Lithium-Liner Compression Pushes LM26 Electrons Above 1 keV
A spherical tokamak compressed inside a lithium liner reached electron temperatures above 1 keV, with ion heating and neutron production measured during the experiment.
Underlying Paper
Compressional heating above 1 keV on the LM26 magnetized target fusion machine
The LM26 device has achieved a peak electron temperature of $T_e = 1180 \pm 65$ eV as measured by filtered X-ray diodes, supported by adjacent Thomson scattering measurements of $T_e = 1090 \pm 40$ eV, with a deuterium ion temperature of $T_i = 459 \pm 33$ eV as inferred from neutron yield. LM26 compresses a spherical tokamak plasma inside an initially 1.76 m diameter solid lithium liner imploded by theta-pinch coils. Radial compression by a factor of 2.75 increases $T_e$ over fivefold, while $T_i$ increases as much as twofold. The integrated observation of magnetic flux compression, electron heating, ion heating, and neutron production in a magnetized plasma compressed by a large lithium liner is an important milestone for magnetized target fusion
LM26 compresses a spherical tokamak plasma inside an initially 1.76 m-diameter solid lithium liner driven by theta-pinch coils. The paper reports a peak electron temperature above 1 keV during liner compression, alongside deuterium ion heating and neutron production. The result combines several measurements rather than relying on one diagnostic alone.
Core Contribution
Filtered X-ray diodes measured a peak electron temperature of eV. Adjacent Thomson-scattering measurements gave eV, providing an independent electron-temperature measurement in the compression experiment. From deuterium-deuterium neutron yield, the authors infer a deuterium ion temperature of eV.
The liner achieved a radial compression factor of 2.75. Across that compression, the paper reports an electron-temperature increase of more than fivefold and an ion-temperature increase of as much as twofold. It presents the combined observation of magnetic-flux compression, electron heating, ion heating, and neutron production as a milestone for magnetized target fusion.
Technical Approach
The analysis compares experimental diagnostics with an integrated simulation model (ISM). Figure 4 places measured poloidal magnetic-field data, AXUV filter-ratio measurements, and electron-density measurements alongside synthetic diagnostic signals from the model. It also shows modeled contributions from thermal transport, compressional heating, Ohmic heating, and electron-ion exchange.
A separate Bayesian profile reconstruction is used for plasma geometry and safety-factor quantities. Together, the magnetic, thermal, density, and neutron measurements provide complementary views of the plasma during compression.
Results and Analysis
Figure 3 summarizes electron-temperature measurements from AXUV and Thomson scattering together with ion-temperature information from neutron yield and ion Doppler spectroscopy, plotted against radial compression. The figure compares these measurements with the ISM model and shows heating to electron temperatures above 1 keV.
The measurements support the conclusion that LM26 produced keV-class electrons while compressing a magnetized plasma in a large lithium-liner system. The neutron measurement and inferred ion temperature add evidence that ion heating accompanied the electron heating. The paper does not establish ignition or a sustained path to substantially higher temperatures; it reports a compression experiment with integrated diagnostic evidence.
Limits
The reported peak electron temperature is above 1 keV, while the ion-temperature inference is lower. The Thomson-scattering and filtered-X-ray-diode measurements are adjacent rather than identical measurements, so they should be interpreted as complementary evidence. The neutron-yield-derived ion temperature is an inference based on the experiment's yield analysis.
Evidence Box
strongKey Claims
- •LM26 achieved electron temperatures above 1 keV during lithium-liner compression
- •Radial compression by a factor of 2.75 was accompanied by more than a fivefold increase in electron temperature
- •Magnetic-flux compression, electron heating, ion heating, and neutron production were observed in the same experiment
Key Results
- •Peak filtered-X-ray-diode electron temperature: 1180 ± 65 eV
- •Adjacent Thomson-scattering electron temperature: 1090 ± 40 eV
- •Deuterium ion temperature inferred from neutron yield: 459 ± 33 eV
- •Radial compression factor: 2.75
Limitations & Caveats
- •The highest reported electron-temperature measurement and Thomson-scattering measurement are adjacent rather than identical measurements
- •The ion temperature is inferred from neutron yield rather than directly measured by that diagnostic
- •The result demonstrates a compression milestone, not ignition or sustained high-temperature operation
