A Wall Current Transformer for Beam Intensity Measurements in the Large Hadron Collider

The Large Hadron Collider (LHC) collides two counter-rotating particle beams to recreate conditions that existed in the first moments after the universe began. Rather than forming a continuous stream, the beam particles are grouped into distinct nanosecond-long bunches. A suite of sophisticated instrumentation allows researchers to monitor the beam to assure an efficient and safe operation of the accelerator. One of the most fundamental measurements is quantifying the number of particles contained within each bunch, referred to as the bunch intensity. Charged particles of bunches travelling in an accelerator constitute an electric current, the integral of which is proportional to the bunch intensity. During the first years of the LHC operation four Fast Beam Current Transformers (FBCT) coupling to the beam current served as the primary bunch intensity monitors. However, their undesired sensitivity to the transverse beam position and their excessively long output pulses deteriorated the accuracy of the LHC bunch intensity measurements. This dissertation reports in detail on the author's research and development work on a new electromagnetic sensor overcoming the FBCT limitations. The Wall Current Transformer (WCT) consists of eight small magnetic cores which are evenly distributed azimuthally around a dielectric insert embedded into the accelerator's vacuum chamber. A conductive screw traversing each core's centre carries a part of the wall current, equal but opposite to the beam current, which induces signals in wires wound ten times around the cores. Such an arrangement is equivalent to eight parallel 1:10 current transformers sensing the whole wall current. Thus, the sum of the secondary-side signals is proportional to the instantaneous bunch current and upon integrating and rescaling results in the bunch intensity. To control its measurement bandwidth, the monitor features large ferrite cores and an RF bypass. A set of analogue front-end electronics adapts the WCT output signals to the used acquisition systems. The WCT's mechanical design enables its assembly around a closed vacuum chamber. The monitor is also fully compatible with the existing infrastructure at the original FBCT location. Prior to any particle beam measurements, the WCT's performance was quantified through extensive laboratory tests. The newly developed sensor demonstrated an excellent output pulse quality, adequate signal levels, sufficiently wide bandwidth, and a negligible longitudinal beam-coupling impedance. Such promising results motivated an installation of one WCT in the LHC to compare it with the FBCT as well as other sensors capable of measuring the bunch intensity. Besides a superior output pulse quality, the WCT was also the only tested monitor not showing any measurable sensitivity to the transverse beam position. Over the following years, additional WCTs replaced the remaining FBCTs and became the main LHC bunch intensity monitors. Although the WCT was optimised for the LHC bunches, it can find application in other facilities as well. Notably, it has already replaced another sensor in the second-largest CERN accelerator. Beyond bunch intensity monitoring, the WCT's ability of measuring the transverse beam position and the longitudinal bunch shape remains to be quantified through further studies.