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The Accelerator Complex

Electron gun and Linac

The ESRF pre-injector is a 200 MeV linear accelerator or linac. It consists of a 100 keV triode gun, a short standing wave buncher section and two 6-metre-long 2/3 accelerating sections. The two RF modulators are equipped with a 35MW Thomson TH2100 klystron and are operated at 10 Hz. The gun is triggered either at 10 Hz or at 1 Hz.

TL1: transfer line 1

The transfer line between the linac and the booster synchrotron is 16 metres long. It includes two 15 degree dipole magnets and seven quadrupoles. Energy selection slits are installed in the straight section between the two dipoles.  The electron beam is guided along the transfer line by two pairs of steerer magnets. The position of the beam is controlled by four fluorescent screen monitors.

Booster synchrotron

The booster synchrotron, with a circumference of 300 metres, has a 10 Hz cycling frequency and a low natural beam emittance at 6 GeV of 1.2.10-7 mrad.

The magnet lattice structure was designed to obtain an equilibrium emittance of the order of 10-7 p m.rad after acceleration to 6 GeV.

TL2: transfer line 2

TL2 measures 66m in length. It comprises 5 AC booster type dipoles connected in series, 14 DC quadrupoles and 17 DC steerers. The beam positions are controlled using fluorescent screen monitors.

Storage ring

The storage ring has a circumference of 844.4 metres. Its goal is to store the 6 GeV electron beam injected from the booster, and deliver it to the 42 beamlines.

The beam is guided onto orbit by 128 bending magnets with a longitudinal gradient and 96 bending magnets with a transfer gradient. The beam is focused by 412 quadrupoles. 192 sextupoles control the energy dispersion of the electrons. All the sextupoles have a dipole and skew quadrupole field for small corrections. 

The storage ring components are arranged in 32 cells with the same magnet distribution.
Each cell has one straight section 5.3 m in length where insertion device sources (IDs) of up to 4.5 m long can be installed.

Two classes of IDs are used: undulators and wigglers. Both are made of permanent blocks which force the electron beam onto a sinusoidal trajectory, stimulating radiation and increasing the brilliance of the beam.

The electrons radiate photons as they pass through the bending magnets and the IDs, and lose energy. To compensate, radio-frequency cavities restore any loss and maintain the energy close to the nominal 6 GeV.

The photon beam is extracted tangentially to the source towards the beamlines.

The essential target specification is expressed in terms of brilliance, which can be maximised by combining the following parameters in an optimal way:

  • the beam energy (6 GeV)
  • the beam intensity (200 mA)
  • the smallest achievable emittance (horizontal = 1.510 -10 mrad and vertical = 5.0*10 -12)
  • the gap, the field and the period of the insertion devices.