Particle Accelerator Activities at the Paul Scherrer Institut

PSI Bericht Nr. 17-07 December 2017 Particle Accelerator Activities at the Paul Scherrer Institut Terence GARVEY Paul Scherrer Institut, Villigen, Switzerland 5232 Division of Large Research Infrastructures Invited paper presented at Les Journées Accélérateurs de la Société Française de Physique, Roscoff, 3rd – 6th October, 2017. Resumé Les Activités Accélérateur à l’Institut Paul Scherrer. L'Institut Paul Scherrer exploite deux complexes d'accélérateurs en tant que ‘centre- serveurs’ pour une grande communauté de chercheurs. Il s'agit de l'Accélérateur de Protons à Haute Intensité (HIPA) et de la Source de Lumière Suisse (SLS). HIPA est un cyclotron à protons de 590 MeV. Il sert à produire des neutrons par spallation pour la recherche en physique de la matière condensée et à produire des muons et d'autres particules secondaires pour la recherche en magnétisme et en physique des particules. Le SLS est un anneau de stockage d’électrons de 2,4 GeV utilisé comme source de rayonnement synchrotron de 3ème génération fournissant des photons pour une large gamme de disciplines scientifiques. En plus de ces deux installations, l'Institut met progressivement en service un laser à électrons libres en rayons X (SwissFEL) qui fournira aux chercheurs des impulsions femto-seconde intenses à partir d’une ligne de rayons X ‘dur’ (ARAMIS) et de rayons X ‘mou’ (ATHOS). L'Institut exploite également un cyclotron supraconductrice à 250 MeV (COMET) aux fins de la thérapie par proton. Le centre de thérapie a récemment été équipé d'un troisième « gantry » rotatif qui est en cours de mise en service. Un « upgrade » du système de radiofréquence de HIPA et des projets pour "SLS-2" seront présentés. L'accélérateur SwissFEL et son état de mise en service seront également décrits. Particle Accelerator Activities at the Paul Scherrer Institut Terence GARVEY, Paul Scherrer Institut, Villigen, Switzerland 5232, Division of Large Research Infrastructures Abstract The Paul Scherrer Institute operates two accelerator complexes as user facilities for a large community of researchers. These are the High Intensity Proton Accelerator (HIPA) and the Swiss Light Source (SLS). HIPA is a 590 MeV proton cyclotron. It is used to produce neutrons by spallation for research in condensed matter physics and to produce muons and other secondary particles for research in magnetism and particle physics. The Swiss Light Source is a 2.4 GeV electron storage ring used as a 3rd generation synchrotron radiation source providing photons for a broad range of scientific disciplines. In addition to these two facilities the Institute is currently progressively bringing into operation an X-ray Free Electron Laser, SwissFEL, which will provide researchers with intense femtosecond pulses from hard (ARAMIS) and soft (ATHOS) X-ray beam lines. The Institute also operates a 250 MeV superconducting cyclotron (COMET) for the purposes of proton therapy. The therapy center has recently been equipped with a third rotating gantry which is currently being commissioned. An up-grade of the HIPA RF system and plans for “SLS-2” will be presented. The SwissFEL accelerator and its commissioning status will also be described. Introduction We will present an overview of the different activities in progress at the Paul Scherer Institut (PSI) in the field of particle accelerators. The treatment is not intended to be ‘complete’ or highly detailed but aims rather to give the reader a flavour of the accelerators currently operating as user facilities, as well as those activities aimed at their improvement, or ‘up-grade’. There are currently two operating user facilities (i) the High Intensity Proton Accelerator (HIPA) and (ii) the Swiss Light Source (SLS). In addition to these two research facilities, PSI operates a superconducting cyclotron (COMET) for proton therapy. There is also a new accelerator currently under construction, the Swiss Free Electron Laser (SwissFEL) linac. As will be seen below, this facility is, in practice, divided between two distinct research sectors; the first, ARAMIS, for research with hard x-rays; and the second, ATHOS, for research with soft x-rays. As the heart of the SwissFEL facility is a radio-frequency (RF) linac, we shall restrict our description of the new machine essentially to the RF linac. We will complete the overview by discussing some of the accelerator research activities which have taken place in parallel with accelerator operation and construction. All that will be described here has been reported previously and an extensive reference list is given for the interested reader. The Ring Cyclotron The oldest accelerator on the PSI site is the Ring Cyclotron of the HIPA complex. This is a 590 MeV, sector- type, proton cyclotron initially built in 1974 as a meson factory for studies in high-energy physics. The cyclotron has an injection chain consisting of an 870 keV Cockroft-Walton accelerator and a 72 MeV injector cyclotron, Injector-II (so named as it replaced an earlier commercial injector cyclotron of weaker intensity). Since 1996, the Ring Cyclotron provides beam to a tungsten target (SIN-Q) producing neutrons by spallation for research in neutron scattering. However, the machine continues to provide beams of secondary particles for precision studies in particle physics at low energies. In addition, muons produced from the decay of these secondary particles are used as probes for the study of the magnetic properties of materials within the Laboratory for Muon Spin Spectroscopy. The nominal current provided by the ring cyclotron is 2.2 mA corresponding to a beam power of ~ 1.3 MW. Approximately two hundred turns are required by the beam to reach full energy. The relative loss of beam power, essentially at the extraction elements is ~ 10-4. These losses result in activation of machine components rendering maintenance difficult. Currently a shut-down of approximately four months each year is required to maintain the machine while limiting radiation exposure to service personnel below regulatory limits. A view of the Ring Cyclotron is shown in Fig.1. The machine consists of 8 main magnets with four 50 MHz accelerating resonators and one 150 MHz “flat-top” resonator interspersed between them. Figure 1: View of the Ring Cyclotron. An up-grade of HIPA has been taking part in stages in order to increase the beam current to 3 mA (1.8 MW beam power). The major part of the up-grade is concerned with a replacement of much of the RF power equipment. The beam-current in cyclotrons is known to scale with the inverse cube of the number of turns required to reach extraction energy [1]. The fewer the number of turns the greater is the distance between proton bunches on adjacent turns and thus the beam can be extracted more cleanly without particle loss. Fewer turns implies the need for increased energy gain per turn. To this end four new copper resonators were installed on the ring cyclotron to increase the energy per turn. The last of these new resonators was installed in 2008. A photo of one such resonator, prior to installation in the ring, is shown in Fig.2a. New resonators for Injector II have also been built, Fig.2b, and installation of these will take place in the near future. A detailed description of the RF up-grade plans for the HIPA complex can be found in [2]. Figure 2a: One of the 50 MHz resonator of the ring cyclotron. Figure 2b: New resonator for the Injector II cyclotron The Swiss Light Source The Swiss Light Source is a 2.4 GeV, 400 mA electron storage ring for the production of synchrotron radiation. Synchrotron radiation sources have emerged as one of the most important tools for the study of the structure of materials at the atomic, molecular and mesosopic scales today [3]. The SLS serves a broad research community in the fields of condensed matter physics, material sciences, biology and chemistry. The storage ring has an injection complex consisting of a 100 MeV S-band linac and a full energy booster synchrotron. The machine has been in operation since 2001 [4]. The full energy booster permits “top-up” operation of the storage ring, whereby current losses due to finite beam life-time can be replenished by injecting additional current at given intervals of time. In practice the injection of ~ 1 mA every ~ 100 seconds maintains the current effectively constant, Fig. 3. The constant beam current, and, consequently, constant heat-load on optical elements of the synchrotron radiation beam lines is an important element in the positional stability of the photon beams received by users. In addition to top-up operation, a fast-orbit feedback system taking information from high precision beam position monitors ensures that the stability of both position (sub-micron) and intensity of the SLS beam satisfies the needs of its user community. The SLS demonstrates an excellent reliability. The availability of the machine is typically 99% of its projected 5’000 hours per year user time with mean times between failures of ~ 135 hours (2015 figures). A schematic of the SLS storage rings and its associated beam lines is shown in Fig. 4. Figure 3. Variation of beam current and vertical beam size in the SLS during user operation. Currently, many operating light sources are planning to up-grade to higher brightness machines through the installation of new lattices employing multi-bend achromats. These new diffraction limited light sources (DLSR) produce electron beams with two orders of magnitude reduction in emittance and with a corresponding increase in photon beam brightness. In order to remain competitive with other sources it is imperative that the SLS should up-grade to a DLSR. A novel lattice layout has been elaborated which allows a factor of 40 emittance reduction on the SLS despite its relatively small circumference (288 m) in comparison with other 3rd generation light sources [5].

Load more