Cumulative beam break-up study of the spallation neutron source superconducting linac

Beam simulations with initial bunch noise in superconducting rf proton linacs

“…Sketch of primary bunch-noise spectrum (bottom) and the beam spectrum as seen by the HOMs (top) for beam pulses with pulse chopping. 1 The induced HOM field may cause a tiny change of the particle speed through the cavity: the integrated contribution of the initial field (the initial V H ) does not remain absolutely invariant. This tiny nonlinearity can be neglected in the context of the voltages.…”

“…Many parameters of SNS, SPL, and Project X do not differ considerably. In the following we assume a generic machine similar to SPL but using for yet not well-defined parameters settings taken over from [1] used for the SNS simulations. In particular, we also use as a worst case a cavity-to-cavity HOM frequency spread of only 0.1 MHz, a longitudinal (R=Q) of 5 up to 50 as well as a transverse ðR=QÞ ?…”

“…These values also match well the ones found for the presently considered high-SPL cavities. We also apply the same safety factor as [1] with a beam current exaggerated up to 10 times the design value.…”

“…Similar to what was done in [1], we assume ðR=QÞ ¼ 50 circuit ( ¼ 100 linac ) and a beam current of 400 mA, about 10 times the design current and, hence, exaggerating here as a safety factor. The HOM frequencies scatter around the center value is Áf ¼ 100 kHz.…”

“…In Ref. [1] the necessity of higher order mode (HOM) dampers on the superconducting rf cavities in the SNS [2] linac has been studied. Even if they were not considered absolutely necessary, to be on the safe side, they have been installed.…”