[Paper Review] Generation of megatesla magnetic fields by intense-laser-driven microtube implosions

M. Murakami, J. J. Honrubia+3 more|arXiv (Cornell University)|Sep 2, 2020

Laser-Plasma Interactions and Diagnostics46 references4 citations

TL;DR

This paper proposes a novel method to generate megatesla (MT) magnetic fields using intense-laser-driven microtube implosions. By pre-seeding a kilotesla-order magnetic field, relativistic ions and electrons undergo Larmor gyromotion, creating ultrahigh spin currents (~10^15 A/cm²) that amplify the field to the MT range via collective motion in a nanoscale central hole, demonstrating a new pathway for ultrahigh-field physics.

ABSTRACT

A microtube implosion driven by ultraintense laser pulses is used to produce ultrahigh magnetic fields. Due to the laser-produced hot electrons with energies of mega-electron volts, cold ions in the inner wall surface implode towards the central axis. By pre-seeding uniform magnetic fields on the kilotesla order, the Lorenz force induces the Larmor gyromotion of the imploding ions and electrons. Due to the resultant collective motion of relativistic charged particles around the central axis, strong spin current densities of ~ peta-ampere/cm2 are produced with a few tens of nm size, generating megatesla-order magnetic fields. The underlying physics and important scaling are revealed by particle simulations and a simple analytical model. The concept holds promise to open new frontiers in many branches of fundamental physics and applications in terms of ultrahigh magnetic fields.

Motivation & Objective

• To overcome the experimental and theoretical limitations of generating magnetic fields beyond the kilotesla range in laboratory settings.
• To address the gap between predicted magnetic fields (~100 kT) and the highest experimentally observed fields (on the kT order).
• To develop a scalable, laser-based method that leverages geometrically confined plasma flows to achieve unprecedented magnetic field intensities.
• To demonstrate the feasibility of generating megatesla-order fields using existing high-power laser technology and structured targets.
• To explore the scaling laws and physical mechanisms governing field amplification through collective particle motion in a pre-seeded magnetic field.

Proposed method

• Utilizes a cylindrical microtube target with an inner radius of 1–10 µm, irradiated by ultraintense femtosecond laser pulses (10^19–10^22 W/cm²).
• Laser-generated hot electrons (1–10 MeV) produce a plasma sheath that drives inward implosion of cold ions via electrostatic sheath acceleration.
• A pre-seeded uniform magnetic field (B₀ ~ kT) is applied along the axial direction to induce Larmor gyromotion of ions and electrons in opposite directions.
• The collective azimuthal motion of relativistic charged particles forms a nanoscale 'Larmor hole' at the center, hosting ultrahigh spin current densities (~10^15 A/cm²).
• The spin currents generate a self-amplified magnetic field (B_c) at the center, reaching the megatesla range via current-driven field amplification.
• Validated through 2D particle-in-cell (PIC) simulations and a simplified analytical model, with scaling laws derived for B_c.max as a function of B₀ and ion flux Ψ.

Experimental results

Research questions

• RQ1 Can intense-laser-driven microtube implosions generate magnetic fields exceeding 1 megatesla?
• RQ2 What is the role of pre-seeded magnetic fields in amplifying the central magnetic field through collective Larmor gyromotion?
• RQ3 How do spin current densities and their spatial confinement scale with laser intensity and target geometry?
• RQ4 What are the key scaling laws governing the maximum achievable magnetic field (B_c.max) in terms of initial field B₀ and ion flux Ψ?
• RQ5 How does the geometry of the microtube target enhance field amplification compared to conventional magnetic flux compression?

Key findings

• The microtube implosion (MTI) concept successfully generates magnetic fields on the megatesla (MT) order, exceeding previous experimental limits by 2–3 orders of magnitude.
• A central 'Larmor hole' with sub-100 nm size forms due to the collective Larmor motion of ions and electrons, confining ultrahigh spin currents of ~10^15 A/cm².
• The maximum magnetic field B_c.max scales with the pre-seeded field B₀ and the total ion flux Ψ, with analytical scaling laws derived from simulation and model validation.
• Two-sided laser illumination (n_B ≥ 2) is sufficient to achieve sufficient uniformity in the ion front, reducing nonuniformity from hot electron diffusion across the target wall.
• Laser systems with pulse energy of 0.1–1 kJ and peak power of 10–100 PW are required for full-scale MTI, which are within reach of current high-power laser technology.
• Polarity switching of the central field is observed depending on the direction and magnitude of the pre-seeded field, indicating a novel and nontrivial magnetic response in the system.