Magnetic fields so strong that they are typically only observed in astrophysical jets and highly magnetized neutron stars could be created in the laboratory, say physicists at the University of Osaka, Japan. Their proposed approach relies on directing extremely short, intense laser pulses into a hollow tube housing sawtooth-like inner blades. The fields created in this improved version of the established “microtube implosion” technique could be used to imitate effects that occur in various high-energy-density processes, including non-linear quantum phenomena and laser fusion as well as astrophysical systems.
Researchers have previously shown that advanced ultra-intense femtosecond (10-15 s) lasers can generate magnetic fields with strengths of up to several kilotesla. More recently, a suite of techniques that combines advanced laser technologies with complex microstructures promises to push this limit even higher, into the megatesla regime.
Microtube implosion is one such technique. Here, femtosecond laser pulses with intensities between 1020 and 1022W/cm2 are aimed at a hollow cylindrical target with an inner radius of between 1 and 10 mm. This produces a plasma of hot electrons with MeV energies that form a sheath field along the inner wall of the tube. These electrons accelerate ions radially inward, causing the cylinder to implode.
At this point, a “seed” magnetic field deflects the ions and electrons in opposite azimuthal directions via the Lorentz force. The loop currents induced in the same direction ultimately generate a strong axial magnetic field.
Self-generated loop current
Although the microtube implosion technique is effective, it does require a kilotesla-scale seed field. This complicates the apparatus and makes it rather bulky.
In the latest work, Osaka’s Masakatsu Murakami and colleagues propose a new setup that removes the need for this seed field. It does this by replacing the 1‒10 mm cylinder with a micron-sized one that has a periodically slanted inner surface housing sawtooth-shaped blades. These blades introduce a geometrical asymmetry in the cylinder, causing the imploding plasma to swirl asymmetrically inside it and generating circulating currents near its centre. These self-generated loop currents then produce an intense axial magnetic field with a magnitude in the gigagauss range (1 gigagauss = 100 000 T).
Using “particle-in-cell” simulations running the fully relativistic EPOCH code on Osaka’s SQUID supercomputer, the researchers found that such vortex structures and their associated magnetic field arise from a self-consistent positive feedback mechanism. The initial loop current amplifies the central magnetic field, which in turn constrains the motion of charged particles more tightly via the Lorentz force – and thereby reinforces and intensifies the loop current further.
“This approach offers a powerful new way to create and study extreme magnetic fields in a compact format,” Murakami says. “It provides an experimental bridge between laboratory plasmas and the astrophysical universe and could enable controlled studies of strongly magnetized plasmas, relativistic particle dynamics and potentially magnetic confinement schemes relevant to both fusion and astrophysics.”
The researchers, who report their work in Physics of Plasmas, are now looking to realize their scheme in an experiment using petawatt-class lasers. “We will also investigate how these magnetic fields can be used to steer particles or compress plasmas,” Murakami tells Physics World.
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