Force-free electromagnetic pulses in a laboratory plasma

A short, intense current pulse (I=150 A, {Δ}t{}0.2 {μ}s, N{}2×{}10¹⁴ electrons) is drawn from an electrode immersed in a magnetized afterglow plasma (n{}2×{}10¹¹ cm^-3, kTₑ{}1.3 eV, B₀=10 G). The induced magnetic field B(r,t) assumes the shape of a helical double vortex which propagates along B₀ through the uniform plasma as a whistler mode. The observations support a prediction of force-free (J×{}B+neE=0) electromagnetic fields and solitary waves. Energy and helicity are approximately conserved.