Laboratório Nacional
de Luz Síncrotron

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SUBSYSTEMS OF THE ELECTRON ACCELERATORS

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To construct a synchrotron light source it is necessary to provide stability conditions for an ultra-relativistic electron beam be stored for several hours emitting radiation.

In addition to the magnetic lattice, the set of magnets that deflects and focuses the electron beam, it is necessary for the storage of the electron beam:

a) an ultra-high vacuum chamber delimiting the transit area of the electrons and allowing the beam to remain stored in a clear environment;

b) radio frequency cavities, used to replenish the energy lost by the electrons in the form of radiation;

c) a set of auxiliary systems that allow the particle accelerator to function as a whole.

The main subsystems that make up the accelerators of a synchrotron light source are described below.

Vacuum System


The vacuum system defines the environment in which the electron beam travels under the influence of electromagnetic fields. This environment should be substantially free of gas molecules, since collisions between the electron beam and gas molecules can lead to loss of stored electrons and a rapid decrease in the beam current.

The average pressure over more than 500 meters long vacuum environment of the Sirius storage ring must be a trillion times lower than the atmospheric pressure. This corresponds to what is called ultra-high vacuum and requires several special techniques for its production.

The vacuum environment is physically delimited by a vacuum chamber. In the Sirius storage ring, the vacuum chamber in the region of the quadrupoles and sextupoles will be cylindrical, with a radius of 12 mm for the free region for the electrons, considerably lower than the values normally used in the current synchrotron light sources. As a comparison, the radius of the vacuum chamber of the UVX storage ring is 30 mm.

The small size of the vacuum chamber brings several consequences, among which is the need for distributed pumping with the use of the NEG (Non-Evaporable Getters) technology. The NEG is a thin film deposited on the inner surface of the vacuum chamber capable of trapping gases, providing a vacuum pumping effect. The technology for deposition of these films was established and tested at LNLS, after agreement with CERN (European Organisation for Nuclear Research).

RF System


In the storage ring,  the RF system replenishes the energy lost by the electron beam, mainly due to the emission of synchrotron light.

The main elements of the RF system are the RF cavities, metal structures that confine electromagnetic fields oscillating in the range of microwave whose fundamental mode of resonance has a longitudinal oscillating electric field in the direction of propagation of the electrons. The RF cavities are part of the vacuum chamber of the storage ring. Passing through the cavity in the correct phase of the oscillating electric field, the electron beam receives the energy needed to replenish what was lost along its trajectory.

Coupled to the cavities, there is a low and high power RF system that includes control circuits, power amplifiers and transmission lines. The total RF power required includes losses on the accelerating cavities, in waveguides and components of the transmission line between the generator and the cavities.

Each superconducting cavity is housed in a cryostat, immersed in liquid helium at a temperature of 4.5 K (equivalent to -268.65°C). The operation of these cavities requires the installation of a complex cryogenic plant for recovery and liquefaction of helium. This system includes a number of devices, such as a liquefier, a reservoir, compressors, high thermal insulation transfer lines and helium pressure and level control in the cavities.

Pulsed Injection System


The pulsed injection system is responsible for the process that allows the beam from the booster to be inserted in the storage ring. In this process, the injected beam must be added to the beam that is already stored in the ring. The current produced by the booster is just a small fraction of the desired current, requiring several injector pulses to reach the final operating current.

During normal operation of the ring, the injection system re-injects the current lost in the course of time due to the finite lifetime of the beam, maintaining practically constant the stored current. This mode of operation is known as top-up injection and requires that the injection process is very precise not to disturb the beam already stored in the ring. This is accomplished using pulsed magnets known as septa and kickers.

The septa are positioned in the region where the vacuum chamber of the transmission line that brings the beam from the booster and tangentially couples to the vacuum chamber of the storage ring. The septa produce a pulsed magnetic field that is strong on the region of the transmission line and drops to zero in a very short distance as not to affect the beam stored during the injection pulse. On the other hand, kickers produce a localized deflection of stored beam to pass near the orbit of the injected beam only during the injection process.

The injection mechanism requires a system responsible for synchronizing the various devices that are part of the injection system as a whole. This system should be capable of generating and distributing signals with temporal resolution smaller than 1 nanosecond, configurable pulse by pulse, with jitter of less than 1 picosecond RMS. The signals to be used to trigger events in both the source and in the beamlines.

Control System


The control system allows all other systems to work within the desired parameters, being responsible for sending, reading and controlling the various parameters of the equipment that make up the synchrotron light source. It is a communication network that interconnects all systems and equipment and provides control and parameters to the high level programs that control the source operation with around eight thousand points of control, with about two thousand microprocessor interfaces linked to about 400 computers.

It is a complex system and the major challenges associated with it is to obtain response times of a few microseconds in the various subsystems within the required for a stable and reliable operation of the accelerators, which implies fast communications systems and without the bottlenecks usually acceptable in conventional digital communication systems. The control system will be developed and implemented using the EPICS (Experimental Physics and Industrial Control System) programming and development environment, which is a standard control systems used in many other laboratories.

Diagnostic and Feedback Systems


The diagnostic and feedback systems are responsible for monitoring and dynamically correcting parameters of the electron and photon beans in different parts of accelerators and beamlines, and are essential to ensure high stability and reliability of the synchrotron light source.

Several equipment for monitoring, diagnosis and correction of the light source parameters are part of these systems, including position monitors for the electron beam; position monitors for the photon beam; monitor of the beam current; monitors for the beam oscillation frequencies and actuators to correct deviations in orbit  and oscillation frequencies.

The reliability and the proper design of diagnostic and feedback systems are fundamental not only to the rapid commissioning of the machine, but also for the operation of the light source in safe, reliable and stable way.

Beam Position Monitors


Among the greatest challenges of the diagnostic system is the design and fabrication of electron beam position monitors, as the precision of the measurement of the beam position must be in the order of tenths of micrometers and the measurement cannot disturb the stored beam. The electronic monitors must have temporal resolution for package position measurements at each turn in the ring (measures back-to-back position), which means about 500,000 precise measurements of the position every second.

The specifications for the stability of the synchrotron light source require a Beam Position Monitor that presents stability levels below 10% of the size of the beam being monitored, i.e., the monitor can only move, either slowly or quickly, less than 100 nm, which means stability better than 0.0001 mm. Temperature variations of the order of one tenth of a degree or mechanical vibrations of the soil and mechanical parts near the monitor can easily deteriorate its mechanical stability.

Orbit Correctors


To maintain the stability of the beam within the tolerances, there are two orbit correction systems dedicated to offset long and short term variations in the orbit of the electron beam. These systems use the position measurements supplied by the position monitor and act on sets of magnets (correctors) to minimize distortions in the beam orbit.

Short-term fluctuations are fixed by the fast orbit correction system, which operates at a rate of 10,000 corrections per second and should be able to mitigate fluctuations of up to 1 kHz in the orbit of the beam. This system requires special features to sources, magnets, control system and for the diagnostic system itself so that it can operate within specifications. The development of this system is being carried out at LNLS and is another major design challenge.

Another very important feedback system suppresses instabilities in the beam acting on each of the 864 electron beam individually. Each beam runs around the storage ring within 1.7 microseconds. This system uses sensors and actuators specially designed for this purpose.

Diagnostic of the Synchrotron Beam


To monitor the light beam delivered to users, X-ray monitors are being developed, which use the synchrotron light itself and that are positioned at the exits of the beamlines. Another major challenge in terms of diagnostic of the beam is the system for measuring the emittance. For being very small, the measurement of Sirius emittance is at the frontier of knowledge. A beam diagnostics beamline is being specially designed for this purpose.