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 the signal from the BPMs is done by dedicated 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 done with the “Fast Orbit Feedback” FOFB system. This feedback starts by reading the beam position given by the BPMs, and computes the current that the corrector magnets need to provide to the beam to reach the ideal orbit. The system reads again the beam position, and the loop starts again. This process is done in a 5kHz rate (2000 times/second), which maintains the beam position within +/-(50, 30) nm. An example of noise correction during the FOFB test is shown in the image, with a comparison of beam position if the FOFB is on/off and while an Insertion Device (ID) moves its gap.

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 picture shows the electron beam distribution along one ALBA revolution time (896 ns). The beam is composed by 440 bunches spaced by 2ns each, and the different bunch current is due to the irregular efficiency along the injection chain.

Synchrotron Radiation Monitors

An image of the electron beam is obtained by using the visible 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 camera.

The image shows the electron beam images at different times during the Booster accelerator ramp, which is done by delaying the camera trigger. 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 using visible light (as in the Booster) is diffraction limited and another set-up must be used: the x-ray 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, the pinhole camera selects the hard X-ray part (around 45 keV) of the synchrotron radiation in order to obtain a transverse image of the electron beam and measure its beam size, from where the beam emittance is inferred. ALBA is equipped with two x-ray pinhole cameras, both providing similar beam images.

The image shown here is a picture of the transvers electron beam obtained using the X-ray pinhole camera at FE21, and the contour lines corresponds to the 2-d fit to obtain the beam sizes and tilt angle.