The LHC microwave mode reflectometer (assembly version) reached operational status by the autumn of 2005. It is now routinely used in the LHC tunnel and on the surface to take data on the beam screens of the individual LHC magnets and also groups of magnets with lengths up to 100 meters. The reflectometer operates in the frequency range from about 4 GHz to 10 GHz and employs mode selective launchers. This paper discusses the operational aspects of the system as well as methods for clutter (fake reflection) elimination and procedures for cross-checks in case of a suspected obstacle or other faul

Borowiec, P

Caspers, Friedhelm

Kroyer, T

Sulek, Z

Williams, L

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsOperational experience with the LHC waveguide mode reflectometerThe LHC microwave mode reflectometer (assembly version) reached operational status by the autumn of2005. It is now routinely used in the LHC tunnel and on the surface to take data on the beam screens of theindividual LHC magnets and also groups of magnets with lengths up to 100 meters. The reflectometeroperates in the frequency range from about 4 GHz to 10 GHz and employs mode selective launchers. Thispaper discusses the operational aspects of the system as well as methods for clutter (fake reflection)elimination and procedures for cross-checks in case of a suspected obstacle or other fault.Large Hadron Collider ProjectAbstractT. Kroyer,  P. Borowiec, F. Caspers, Z. Sulek, L. R. WilliamsLHC Project Report 907CERN,CH-1211 Geneva 23,SwitzerlandGeneva, June 2006CERN, Geneva, SwitzerlandPresented atEPAC'06, Edinburgh, UK, June 26-30, 2006OPERATIONAL EXPERIENCE WITH THE LHC WAVEGUIDE MODEREFLECTOMETERT. Kroyer, P. Borowiec, F. Caspers, Z. Sulek, L. R. Williams, CERN, Geneva, SwitzerlandAbstractThe LHC microwave mode reflectometer (assembly ver-sion) reached operational status by the autumn of 2005. Itis now routinely used in the LHC tunnel and on the surfaceto take data on the beam screens of the individual LHCmagnets and also groups of magnets with lengths up to 100meters. The reflectometer operates in the frequency rangefrom about 4 GHz to 10 GHz and employs mode selectivelaunchers. This paper discusses the operational aspects ofthe system as well as methods for clutter (fake reflection)elimination and procedures for cross-checks in case of asuspected obstacle or other fault.INTRODUCTIONIn several accelerators foreign objects were found in thebeam aperture. In LHC, such obstacles would cause majordisturbances. Therefore the detection and localization ofsuch obstacles during LHC installation is highly desirable.Among a number of considered techniques, microwave re-flectometry was chosen as a promising approach for quickand sensitive inspection of single beam screens and stringsof interconnected magnets. This method is non-invasive,that is, there is no need to insert any tools into the beamscreen, which is problematic for cleanliness and vacuumreasons. At present, reflectometry is used routinely duringthe installation of LHC, in parallel with visual inspectionmethods.System overviewIn Fig. 1 a sketch of the measurement set-up is shown.Measurements are done in the frequency domain on a net-work analyser. In the beam screen, waveguide modes areexcited by custom couplers. Any discontinuity in the beamscreen gives rise to reflections, which are displayed afterappropriate signal treatment.For the routine inspections, two waveguide modes areused, namely the first TE mode and the first TM mode.The use of two modes makes two independent measure-ments possible and thus provides redundancy and/or addi-tional information about the properties of an obstacle. Thecouplers used to excite these modes have been designed,simulated and implemented. Besides having suitable RFproperties these couplers must also be short enough to fitin the space between two installed magnets, about 15 cm.For more detail on the device the reader is referred to [1].ObstacleCommunication, control & signal processing (Visual Basic, Matlab)Pipe under testCouplerNetworkanalyserFigure 1: System overview of the LHC reflectometer.Please note the beer bottle is only one example of an obsta-cle – fortunately it would not fit into the LHC beam screen.PRACTICAL APPLICATIONUsing microwave reflectometry one cannot see obstaclesas such. What is measured are the discontinuities in the RFproperties of the beam screen. We refer to an obstacle as anobject that in some way disrupts the continuity of the beamscreen, such as a change in• the beam screen geometry - a deformation or a pro-truding metallic object• permittivity - a dielectric object• permeability - a ferromagnetic objectDue to the small penetration depth in metals (≈1 µm incopper in the operational frequency range) for metallic ob-jects the surface and shape are relevant, while for dielectricand ferrite objects it is the volume. To give an example,it has been found in simulations and measurements that ametallic M3 nut (outer diameter: 5.6 mm, height 2.6 mm)lying in the center of the flat beam screen side gives a re-flection of about S11 = −34 dB for both modes. Whenthe nut is moved to the round side of the beam screen, thissignal reduces to −40 dB for the TM mode and to −45 dBfor the TE mode. This variation of the sensitivity over theazimuth is due to the modes’ field pattern [1].Typical measurement dataFor LHC magnets the operational frequency range waschosen as a trade-off between high spatial resolution (largebandwidth) and a combination of low insertion lossesand low spurious-mode excitation, which requires lim-ited bandwidth. The TE mode is operated between 4 and6.6 GHz and the TM mode between 6 and 10 GHz. A res-olution (6 dB peak width) of about 5 cm was obtained.Fig. 2 shows typical data from a single LHC dipole in-stalled in its slot in the tunnel. A few centimeters afterthe reflection from the coupler, the signal decreases to thenoise floor, which is in the range of −55 dB for the TE and0 5 10 15−80−70−60−50−40−30−20−100Distance from coupler [m]S 11 [dB]TETM75 cm patternCouplerEnd ofmagnetModeconversionFigure 2: Data from a single fully assembled dipole withthe TE mode (thin black) and TM mode (thick red).Waveguide dispersion and attenuation were removed nu-merically.−65 dB for the TM mode. Since the TE mode has also az-imuthal wall current components going “across” the beamscreen’s pumping slots, it can couple to the outside, i.e. tothe space between the beam screen and the cold bore. Asa result of this a structure with a period of 50 and 75 cmcan be seen. These patterns are related to the periodicity ofthe pseudo-random pumping slot pattern (50 cm) and to thedistance between the beam screen support rings (75 cm). Inpractise, this effect may limit the dynamic range achievablewith the TE mode and complicate the interpretation of thedata. Techniques for reducing these spurious periodic sig-nals are currently under development.The TM mode on the other hand is only weakly affectedby the pumping slots. However, since it is a higher ordermode, it is prone to mode conversion. Close to the endof the dipole in Fig. 2, a peak stemming from mode con-version to the TE mode can be observed. Other spuriouseffects that have to be coped with include:• Reflections from cable connectors and internally inthe network analyser• Multiple reflections between interconnects• Power “trapped” in interconnects for measurementsover chains of magnets• Coupling to beam positions monitorsFor these reasons, a number of checks have to be performedto exclude false indications.• The TE mode patterns can be correlated to magnetsthat were found to be clean• Mode conversion and connector reflections can beidentified by examining the peak’s dispersion charac-teristics• Multiple reflections are related to the spacing of thepreceding peaks.Repeating the measurements on single dipoles within sev-eral weeks showed an excellent reproducibility of the re-sults. The differences between subsequent measurements0 1 2 3 4 5 6 7−80−70−60−50−40−30−20−100Distance from coupler [m]S 11 [dB]Clean quadrupoleObstacle at 0.35 m from couplerFigure 3: TE mode signal from a quadrupole containing ametallic obstacle compared to reference data from a cleanmagnet. Similar data were found for the TM mode.were generally of the order of −70 dB. This shows thatthe traces as plotted in Fig. 2 are not produced by noisebut represent a characteristic signature of the magnet. Onlythe spurious peaks due to mode conversion may vary some-what, since the electric contact between the coupler and thebeam screen has some impact on this effect.For measurements over longer sections a rise in noise floordue to multiple reflections was observed. This effect even-tually limits the range of operation, since line attenuationis rather low compared to the system’s dynamic range.OBSTACLES DETECTED IN LHCMAGNETSIn the beginning of installation in the LHC tunnel allmagnets were checked visually before being intercon-nected. Reflectometry was used only for final checks oflong chains of interconnected magnets, e.g. over 100 m.The expected signal of a metallic M3 nut, i.e. −40 dB, wastaken as a preliminary acceptance criterium. A number ofsmall isolated peaks (below −40 dB) were detected, butcould not be confirmed visually. However, visual inspec-tions are difficult even on a single magnet due to multiplelight reflections inside the beam screen.The signal from the first major obstacle found is shownin Fig. 3. Since the object was very close to the end ofthe quadrupole, it could be easily seen optically and re-moved. The object was made of steel, about cylindrical inshape with ≈12 mm length and ≈5 mm diameter. It wasidentified as belonging to a tool that is used during magnetassembly.A very interesting case is shown if Fig. 4. In reflectometrywith both the TE and the TM mode a distinct peak right inthe middle of the magnet was observed; visual inspectionyielded suspicious results, too. After blowing dry nitrogeninto the not yet interconnected quadrupole a piece of metalswarf as shown on the right side in Fig. 5 was recuper-ated and the suspicious peak disappeared. Since this peak0 1 2 3 4 5 6 7−80−70−60−50−40−30−20−100Distance from coupler [m]S 11 [dB]First measurementAfter blowing withcompressed nitrogenFigure 4: TM mode signal from a quadrupole before andafter removal of a metal swarf as shown in Fig. 4.Figure 5: Some of the obstacles found in magnets in thetunnel.had been below −40 dB, the initial assumptions had to berevised. Now every peak clearly above the noise floor isconsidered as a potential obstacle and cross-checked withadditional measurements, e.g. from the other side of themagnet. In case of strong suspicion, blowing with com-pressed nitrogen or endoscopy is used on the magnet underexamination.In some cases significant changes in the reflected signalwere observed after such an intervention, which can be in-terpreted as stationary obstacles that changed their shapeand therefore their “cross-section”.Suspected obstacles are classified according to• the signal strength for each mode• whether the obstacle was detected from both sides ofmagnet• whether the signal changed after an intervention, suchas blowing with compressed nitrogen.As of mid June 2006, about 400 out of 500 installed mag-nets (which corresponds to about 25% of the LHC ringmagnets) in the tunnel have been checked using reflectom-etry. 5 metallic obstacles have been removed and there areabout 20 remaining suspicious cases, a third of them withstrong indications. In addition to that, several small plasticpieces were found by optical inspections (see Fig. 5, in thecenter). Due to their tiny volume (≈1 mm3) they did notgive a signal above the noise floor in reflectometry.ACTION TAKEN TO AMEND THESITUATIONThe obstacle problem is twofold. First, all significant ob-stacles already in the tunnel have to be removed. On singledipoles endoscopy and blowing with compressed nitrogenare the methods of choice, however there are ongoing dis-cussions about what to do with suspected obstacles in in-terconnected magnets.Second, it must be avoided that more magnets containingobstacles arrive in the tunnel. Careful analysis of the ob-jects found so far showed that they can be grouped in twocategories. A few probably found their way into the beamscreen during the magnet transport and assembly at CERN.The majority of cases however are pieces of metal swarfcoming from the beam screen manufacturing process andsurviving all cleaning processes. The plastic pieces areprobably fragments ripped off the plastic sheets in whichthe beam screens are wrapped when they arrive at CERN.This has triggered the following improvements:• The beam screen cleaning procedure is being re-vised; controlled brushing of possible obstacles be-fore chemical cleaning is under discussion as well asinspections of the beam screens before their insertioninto the magnets.• All beam screen equipped magnets will be inspectedvisually and by reflectometry during the final prepa-rations on the surface. Thereafter the beam tubes willbe sealed and kept closed right until magnet intercon-nection.CONCLUSIONThe LHC microwave reflectometer has been developedfor inspection of the beam screens. It is now exploited as astandard tool during installation over lengths up to 100 m.Due to a number of spurious effects, careful analysis of thedata is necessary. Together with visual methods reflectom-etry has been successfully applied to detect several obsta-cles of all kinds in the magnets. The discovery of theseobjects has led to improvements in the beam screen clean-ing and inspection process.ACKNOWLEDGEMENTSWe would like to thank the AB-RF workshops, NicoKos, Alexis Vidal, Patrick Coly and Paul Cruikshank forhelp. Thanks to Elena Shaposhnikova, Flemming Peder-sen, Trevor Linnecar and Steve Myers for support and fruit-ful discussions and to Alain Poncet, Philippe Lebrun andLyn Evans for support.REFERENCES[1] Kroyer, T., Application of Waveguide Mode Diagnosticsfor Remote Sensing in Accelerator Beam Pipes, CERN-THESIS-2005-061, 2005

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