Beam breakup instability suppression in multicell superconducting rf guns

Abstract

In this paper, we provide an analytical description for transverse coupling impedances with dipole high order modes (HOMs) and beam breakup (BBU) instability suppression of relativistic, high-current beams undergoing strong acceleration, such as those typically produced by rf photoinjectors. The model adopted is based on the accepted theory of coupling impedances extended to the case of beams characterized by a fast transition due to strong acceleration, from the nonrelativistic to the relativistic regime in which a bunch trajectory may not be rigidly directed parallel to the axis of the electric field. The trajectory oscillations in a dipole HOM field in the transverse plane are effective in perturbing the bunch energy, which causes an increase of the coupling impedance up to the BBU instability. This BBU instability analysis is based on equations obtained by Volkov [Phys. Rev. ST Accel. Beams 12, 011301 (2009)] in which external focusing due to both the applied transverse electric (TE) HOM and the accelerating fundamental mode as well as dipole HOM damping by external loads are investigated. This analysis is valid in the limit in which the weak nonlinearity of applied fields near the axis may be neglected. The solution suggests a means of enhancing the BBU threshold current. The possibility of obtaining BBU instability suppression in a long, integrated photoinjector and linac structure, such as the superconducting rf source at Forschungs Zentrum Dresden (FZD), are numerically examined. New designs of photoinjectors that provide BBU instability suppression of all dipole HOMs to enhance the threshold currents are suggested.

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

“…The BBU instability, a transverse magnetic coupling which destroys the beam quality, was discovered by the authors of Refs. [8,9] and has been studied extensively in both rf and induction accelerators [5,6,[10][11][12][13][14][15][16][17][18][19][20]. The Advanced Test Accelerator demonstrated the highest intensity transport in an induction accelerator to date with a 10 kA, 50 MeV electron beam utilizing phase mix damping through a laser ionized channel to suppress the BBU [12,13,21].…”

Considerations for a TeV collider based on dielectric laser accelerators

“…where it has been suggested to damp and detune the structure (DDS) in order to suppress hybrid high order modes (HOM) that leads to BBU instability. Later when the energy recovery linac (ERL) was in focus BBU was investigated in this configuration [59] and further for superconducting RF gun [60]. Without exception, the acceleration structure is initially azimuthally symmetric and the hybrid modes are excited due to transverse offset or asymmetry of the beam or to the coupling of input or output arms.…”

Considerations for a TeV Collider Based On Dielectric Laser Accelerators

“…During the years BBU attracted attention every time a new acceleration paradigm came to serious consideration: this was the case for NLC [57] and CLIC [58], where it has been suggested to damp and detune the structure (DDS) in order to suppress hybrid high order modes (HOM) that leads to BBU instability. Later when the energy recovery linac (ERL) was in focus BBU was investigated in this configuration [59] and further for superconducting RF gun [60]. Without exception, the acceleration structure is initially azimuthally symmetric and the hybrid modes are excited due to transverse offset or asymmetry of the beam or to the coupling of input or output arms.…”