Low- to medium-β cavities for heavy ion acceleration

Abstract

Acceleration of low- and medium-β heavy ions by means of superconducting (SC) linear accelerators (linacs) was made possible by the development, during four decades, of a particular class of cavities characterized by low operation frequency, several different shapes and different electromagnetic modes of operation. Their performance, initially rather poor in operating accelerators, have steadily increased along with the technological progress and nowadays the gap with the high-β, elliptical cavities is close to be filled. Initially confined to a very small number of applications, this family of cavities evolved in many directions becoming one of the most widespread in linacs. Nowadays it is present in the majority of superconducting radio-frequency ion linac projects worldwide. An overview of low- and medium-β SC cavities for heavy ions, focused on their recent evolution and achievements, will be given.

Overview of the IFMIF/EVEDA project

“…• the growing maturity of superconducting resonators for light hadrons and low-β beams (typically 0.03 < β < 0.2 [60]) achieved in recent years [61].…”

“…The use of superconducting cavities would allow an increase of the beam aperture, with a beneficial impact on beam losses and equipment activation. The demonstration of the feasibility for 0.2 < β < 0.6 proton beams [60] paved the way for a new operational window at even lower β, in a more reliable manner than Alvarez-type-based linacs for high currents. In the existing machines, the most used resonator type is the quarter-wave resonator (QWR), preferred for its relatively low cost, easy mechanical assembly, and high performance at lowβ [61]; however, the electric and magnetic dipole field components induced by the asymmetry of its shape, might cause beam undesired vertical steering.…”

“…The validation of the tuning and stabilization procedures was established following low power tests on an aluminium real scale RFQ, which determined the mode spectra and the electric field distribution with the bead-pulling technique; this was successfully carried out over summer 2016 in Rokkasho Fusion Institute [72]. The ultra-high vacuum wished performance under beam operation is achieved with cryopumps, profiting from their high pumping capacity for H. SC technology can be efficiently used in pulsed proton high-power linacs as demonstrated at SNS; it can also be used in CW mode with low-β protons [60] as recently demonstrated [63,64]. The baseline configuration defined in historical concepts of the IFMIF for the deuteron beam acceleration from 5 to 40 MeV relied on a DTL.…”

“…The optimal amplitude and phase stability of the beam is essential for an efficient beam transfer and minimization of beam losses. Microphonics, the changes in cavity frequency caused by coupling to vibration sources from the external world, might be encountered; typically they are enhanced at low frequencies in CW mode [56,60,62], but solutions could be implemented upon the identification of the source. The non-relativistic nature of low-β proton beam leads to a higher influence of the cavity field fluctuations driven by phase slippage as the beam traverses the consecutive cavities.…”

Impact of geometry on the magnetic flux trapping of superconducting accelerating cavities

“…One technical difficulty appears when one tries to shield cavities with a different geometry, namely, low-beta structures [16]. Their relatively large dimensions can significantly increase the mechanical complexity and cost to fabricate ideal magnetic shields [17] and thus can practically limit field attenuations around the cavities.…”