The LINear ACcelerator (LINAC) generates electron pulses at 90 keV and accelerates them up to 100 MeV. Afterwards, the beam is transferred and injected into the Booster synchrotron where the acceleration process continues. The ALBA pre-injector was purchased as a turn-key system and is in operation since 2010.

A 90kV DC thermionic electron gun generates bunches of electrons of a length of 2 ns. These electrons are extracted from a metal (tungsten impregnated of BaO) heated at 1,200 degrees. One can generate either one single bunch (SBM) or a train of several bunches (MBM), as it is shown next. The typical injection pattern consists of trains of 32 bunches, which are generated at a rate of 3 times per second. The maximum charge per bunch at the end of the LINAC is 0.25 nC.

Signals from the Fast Current Transformers installed along the LINAC for the two injection modes: MBM (train of 54 bunches) and SBM (16 single bunches separated by 64 ns).

Electrons pass a bunching region that reduces its bunch length and increases its energy. The bunching system comprises two pre-buncher cavities, one working at a sub-harmonic frequency (500 MHz) and one at 3 GHz, and a 22-cell standing-wave buncher. After the bunching system the beam energy is 16 MeV. Two travelling wave accelerating structures increase the beam energy up to more than 100 MeV. Each accelerating structure is made of 96 cells and works in the 2Π/3 mode and at a constant gradient of 10-15 MV/m. The beam focusing is ensured by the use of shielded solenoids at the bunching part and a triplet of quadrupoles between the two accelerating structures.

Two klystrons (TH2100) are used to feed the RF power to the LINAC cavities at 3 GHz by means of RF waveguides. Thanks to a switching system, if one of the klystrons fails the LINAC can keep providing a reduced beam of 67 MeV.

ALBA LINAC scheme with the RF distribution and a picture of the LINAC inside the bunker.

Under nominal conditions the LINAC is operated at 110 MeV in top-up mode. Recently it has been proven that, if needed, a LINAC beam of 67 MeV can be as well injected and accelerated in the Booster.

The energy spread of the LINAC beam is below 0.5% and its normalized emittance below 30 mm mrad.

Energy spread measurement of a 110 MeV and 1nC beam taken in a dispersive region. On the right are plotted the emittance measurements from the same beam for the x and y axis, taken by means of the quadrupole scan technique.

Specifications of the LINAC beam parameters

The ALBA booster is a synchrotron accelerating electrons delivered by the LINAC from an energy of 100 MeV to 3 GeV. During the energy ramping, the magnetic fields are adapted to the corresponding energy of the electrons. At 3 GeV the electron beam is extracted to be sent to the storage ring and the magnetic fields are restored to their initial values. This cycle is repeated 3 times per second.

The booster is located in the same tunnel as the storage ring. Its large circumference of 249.6 m, the large number of dipole magnets (40) and the magnetic lattice based on dipoles with integrated gradient, provide an equilibrium emittance as low as 10 nm∙rad. Such a small emittance, and electron beam size, provide high efficiency injection for the so called top-up operation. In this mode electrons are injected every 5-10 minutes into the storage ring, without interrupting the experimental data acquisition and keeping the intensity of the circulating electron beam constant.

The following table summarizes the ALBA booster main parameters.

The magnetic design of the ALBA storage ring (SR) is based on a modified Chasman-Green (Double Bend) lattice. The basic unit cell (repetitive structure) is an arrangement of two bending magnets accompanied by quadrupoles that produce non-zero dispersion in the straight lines between cells. Despite dispersion contributes to the radiation apparent beam size, the ALBA lattice design minimizes it while maximizing the available space in the straight sections.

The straight sections at ALBA are called LSS (long), MSS (medium) and SSS (short). The unit cell has MSS with small beta values which has very good properties to allow the installation of insertion devices, however they are not long enough to accommodate the injection straight. So, modified cells, called matching cells, are used to accommodate LSS. The following table summarizes the main parameters of the ALBA storage ring.