Brightness of the synchrotron radiation for various ID's and bending magnets as a function of photon energy
The third generation storage ring has been in operation since October 1993. It has been optimised and is continuously upgraded to provide the scientific community with photons in the energy range from a few to several tens of KeV with spectral brightness of up to 1019 photons/s/mm2/mrad2/0.1%bw.
A Free Electron Laser (FEL) is now in development and operates on the ring. New developments in the field of FELs are now under way based on the 1 GeV linac, to keep ELETTRA at the forefront of fourth generation photon sources.
Brightness of the synchrotron radiation for various ID's and bending magnets as a function of photon energy
Synchrotron radiation is produced when electrons travelling at relativistic speeds are deflected in magnetic fields. The storage ring is made up of four types of magnets: bending magnets that deflect the circulating electron beam into a closed circular path, quadrupoles that focus the beam, sextupoles that compensate chromatic and non-linear effects and steerer magnets that perform small adjustments to the circular trajectory. The arrangement of magnets forms a lattice of magnetic confinement elements. The lattice used for ELETTRA is an expanded Chaseman Green type also known as a double bend achromat. The ring is made up of twelve identical groups of magnets forming a ring roughly 260 m in circumference. A characteristic of third generation synchrotrons is the space that is made available in the lattice, the so called long straight sections, to install insertion devices of lengths up to 4.5 m. These are the principal sources of high brightness photons and are composed of arrays of magnetic poles that force the circulating electrons along serpentine trajectories. Insertion devices are of many types (electromagnets or permanent magnets) and depending on the magnetic configuration can be made to produce linear to circular polarised light. The wavelength of the light is tuneable by changing the magnetic field acting on the electron beam. For the electromagnets this is performed by changing the current flowing in the coils, whilst for the permanent magnets the field is changed by varying the distance between the top and bottom magnet arrays. One of these sections is used for injecting electrons into the storage ring.
General layout of the accelerator complex showing the position of the linac, transfer line and storage ring
Magnets of the storage ring
Typical insertion device
The electrons are generated in a LINAC (LINear ACcelerator) and then injected in the ring. Electrons start off from a ceramic disc that is heated to very high temperature. An electric field of up to 100 kV draws out the electrons that are then accelerated through various radio-frequency structures that make up the linac. The linac is composed of the Gun (that houses the ceramic disc), a low energy bunching section and several high-energy sections. The overall length of the linac is 66 m and contains seven high-energy sections each 6m long. Between accelerating sections quadrupole magnets keep the beam focused. The linac operates at 3 GHz and generates a pulse of electron bunches that are accelerated to a final energy which can be as high as 1.2 GeV. In order not to lose electrons as they are accelerated the entire linac is under vacuum, as is the transfer line and storage ring. In this way collisions with gas molecules, that would otherwise cause the loss of electrons, are avoided.
The cathode (part of the gun)
The linac tunnel showing SLED cavities,
accelerating sections and magnets
The electrons exiting the linac are then transported to the inner side of the storage ring by a transfer line (a series of deflection and focussing magnets). Both the linac and transfer line are below ground so as not to interfere with beamlines in the experimental hall. The storage ring is filled by a multi-turn injection process whereby pulses of electrons are gradually fed into the ring ten times a second until the desired current is achieved. Filling the ring to high currents takes several minutes.
The transfer line
The electrons circulating in the ring do so in a metal vacuum chamber. The vacuum that is maintained in the ring must be of very high quality, since unlike the linac and transfer line where an electron passes through once, in the ring the electrons travelling close to the speed of light traverse a given point more than a million times in one second. To maintain a long beam lifetime we must therefore reduce the chance of electrons colliding with gas molecules. The situation is further complicated by the copious emission of synchrotron radiation - around 90 kW of power just from the bending magnets. The unused radiation must be absorbed in special places otherwise chamber deformation and photo-electron release of surface gasses will occur.
A bending magnet vacuum chamber
An aluminum vacuum chamber
(bending and insertion device)
The brightness of the photon beam is derived from the small transverse size and divergence of the electron beam. A parameter that encompasses these dimensions is the emittance defined as the area occupied by the beam in phase space. To obtain a small emittance the beam is strongly focused by the ring quadrupoles. Furthermore the bending magnets have a gradient to provide additional focussing. The use of strong focusing magnets leads to increased chromatic aberrations simply because the beam contains electrons with a distribution of energies (up to a few percent of the total energy). If uncorrected these chromatic effects will limit the current that can be stored to a few mA. Sextupole magnets, placed at points in the ring where electrons with different energies travel different paths (dispersion regions), are used to compensate these unwanted effects. The story does not end here though because the chromatic sextupoles themselves introduce non-linear motion of the electrons that may lead to particle loss. This effect is in turn compensated by additional sextupoles place at points in the ring where the electrons travel the same path even if they have different energies.
SRPM image and the dimensions of the beam
The energy lost by the electrons when emitting synchrotron radiation is compensated by radio-frequency cavities. Four single cell cavities are used and have been positioned between dipole magnets in the dispersion region thereby allowing maximum use of the long straight sections for insertion devices. The cavities, operating at 500 MHz, produce a longitudinally bunched beam, simply because only those electrons arriving at the right time will be accelerated and the rest lost. The maximum number of electron bunches, separated by 2 ns, that can fit in the ring circumference is 432. There is a great deal of flexibility in filling the ring: from one bunch to any combination. The usual mode of operation is multi-bunch where roughly 95% of the ring circumference is filled with electron bunches. The summed voltages of the cavities determines the (longitudinal) energy acceptance and electrons having energies outside the acceptance are lost. The energy acceptance is an important parameter and partially governs the lifetime of the stored beam. This arises because collisions between electrons within the highly dense bunches (a consequence of low emittance) transfer energy from the transverse plane to the longitudinal (the Touschek effect). The lifetime is therefore essentially determined by the quality of the vacuum and the quality of the emittance.
The beam train seen on an oscilloscope
The rf cavities (SLS type)
Users are sensitive to variations in beam parameters. Unwanted motion of the electron beam translates as an effective emittance growth or worse still as jumps in intensity and loss of brightness. The disturbances have different time scales ranging from months to milliseconds and require different techniques for their suppression or control. Although much is done to passively control instabilities some residual effect is unavoidable. The slowest instabilities affect the orbit of the beam and are mainly due to changes in temperature (of buildings, electronic components, ring equipment, etc…). To control the orbit Beam Position Monitors - BPM's are used to provide information to orbit correction programs. The resolution of the BPM's has to be good ( a few microns) to enable effective control of the beam that has typical dimensions of tens of microns. The faster instabilities require feedback systems.
The BPM mounted on a quadrupole
The energy of the circulating electrons can be varied up to 2.4 GeV. A typical annual operating schedule allocates about 25% of beam time at this energy and the remaining time at 2.0 GeV. The storage ring operates on a twenty-four hour basis for up to 6500 hours a year (about 74% of the year). These hours are distributed into so called Runs, i.e., blocks of time that usually last seven to ten weeks. A Run is further split into periods lasting about a week for the production of light for the Users interspersed by one or two days of machine dedicated studies. Machine studies, performed by accelerator physicists and engineers, are all geared towards bettering the quality of the light and the commissioning of new systems. The Runs are separated by the Shutdown periods that usually last from one to four weeks. During Shutdowns maintenance of systems and the installation of new equipment is performed and is an essential activity in the life of the facility.
Histogram of the operating hours since commissioning