Dense Plasma Focus
A coaxial gun that pinches its own plasma into a millimetre-wide hot spot — for an instant, a star is born inside a steel barrel.
Stores energy in a capacitor bank (typically 10–100 kJ at 20–40 kV), discharges it across two coaxial electrodes, and accelerates a current sheath axially down the gun. At the end of the inner electrode the sheath rolls off and pinches radially into a 1 mm × 10 mm filament — the 'pinch' or 'focus' — at temperatures of 1–10 keV and densities of 10²⁵ m⁻³ for ~100 ns. In deuterium fill, the pinch produces 10⁹–10¹² neutrons per shot. In hydrogen-boron-11 fill, advocates claim aneutronic alpha emission, but no peer-reviewed independent confirmation of net-positive p-B11 fusion has been published.
(1) A capacitor bank charges to 20–40 kV. (2) A spark-gap switch dumps the bank into a coaxial electrode pair (inner anode, outer cathode) inside a vacuum chamber backfilled with ~5–20 mbar of deuterium or D-D-T. (3) Surface flashover ignites a current sheet at the breech end. (4) Magnetic pressure (J × B) pushes the sheet axially along the gun in 1–3 µs ('rundown phase'), gathering plasma like a snowplow. (5) At the muzzle the sheet rounds the end of the anode and is squeezed inward by the azimuthal magnetic field, collapsing into a 1 mm-diameter, 10 mm-long pinch with current density ~10¹¹ A/m². (6) The pinch is unstable — Rayleigh–Taylor and m=0 sausage modes terminate it after 50–200 ns — but during that brief window plasma temperatures reach 1–10 keV and densities 10²⁵ m⁻³, comparable to inertial-confinement-fusion regimes for a moment. (7) Neutrons (D-D), X-rays (bremsstrahlung from kV electrons), ion beams, and (for B-11 fills) potentially alpha particles emerge from the disrupting pinch. The neutron output is genuine and reproducible; the proton-boron-11 (p-B11) fusion path advocated by LPPFusion and others is intriguing but unverified at energy-positive scale.
In plain terms
Imagine a lightning bolt that races down the inside of a metal pipe, falls off the end, and squeezes itself into a knot the size of a grain of rice. For a few billionths of a second that knot is hotter than the Sun's core and emits neutrons and X-rays. That's a dense plasma focus — controlled lightning that ties itself in a fusion-relevant knot.
Also called: DPF, Mather-type plasma focus, Filippov-type plasma focus.
Why it matters
- Compact, repetitive 10⁹–10¹² /shot neutron source for activation analysis, neutron radiography, and explosives detection.
- X-ray source for EUV lithography prototyping at 13.5 nm.
- Test bed for high-energy-density-plasma physics, particularly Rayleigh–Taylor instability seeding.
- Recurring (and controversial) commercial-fusion candidate, especially via aneutronic p-B11 fuels.
Where it lives
- PF-1000 (IPPLM Warsaw, 1 MJ class)
- FN-II / NX-2 (SE Asia academic)
- LPPFusion FF-2B and FF-3 (commercial p-B11 effort)
- Various university devices (University of Illinois, Texas A&M)
Key numbers
- Capacitor bank energy: 10–1000 kJ
- Discharge voltage: 20–40 kV
- Pinch density: ~10²⁵ m⁻³
- Pinch temperature: 1–10 keV
- Neutron yield (PF-1000): up to 10¹² /shot
Surprising facts
- A dense plasma focus achieves plasma temperatures of 1–10 keV (10–100 million kelvin) inside a steel cylinder you can stand next to during operation.
- A 1 MJ class DPF (PF-1000) emits up to 10¹² neutrons per shot, comparable to large inertial-confinement-fusion shots from much smaller hardware.
- The pinch column collapses at ~0.5 % the speed of light over its final nanoseconds — a millimetre-thick filament of plasma compressed at velocities normally associated with stellar processes.
- LPPFusion's p-B11 'aneutronic fusion' claim — that a DPF could one day burn proton-boron-11 to produce charged-alpha output without neutrons — is repeatedly stated by the company but has not been confirmed by independent peer-reviewed measurement at energy-positive scale.
Open questions
- Anomalous neutron-yield scaling break: DPF neutron yield scales roughly as the fourth power of stored energy up to ~1 MJ, then plateaus. Multiple groups have proposed mechanisms (m=0 instability cutoff, ion-beam saturation, electrode geometry) but none fully accounts for the break across all geometries.
- Beam-target vs. thermal fusion ambiguity: A long-running debate is whether DPF neutrons come from a thermal fusion plasma or from accelerated ion beams hitting cold gas. The truth appears to be a mixture that depends on operating regime, but the relative weighting is not closed and bears directly on whether p-B11 net-energy claims are physically meaningful.
A short history
Independently developed by N. V. Filippov (USSR, 1958, short coaxial geometry) and J. W. Mather (Los Alamos, 1965, long coaxial geometry). Used through the 1970s–80s as a workhorse plasma-physics device. PF-1000 (Warsaw, commissioned 1990s) is the largest research DPF still operating. LPPFusion (originally Lawrenceville Plasma Physics) was founded by Eric Lerner in the early 2000s to commercialise a p-B11 DPF reactor; they have published progress reports but no peer-reviewed independent confirmation of net-positive fusion. As of 2026, the dense plasma focus is well-established as a neutron and X-ray source and remains a speculative — not yet demonstrated — fusion-energy candidate.
Sources
- Mather, Phys. Fluids (1965) — coaxial plasma focus (https://aip.scitation.org/doi/10.1063/1.1761310)
- Bernard et al., review of plasma-focus devices (1998) (https://www.sciencedirect.com/science/article/pii/S0370157398000048)
- PF-1000 IPPLM Warsaw programme (https://www.ipplm.pl/)
- LPPFusion FF-2B documentation (https://lppfusion.com/)