Beam-based Alignment of Clic Accelerating Structures Utilizing Their Octupole Component

Alignment of the accelerating structures is essential for emittance preservation in long linear accelerators such as the Compact Linear Collider (CLIC). The prototype structures for CLIC have four radial waveguides connected to each cell for damping wakefields and this four-fold symmetry is responsible for an octupole component of the radio-frequency fields, phase-shifted 90 degrees with respect to the accelerating mode. The octupole field causes a nonlinear dependence of the transverse beam deflection with respect to the position within the accelerating structure. By transversely moving the beam with two upstream steering magnets, and observing the deflection with beam position monitors or screens, the electromagnetic center of the structure can be found. We discuss the applicability of this method for aligning the beam in the accelerating structures.