Abstract
The image displacement instability (IDI) has been measured on the 20 MeV Axis I of the dual axis radiographic hydrodynamic test facility and compared to theory. A 0.23 kA electron beam was accelerated across 64 gaps in a low solenoid focusing field, and the position of the beam centroid was measured to 34.3 meters downstream from the cathode. One beam dynamics code was used to model the IDI from first principles, while another code characterized the effects of the resistive wall instability and the beam break-up (BBU) instability. Although the BBU instability was not found to influence the IDI, it appears that the IDI influences the BBU. Because the BBU theory does not fully account for the dependence on beam position for coupling to cavity transverse magnetic modes, the effect of the IDI is missing from the BBU theory. This becomes of particular concern to users of linear induction accelerators operating in or near low magnetic guide fields tunes.
Highlights
Understanding instabilities that can cause unwanted behavior for a beam of charged particles in an accelerator is important for both new accelerator designs and characterizing stable parameters of existing machines
The raw data for the low magnetic guide field case presented in Fig. 5(B) shows that the erosion of the beam current occurs at the rear of the pulse, which indicates the presence of the beam breakup (BBU) instability at the end of the accelerator
The beam centroid measurements made on Axis I appear to be the first of their kind and compare well with analytical theory and computational analysis for the image displacement instability (IDI)
Summary
Understanding instabilities that can cause unwanted behavior for a beam of charged particles in an accelerator is important for both new accelerator designs and characterizing stable parameters of existing machines. There must be a discontinuity within the accelerator walls such as accelerating gaps This latter point makes the instability especially concerning for users of linear induction accelerators (LIAs) [4,5,6] and for present designers of new LIAs such as the Los Alamos National Laboratory, the Institute of Fluid Physics (China Academy of Engineering Physics) [7], and the Budker Institute of Nuclear Physics (Russian Academy of Sciences) [8]. Measurements is that it appears the IDI amplifies the BBU instability, as the present BBU theory predicts a significantly slower BBU growth rate compared to these measurements
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