Advanced Concepts and Technologies for Heavy Ion Synchrotrons

New concepts and technologies, relevant for a new generation of heavy ion synchrotrons with novel features, are developed and studied at GSI. To reach the intensity goals for heavy ions of the FAIR project [1], several of these approaches had to be implemented already in the FAIR synchrotrons SIS18 and SIS100. Other technical advances were achieved in the FAIR preparatory phase in preparation of the heavy ion synchrotron SIS300. Beyond these new concepts and technologies realized in the FAIR synchrotrons, new advanced accelerator R&D is conducted with the potential of a major impact on several large synchrotrons world-wide. DYNAMIC VACUUM AND INTERMEDIATE CHARGE STATE HEAVY ION BEAMS During operation with intermediate charge state heavy ions, the main intensity limitation is determined by charge exchange process in collisions with residual gas atoms and molecules [2]. Beam loss originated by such processes starts to dominate the overall loss budget significantly earlier than any space charge or current dominated phaenomena. The cross section for ionization of intermediate charge state heavy ions in the energy range of SIS100 [3], is about a factor of Hundred higher than for highly charged heavy ions. The issue with charge exchange driven loss becomes significant and develops into an instability as the residual gas pressure in the machine is no longer static but becomes strongly dynamic with local variations of up to two orders of magnitude. The main driver for the vacuum dynamics is the beam itself. Systematic processes (e.g. injection/extraction processes, Rf capture losses etc.) and especially in case of intermediate charge state heavy ion operation, charge exchange processes, are the initiators of a strong residual gas pressure dynamics. A single ion impact on the surface of an accelerator component is able to release up to 104 bound atoms and molecules. In order to be able to conduct self-consistent simulations of the spatial and time resolved development of the pressure evolution and charge exchange processes in circular accelerators, the STRAHLSIM code [4] has been developed. STRAHLSIM accounts for the following features: the machine lattice, the machine cycles, the atomic physics cross sections for projectile ionization and capture, the cross sections for target ionization, the properties of the conventional UHV system, the desorption yields of different materials, and the pumping properties of NEG and cryogenic surfaces. Figure 1: Top: Charge exchange processes at collisions with residual gas particles drive local pressure bumps. Bottom: The SIS100 charge separator lattice provides a peaked loss distribution for ionized projectiles with peaks in the middle of each doublet group (red lines). This loss distribution enables a control of the desorbed gases. The control and stabilization of the dynamic vacuum and the minimization of beam loss by charge exchange processes are key developments for the FAIR synchrotrons SIS18 and SIS100 [5]. In SIS100, the main technical approach for stabilizing the dynamic vacuum is the charge separator lattice (Fig. 1) coupled with the application of cryo-pumping. The usage of superconducting magnets in SIS100 is mostly driven by the need for a cryogenic UHV system which acts as a super-pump and stabilizes the dynamics of the residual gas pressure. Besides the superconducting magnets themselves, there is a number of devices making use of the LHe as coolant. All magnet chambers are actively cooled with LHe (Fig. 2). Figure 2: Thin wall, rib reinforced, LHe cooled quadrupole chamber. Even during fast ramping and corresponding inductive heating by the changing magnetic field, their surface temperature has to be kept at 10 K. The chambers are cooled via separate process lines, connected to an 12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW Publishing ISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-MOPAB175 MOPAB175 C on te nt fr om th is w or k m ay be us ed un de rt he te rm s of th e C C B Y 3. 0 lic en ce (© 20 21 ). A ny di st ri bu tio n of th is w or k m us tm ai nt ai n at tr ib ut io n to th e au th or (s ), tit le of th e w or k, pu bl is he r, an d D O I 594 MC4: Hadron Accelerators A04 Circular Accelerators individual auxiliary supply header. The decoupling of the UHV system cooling from the magnet cooling, enables independent thermal cycles, e.g. to recover the UHV system from condensed and adsorbed gases. In addition to the cryogenic magnet chambers and with the purpose of providing sufficient pumping power for light atoms, e.g. for H2 and He, a large number of cryosorption pumps is foreseen (Fig. 3). The cryosorption pump uses a LHe cooled charcoal to provide large pumping power for light residual gas atoms. In order to minimize and control the pressure bump generated at the main loss positions for ionized projectiles, special cryo ion catchers [6, 7] have been developed (Fig. 3). Figure 3: Left: Cryosorption pump using charcoal for H2 and He-pumping. Right: Series cryogenic ion catchers with included low desorption beam absorbers. The cryo ion catchers contain a block which dumps the ionized projectiles outside the machine acceptance, surrounded by a cryogenic surface. To minimize the release of particles, the Cu-blocks have a low desorption yield Au-coating. The block is kept on an intermediate temperature by means of its connection to the shield cooling system. This assures that the block itself does not act as a cryopump and no residual gas molecules stick to its surface. The surrounding vacuum vessel provides a cryogenic surface with a stable temperature of 4.5 K free from inductive heating by the field of the neighbouring quadrupole magnets. FAST RAMPED SUPERCONDUCTING MAGNETS For SIS100, fast ramped superferric dipole magnets providing a field of 1.9 T with a maximum cycle frequency of 1 Hz and a ramp rate of 4 T/s have been developed [8]. The magnets are operated at 4.5 K and use a Nuclotron type cable. In this cable the superconducting NbTi-strands are wrapped around a tube which is cooled by a two phase forced helium flow. The coil is hold mechanically tight by the cold iron yoke and cooled in series with the yoke. The magnets create heat when they are ramped due to hysteresis and eddy current effects. With respect to the original Nuklotron magnets, the AC loss could be significantly reduced. In order to enable continuous operation with triangular cycles, a curved, single layer coil dipole has been developed. The coil is made of a high current cable with lower hydraulic resistivity and reduced AC losses using new NbTi strands with a Cu-Mn interfilamentary matrix. The curved 3 m long magnet has a bending angle of 3 1/3°. For the planned second stage synchrotron SIS300, a first full size prototype of a curved dipole has been developed at INFN and tested at LASA in Milan, Italy [9]. The 7 m long magnet provides a magnet field of 4.5 T and is ramped with 1 T/s. This challenging magnet has also the particular characteristic to be geometrically curved with a radius of curvature of 66.67 m and a Sagitta of 114 mm. A second enhanced collared coil has been built in the frame of the European CRISP project. The R&D program included: a) the development of low loss superconducting wires and cables, b) the construction of a curved dipole coil winding and c) the construction of the complete dipole. At IHEP in Protvino, Russia, corresponding fast ramped SIS300 prototype quadrupole magnets with enhanced low loss cable has been built and successfully tested. In order to further advance the technologies for fast ramped s.c. magnets, in the frame of the EU IFAST program, GSI and others aim for the development of a new multi-layer HTS cable in conduit. This cable shall deliver the performance needed for a fast ramped SIS400 dipole magnet. FLEXIBLE OPERATION AND HEAT LOAD MANAGEMENT Since SIS100 shall be operated similar to a normal conducting synchrotron, special concepts had to be developed to assure a stable cooling of the magnet string and an efficient operation of the central cryogenic facility. The heat load to the cryogenics system varies significantly between the different operation modes, e.g. pure triangular cycle and cycles with long extraction plateau for slow extraction. Three independent electrical systems power the dipole and three quadrupole families with different current cycles, generating different heat loads in the corresponding units of each family. The individual and parallel magnet cooling circuits of each unit, are hydraulically adjusted to each other by means of mass flow restrictors with reference to a selected high load cycle [10]. Nevertheless, due to operation with quite different cycles and the fact, that each quadrupole circuit is in general performing a different ramp, the goal of reaching 100 % of gaseous He in the return header cannot always be reached. Therefore, in order to maintain an efficient operation of the cryogenic system, it is planned to adapt the supply header pressure according to the mean load value of the upcoming cycles. Furthermore, liquid Helium pumps are foreseen in the feed boxes to pump remaining liquid from the return header to the supply line. Furthermore, each of the parallel hydraulic magnet circuits is equipped with heaters, which provide auxiliary heat to the different parallel cryogenic circuits and act as valve in low loss cycles or during transition phases. LASER COOLING OF HEAVY ION BEAMS High-quality ion beams can be obtained by means of electron cooling and/or stochastic cooling. At intermediate kinetic energies these methods work very well. But at very 12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW Publishing ISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-MOPAB175 MC4: Hadron Accelerators A04 Circular Accelerators MOPAB175 595 C on te nt fr om th is w or k m ay be us ed un de rt he te rm s of th e C C B Y 3. 0 lic en ce (© 20 21 )