Detection of an Electron-scale Dissipation Layer near the X-line during Magnetic Reconnection in Laboratory

Despite its disruptive influences on the large-scale structures of space and solar plasmas, the crucial topological changes and associated dissipation during magnetic reconnection take place only near an X-line within thin singular layers. The collisional Sweet-Parker model, where electrons and ions flow together through a single geometrically thin and long dissipation layer, fails to explain the observed fast reconnection rates. When electrons and ions are allowed to move separately in modern collisionless models, it has been predicted that ions, despite their heavy mass, exhaust efficiently through a thicker, ion-scale dissipative layer while mobile electrons can evacuate through a thinner, electron-scale dissipation layer, allowing for efficient release of magnetic energy. While ion dissipation layers have been frequently detected, the existence of election layers near the X-line and the associated dissipation mechanism is still an open question, due to extremely rare encounters in space, despite their crucial importance in determining magnetic topology and dissipations. Here we report the first definite evidence of electron dissipation layers near the X-line detected in a reconnecting laboratory plasma. The measured electron layers display properties strikingly similar to predictions by 2D PIC simulations, including their geometrical shape, insensitivity to ion mass, and sensitivity to the boundary conditions, but disagreeing on the electron layer thickness. The electron layers are 4 to 6 times thicker in the laboratory than those in simulations, beyond thickening due to residual electron-ion collisions. These results effectively rule out all 2D mechanisms operative in the simulation model, including the usually hypothesized electron inertia effects, as a main mechanism for dissipations within the electron layer, and thus strongly, although indirectly, support that 3D effects, such as wave-particle interactions, are responsible for fast magnetic reconnection.