Penetration of electrons into helical magnetic field configuration

Summary form only given, as follows. Experimental studies on injecting an electron beam inside a helical magnetic field via stochastic (chaotic) field region have been conducted on the Compact Helical System (CHS) device. Electrons are launched from a LaB/sub 6/ cathode located in the stochastic region. Remarkably, despite launching from the outside of the separatrix, the electrons travel across the separatrix and furthermore, penetrate deeply inside the closed field region. The length of the penetration is about 20 cm, while the Larmour radius of electrons is about 0.2 mm in experiments. In consequence of that, a strong radial electric field up to 10 kV/m is established in the boundary layer of the helical field configuration. Such penetration of electrons strongly depends on the emission current of electrons. In fact, measured profiles of the space potential inside the separatrix show a transition-like jump when the emission current from the gun exceeds a critical value. Simultaneously, the electron flux measured inside the separatrix increases significantly within 50 ps. This is shorter than the electron-neutral collision time. Thus, the observed penetration is categorized as a collision-free phenomenon. Actually, the signal of the measured electron flux shows fluctuations with 40-60 kHz frequency. No dependence on the initial direction of launched electrons is observed in experiments. Electrons penetrate even when being launched quasi-parallel to the magnetic field. Also, no penetration of electrons appears when the gun is distanced about 5 cm outside the separatrix. This corresponds to the scale length of the stochastic layer around the separatrix. Thus, these results suggest the existence of some collective motion of electrons induced around the separatrix with increasing the electron flux. As a result, the collective motion might cause the penetration of electrons inside the separatrix.