THE LONGITUDINAL WALL IMPEDANCE INSTABILITY IN A HEAVY-ION FUSION DRIVER

For more than ten years [J. Bisognano, I. Haber, L, Smith, IEEE Trans, Nucl, Sci, NS-30, 2501 (1983)], the longitudinal wall impedance insta bility was thought to be a serious threat to the success of heavy-ion driven inertial confinement fusion. This instability is a ''resistive wall'' instability, driven by the impedance of the induction modules u sed to accelerate the beam. Early estimates of the instability growth rate predicted tells of e-folds due to the instability which would mod ulate the current and increase the longitudinal momentum spread and pr event focusing the ion beam on the small spot needed at the target. We have simulated this instability using an r-z particle-in-cell code wh ich includes a model for the module impedance. These simulations, usin g driver parameters, show that growth due to the instability is smalle r than in previous calculations. We have seen that growth is mainly li mited to one head to tail transit by a space-charge wave. In addition, the capacitive component of the module impedance, which was neglected in the early work of Lee [E, P. Lee, Proc, Linear Accelerator Confere nce, (UCRL-86452), Santa Fe, NM, 1981] significantly reduces the growt h rate. We have also included in the simulation intermittently applied axial confining fields which are thought to be the major source of pe rturbations to seed the longitudinal instability. Simulations show the beam can adjust to a systematic error in the longitudinal confining f ields while a random error er;cites the most unstable wavelength of th e instability. These simulations show that the longitudinal instabilit y must be taken into account in a driver design, but it is not the maj or factor it was once thought to be, (C) 1997 American Institute of Ph ysics.