Magnets - bending and focussing the beam

Each of the 48 bending magnets at Diamond is approximately one metre long and consists of coils wrapped around the terminals of a ferromagnetic yoke. The yoke has a “C” shaped cross section viewed along the electron’s direction of motion. When an electric current flows through the coils it induces a magnetic field in the yoke and in the vacuum gap between the terminals of the yoke.

Electrons travelling through the vacuum gap experience a uniform, vertical magnetic field, which causes them to follow a bending path.

A sextupole magnet

To maintain a 3GeV electron beam in the desired orbit at Diamond requires a magnetic field strength of 1.4 Tesla (approximately twenty thousand times stronger than the earth’s natural magnetic field, or a 100 times stronger than a typical bar magnet). To generate such a field in the fifty millimetre minimum vacuum gap in Diamond’s bending magnets requires a coil with forty turns and over 1,300 amps of current! A large fraction of the energy used by the machine is taken up by the power supplies for the coils.

The spectral distribution (the number of photons with a given energy) of the bending magnet is a smooth continuum. The critical energy, proportional to the magnetic field and the square of the electron energy, is defined to be the energy at which half the power is radiated above, and half below. At Diamond, x-rays with an energy of eight thousand electron volts (8 keV) are the most abundantly produced by the bending magnets.

Diamond also uses quadrupole and sextupole magnets to keep the electron beam focussed and in the correct orbit.