Longitudinal Accumulation of Rare Isotope Beams in the NESR


A high intensity of rare isotope beams is required by the internal experiments in the NESR [1] and in particular by the electron-ion collider mode [2]. It is therefore planned to stack the RIBs longitudinally at injection energy i.e. in the range 100-740 MeV/u. The stacking will be supported by electron cooling. The favored method of longitudinal beam accumulation is based on a barrier bucket RF system in combination with electron cooling. A broadband barrier bucket (BB) system is foreseen, which provides single sine waves of 200 ns period (5 MHz). Four cavities, each driven by a 3.5 kW solid state amplifier, result in a total voltage of 2 kV. According to dedicated beam dynamics simulations [3], this voltage is sufficient to compress cooled beams. The stacking cycle time, i.e. the time between 2 successive injections, could be about 2 s, provided that the quality of the injected precooled beam from the CR allows cooling times below 1 s. This is demonstrated in Fig. 1 for a beam of Sn ions at 740 MeV/u (0.9 μs revolution period). At t=0 a bunch is injected between the BB sine pulses of 100 ns period (10 MHz). The beam immediately debunches because the barrier voltage is not sufficient to capture the particles. The BB pulses are decreased and switched off at t=0.2 s, while the beam is being continuously cooled. For the injected beam, an initial emittance of 0.5 mm mrad and momentum (energy) spread of 1.3 × 10−3 (1.5 MeV/u) was assumed. They correspond to the 2σ design values for the pre-cooled beam in CR, taking into account the transfer through the RESR to the NESR as well as an additional 30% increase of the longitudinal emittance due to diffusion processes. Parkhomchuk’s formula [4] is used for the cooling rate, for an electron beam density of 3.2× 108cm−3 (1 A, 5 mm beam radius), a magnetic field strength of 0.2 T in the cooling section and an effective electron velocity corresponding to magnetic field misalignments of 5×10−5. The resulting cooling time is about 0.8 s. Then, the BB pulses are adiabatically introduced into the beam and increased to 2 kV. One stays stationary while the other is shifted in phase to compress the cooled beam. At t=2 s a new bunch is injected. The process will be repeated until the required intensity of the accumulated beam is reached. With such stacking cycle times the experiments in the NESR can take full advantage of the planned cycle time of 1.5 s of SIS100, where the primary heavy ion beam is accelerated. As an alternative, a h=1 RF system for bunching of the circulating beam and injection of a new bunch onto the unstable fixed point in longitudinal phase space is considered [5]. The RF voltage is raised so as to confine the bunch in a small fraction of the ring circumference. A new bunch ∗ c.dimopoulou@gsi.de is injected onto the free part of the circumference. Then the RF voltage is decreased to let the beam debunch. Continuous application of electron cooling maintains the stack and merges it with the freshly injected bunch. The present choice is a ferrite-filled RF cavity with a peak voltage of 15 kV in c.w. operation and with a frequency swing in the range 1-2.6 MHz [1]. It should be possible to modify the cavity for operation at frequencies down to 600 kHz, and thus cover the full range of injection energies at h=1, with a moderate peak voltage up to 1 kV.

1 Figure or Table

Cite this paper

@inproceedings{Katayama2007LongitudinalAO, title={Longitudinal Accumulation of Rare Isotope Beams in the NESR}, author={Tomoki Katayama and Tetsuro Kikuchi and Dieter M{\"{o}hl and Catherine Dimopoulou and Peter Beller and Brandon Franzke and U. Laier and Meredith Steck}, year={2007} }