The LEBT (Low Energy Beam Transport) of an accelerator takes the beam extracted from the ion source and prepares it for injection on the next accelerating section, the RFQ. The ESS-Bilbao LEBT is composed of two solenoids placed at fixed positions, producing tunable magnetic fields. The solenoids have a smaller internal radius (involving more turns) at their ends than in their centres. This way, the magnetic field profile along the axis is flatter than the one achieved with an uniformly shaped solenoid; which would present a typical bell-shaped magnetic field profile. Besides, the variable radius approach creates a magnetic field that remains confined within the solenoid limits, avoiding perturbations on any nearby elements (e.g. other solenoids and the vacuum pump).
In order to save beam-line space, each solenoid includes a set of two crossed (x/y) dipoles of the cos θ type. The dipoles are capable of steering the beam to correct for misalignment of the beam line components, reaching a deflection up to ±4° of the protons. The presence of the dipoles limits the aperture to 100 mm. Although the complete LEBT is equipped with three diagnostic boxes; one before the first solenoid, one between solenoids and one after the second solenoid; at this first stage we use only one solenoid and two boxes (Figure 2). The first box is equipped with an AC Current Transformer (ACCT1), a double-wire Wire Scanner (WS1) and a retractile beam collimator (BC) with a 5 mm radius hole to create a pencil beam. The second box contains a second ACCT2, a second WS (WS2), a quartz window for fluorescence measurements and a retractile beam shutter that protects the quartz. The two wires of the WS are at 45° from the horizontal and vertical directions. A Princeton Instrument CCD camera complements the quartz window in the exit port of the diagnostic box, for 2D profile photographs and pepper-pot measurements. We used the camera also to record the Beam Induced Fluorescence (BIF) by mounting it in the side port of the first box.