Magnetic Measurement

CERN is in the process of constructing the world’s largest particle collider, the LHC (Large Hadron Collider). The facility will operate on an array of superconductive magnets, interconnected to form a circle. With over 2000 of these magnets, quality control becomes a serious issue. All magnets must be calibrated against reference magnets to ensure operation is to specification.

CERN has developed a device for this purpose. The apparatus is a rotating measurement system, composed of several shaft segments connected together (see Appendix). The segments contain coils that rotate with the shaft; rotation is provided by a Twin Rotating Unit or TRU. Upon rotation, the magnetic field induces current in the coils; the output signals from the coils give the magnetic flux, as a function of angular position. This data is gathered for all dipole and quadrupole magnets used in the LHC, and checked against data collected from reference magnets. (Here forth, this measurement device will be referred to as a Rotating Coils Unit or RCU).

There exist two different length RCU’s (one for dipoles and one for quadrupoles), made up of different length shaft segments (1.2m and 0.75m, respectively). The segments use ceramic shafts (Al2O3) to support the coils and other components (see Figure 2‑1). These shafts are manufactured to tight geometric tolerances, and further, the segments themselves are manufactured with care.

Figure – Schematic of a shaft segment showing main components: (a) ball bearing, (b) bronze cage with roller, (c) Ti-bellow, (d) tangential coil, (e) central coil, (f) dowel pin, (g) ceramic support, (h) SiN Flange, (i) cable connector, (j) anticryostat, (k) cold bore.

Currently, RCU’s rotate 3 times in each opposite direction before reaching a constant speed (1 Hz maximum) for data acquisition, a process that takes about 15 seconds. However, faster measurement times are desired. The revised method of operation calls for continuous rotation of the RCU at 2 to 3 Hz (10 Hz maximum), resulting in a measurement time of less than 1 second. Still, problems may arise from this new operation method: mechanical vibrations during rotation may cause measurement errors. At present, a maximum rotation rate of 1 Hz is used in order to avoid such problems, however, since the segments have never been balanced before (just carefully manufactured), they may be inevitable at higher speeds.