Correcting the beam centroid motion in an induction accelerator and reducing the beam breakup instability

Abstract

Axial beam centroid and beam breakup (BBU) measurements were conducted on an 80 ns FWHM, intense relativistic electron bunch with an injected energy of 3.8 MV and current of 2.9 kA. The intense relativistic electron bunch is accelerated and transported through a nested solenoid and ferrite induction core lattice consisting of 64 elements, exiting the accelerator with a nominal energy of 19.8 MeV. The principal objective of these experiments is to quantify the coupling of the beam centroid motion to the BBU instability and validate the theory of this coupling for the first time. Time resolved centroid measurements indicate a reduction in the BBU amplitude, hξi, of 19% and a reduction in the BBU growth rate (Γ) of 4% by reducing beam centroid misalignments ∼50% throughout the accelerator. An investigation into the contribution of the misaligned elements is made. An alignment algorithm is presented in addition to a qualitative comparison of experimental and calculated results which include axial beam centroid oscillations, BBU amplitude, and growth with different dipole steering.

Beam Breakup in an Advanced Linear Induction Accelerator

“…Several BBU experiments have been recently performed on DARHT-I [13], [14]. The magnetic tune and beam envelope calculated by XTR for one of these experiments, with a 2.9-kA beam, is shown in Fig.…”

“…Furthermore, for practical reasons, the magnetic focusing field tune 1 for the present design may be quite different compared with that was assumed in [7]. Finally, even though it has been experimentally validated [12]- [14], the asymptotic formula for BBU growth may be questionable for this assessment because of the following.…”

Beam breakup in a solid-state powered linear induction accelerator

“…Theory [5,6,7] predicts that, after a large number of cells (N), the maximum amplitude of the BBU asymptotes to I is the beam current in kA, tr Z is the transverse coupling impedance in Ohms/cm, B is the solenoidal guide field in kG, and indicates an average over the cells. This scaling of BBU growth has been experimentally validated [3,8,4,9], and is unmistakable in computer simulations based on models of beam interaction with the cavities [10].…”

Diagnosing the DARHT electron beam-target interaction and hydrodynamic expansion [Slides]

“…In general, the increased applied ⃗ B field along the length of the LIA is used to reduce the growth of the Beam Breakup (BBU) instability. Different accelerator tunes are required for different cathode sizes (and thus different beam currents); to minimize BBU, the a higher ⟨B⟩ is required for higher current beams compared with lower current beams [89,93]. Figure 3.2 demonstrates a tune designed for stronger (smaller beam envelope) transport through Axis-I for a 50 mm cathode.…”