Measurement of the Beam Position in the TESLA Test Facility Linac

Abstract

The transverse beam position has to be measured at each superconducting quadrupole in the TESLA Test Facility Linac with a resolution of better than 10 μm. Therefore, a cylindrical cavity excited in the TM110-mode by an off-center beam was chosen, also because of the limited longitudinal space and the cold environment. The amplitude of the TM110 and its phase with respect to a 1.517 GHz reference signal are measured in a homodyne receiver. For the experimental area, stripline monitors having a resolution of better than 100 μm were developed. The averaged position of the whole bunch train of Injector I is measured using the amplitude-to-phase conversion. This paper summarizes the designs and some ‘beam position measurements’ in the laboratory. Introduction In order to establish a technical basis for a superconducting linear collider, the TESLA Test Facility is an essential part of the development of injectors, accelerating cavities, cryostat and new diagnostic techniques [1]. Because of special requirements three different types of beam position monitors will be used in the TTF: Buttons (Injector I), cylindrical cavities (inside the cryostats, attached to the quadrupoles) and striplines (experimental area). The purpose of this note is to discuss the cavities and the striplines, to describe the electronics, and to present measurement results on both types. Unfortunately, due to the delay in the installation of other components we still do not have any operating experience. TM110-Cavity For the alignment of the quadrupoles a single circular cavity was designed because of the limited longitudinal space and the desired resolution of 10 μm in a cold environment. The amplitude of the TM110-mode excited by the beam in the cavity yields a signal proportional to the beam displacement and the bunch charge. Its phase relative to an external reference gives the sign (up/down, left/right). Both TM110-polarizations have to be measured to get the xand y-offset. After cooling down, the seventh harmonic of 216.7 MHz has to be within the TM110-bandwidth to avoid an active tuning system inside the cryostat. The antennae are replaceable to allow a pre-tuning by adjusting the coupling before cooling down. In addition, two (‘warm’) cavities working at room temperature were built. Their temperature is stabilized in a thermostat and can be changed to tune the monitor slightly (about 20 kHz/K). In both cases CrNi was chosen as the cavity material to measure individual bunches spaced at 1 μs (Injector II). Most of the parameters given in Table I were calculated with URMEL, whereas the resonant frequencies and the coupling factors were measured at room temperature. Table 1: Design and measured parameters (at 25 ◦C). parameter ‘cold’ cavity ‘warm’ cavity cavity radius R0 115.2 mm 117.0 mm cavity length l 52.0 mm 52.5 mm beam pipe radius 39.0 mm 29.75 mm loss factor k110 0.24 V/pC 0.23 V/pC unloaded Q11