Beam Position Monitors and Orbit Control

Beam Position Monitor devices provide a measurement of the beam's transverse position. ALBA uses button-type feed-throughs for all BPM blocks both at the injector chain and at the Storage Ring. The signal characteristics captured by the buttons mainly depend on the beam current, beam filling and on the BPMs and button's geometry.

Processing of the signal from the BPMs is done by edicated electronics (Libera Brilliance). These electronics provide positional data at different rates than those rates used for machine studies, machine protection system, post-mortem analysis and orbit control. The beam orbit control is the main role of the Beam Position Monitors. The main goal of the control loop is to maintain the beam position to be within 10% of the beam size and divergence at the synchrotron-light source point. That means that the beam will be within 13µm/0.6µm in the horizontal/vertical position and below 5µrad/0.5µrad in horizontal/vertical divergence. An example of noise correction during the FOFB test is shown in the image, with a comparison of beam position while closing an In-Vacuum Undulator gap, with, and without, FOFB running.

Beam Current Monitors

Both the Booster and the Storage Ring are equipped with a Fast Current Transformer (FCT) and a DC Current Transformer (DCCT). The FCT is a non-interceptive current transformer that measures the beam intensity from the magnetic flux induced in a toroidal core (coil) by a primary winding (the electron beam). But its useful measurement bandwidth is limited to between 1.5 kHz and 2 GHz. For this reason a DCCT, which is able to measure signals down to the level of DC by the use of active feedback with an RMS resolution of 5 μA, is used. The combination of these two current transformers provides us with the total beam current and the longitudinal beam distribution. The electron beam shown below is composed by 10 groups of 32 bunches each, where each bunch is separated by 2ns.

Booster Synchrotron Radiation Monitors

An image of the electron beam is obtained by using the light produced when the electron beam traverses a bending dipole. In the Booster, this is done with the Synchrotron Radiation Monitors. The radiation leaves the vacuum chamber through a sapphire window, and the visible part is chosen by using a silicon mirror located at atmospheric pressure. A tele-photo lens focuses the light onto the CCD, where the beam image is formed. By controlling the CCD trigger, we can obtain the evolution of the beam size along the Booster ramp. The image shows the electron beam images at different times during the Booster accelerator ramp. The horizontal and vertical scales of each picture are mm.

X-ray Pinhole Camera

In the storage ring, the beam size is so small that imaging, as in the Booster SRM, using visible light is diffraction limited and another set-up must be used: the pinhole camera. In this well-known system, light from a scene passes through a small aperture (pinhole) and projects an inverted picture of the scene in the image plane of the system.
At ALBA, this pinhole selects the hard X-ray part (around 45 keV) of the synchrotron radiation in order to obtain an image of the electron beam and measure its beam size, from where the beam emittance is inferred. The image is an example of the electron beam obtained using the X-ray pinhole camera. By subjecting the image to analysis, the beam size and beam emittance are obtained.