid lithium liners are manufactured entirely in-house, with masses ranging from 153 to 218 kg across the shot series and fabrication conducted under argon atmosphere to prevent corrosion. The paper presents the eleven shots as a dataset establishing trends and validating models. The authors describe the results as providing “a basis for planned improvements to the LM26 facility that will enable the compression of magnetised plasma to increasingly higher densities and temperatures.”

The paper’s role in a crowded week for General Fusion

The arXiv posting on June 22 arrived less than two weeks before General Fusion’s shareholder vote on its proposed merger with Spring Valley Acquisition Corp. III (NASDAQ: SVAC), scheduled for July 6. According to SEC filings and the company’s own announcements, the transaction carries an implied equity value of approximately one billion US dollars and a committed PIPE of $107.5 million. The day after the paper was posted, General Fusion also announced a framework agreement with Italian energy developer Renexia covering site evaluation and potential MTF plant construction in Italy.

The paper is a substantive scientific document and the peer review process remains independent of these commercial developments. General Fusion states that the 0.72 keV result demonstrates “significant progress toward” its next milestone of 1 keV electron temperature, while the paper itself frames the current results as the foundation for the upgrades required to reach higher conditions. Those two characterisations are compatible, but they are not identical, and specialist readers will find the distinction worth holding onto as LM26 continues its compression campaign.

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