Measurement of the electron--ion-hose instability growth rate

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 (dm) was seen to be an exponential function of the beam pulse duration (t): dm=dm,0 exp(2πGLt/tc), where Tc is the time required for one channel oscillation and GL is the growth rate (for beam oscillations less than the channel radius). With beam radius equal to channel radius (rb=rc), and channel density equal to half the beam density, GL=0.75±0.15. Here GL was seen to scale with the square root of channel ion mass when data from channels of O2, N2, and H2 were compared. Also, GL was seen to increase as rb was increased (with the initial beam emittance kept the same). A fivefold decrease in growth rate was observed for t>tc and dm>rc. 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