The LINear ACcelerator (LINAC) of ALBA 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. 


Schematic layout and picture of the ALBA LINAC.

A 90kV DC thermionic electron gun generates bunches of electrons of a length of 2ns. These electrons are extracted from a metal (tungsten impregnated of BaO) heated at 1200 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 64ns).

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 

  Parameter at LINAC exitSingle Bunch ModeMulti Bunch Mode 
Number of Bunches 1 to 6 [18...512]
Pulse Length < 1ns (FWHM) [36 … 1024] ns
Bunch Spacing 6 … 256 ns 2ns
Charge Q ≥ 1.5 nC 3 ≤ Q ≤ 4 nC  
Energy ≥ 100 MeV ≥ 100 MeV
Relative energy spread ≤ 0.5 % (rms) ≤ 0.5 % (rms)
Norm. Emittance (1σ) ≤ 30 π mm mrad
Energy Variation Pulse-to-Pulse 0.25% (rms)
Beam Position Stability <10% of beam size
Jitter pulse-to-pulse ≤ 100 ps (rms)