Measurement of the electron–ion-hose instability growth rate

Abstract

The growth rate of the ion‐hose instability has been measured for a 2.5 MeV, 1 kA, 1 μsec electron beam following plasma channels of O₂, N₂, and H₂ (in the ion‐focused regime). The amplitude of transverse oscillations of a given beam segment was seen to grow, saturate, and damp as the segment traveled. The offset amplitude at saturation (d_m) was seen to be an exponential function of the beam pulse duration (t): d_m=d_m,0 exp(2πG_Lt/t_c), where t_c is the time required for one channel oscillation and G_L is the growth rate (for beam oscillations less than the channel radius). With beam radius equal to channel radius (r_b=r_c), and channel density equal to half the beam density, G_L=0.75±0.15. Here G_L was seen to scale with the square root of channel ion mass when data from channels of O₂, N₂, and H₂ were compared. Also, G_L was seen to increase as r_b was increased (with the initial beam emittance kept the same). A fivefold decrease in growth rate was observed for t>t_c and d_m>r_c. The decrease in growth rate may be due to the strongly anharmonic potential outside the channel or detuning from radial oscillations. Agreement was obtained between the data and models.