26-October-2006 PEP-II MAC Session HOM measurement and analysis S. Weathersby, A. Novokhatski HOMs in LER region 4: overview, history Collimator wake fields.

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Presentation transcript:

26-October-2006 PEP-II MAC Session HOM measurement and analysis S. Weathersby, A. Novokhatski HOMs in LER region 4: overview, history Collimator wake fields Design of HOM selective absorber Experimental results LER abort spoiler investigations IP Q2 bellows

26-October-2006 PEP-II MAC Session Region 4 overview and history LER region 4 straight to arc transition temperatures unusually high: bellows, valves, pumps Burned antechamber TSP feedthru indicated HOMs coupling into accelerator components Temperature response to beam position at upstream collimator (10-15 meters away) Installed water cooled absorber in TSP pump chamber

26-October-2006 PEP-II MAC Session RF cavities 100 m TFB & LFB kickers 50 m Collimator 15 m HOMs in region 4

26-October-2006 PEP-II MAC Session Hot spots and diagnostics

26-October-2006 PEP-II MAC Session Antechamber absorber

26-October-2006 PEP-II MAC Session Recent developments Collimator wake field studies indicate presence of traveling waves: predominately dipole and quadrupole Design of selective HOM absorber: Intercept and dissipate harmful modes. Minimize beam impedance. Selective HOM absorber built and installed. Experimental data is encouraging

26-October-2006 PEP-II MAC Session downstream upstream Downstream electric field snapshots after bunch has passed 1.3 cm, 14 nC Gaussian bunch length ~0.6 m height ~ 3 cm Collimator wake fields downstream MAFIA Time domain

26-October-2006 PEP-II MAC Session downstream upstream Power propagation follows bunch Beam arrival

26-October-2006 PEP-II MAC Session How do we remove HOMs? The selective HOM absorber (trap) Devise a means to intercept traveling dipole/quadrupole HOMs before arc transition. Avoid disruption to beam image currents (monopole) Safely dissipate absorbed EM energy Ceramic tiles Coupling slots

26-October-2006 PEP-II MAC Session p1p2 FT Normalize to 1in Dipole mode example cuttoff Scattering parameters s 11, s 21 Port signals Analysis 1out/1in=s11 2out/1in=s21

26-October-2006 PEP-II MAC Session Optimal design Slot length: 70 mm Slot width: 6 mm Absorber thickness: 16.7 mm Slot length affects degree of coupling Slot width broadens spectra Absorber thickness lowers peak frequencies

26-October-2006 PEP-II MAC Session Experimental results Mechanical design and installation Experimental data Ceramic absorbing tiles Coupling slots Bellows cavity coupled to absorber Water cooling

26-October-2006 PEP-II MAC Session Tiles braised to water cooled copper support columns Ready for install Production

26-October-2006 PEP-II MAC Session Installation May 17, 2006

26-October-2006 PEP-II MAC Session Selective absorber installed 15 m More current, half the HOM heating

26-October-2006 PEP-II MAC Session 20 m Other bellows A

26-October-2006 PEP-II MAC Session Valve temperatures 14 m

26-October-2006 PEP-II MAC Session 20 m A At burned antechamber site

26-October-2006 PEP-II MAC Session Power at antechamber absorber

26-October-2006 PEP-II MAC Session Region 4 HOM Summary HOMs from collimators propagate large distances and infiltrate beam line structures with harmful RF energy Dipole and quadrupole modes predominate Selective absorber designed to specifically remove dipole and quadrupole HOM power from the beam chamber while presenting minimal disruption to beam image currents Selective absorber built and installed. Reduction in downstream temperature profiles

26-October-2006 PEP-II MAC Session LER abort spoiler investigations Diffuse aborted beam to protect abort window Large power measured during x4 bucket pattern at 500 mA (stored beam) Recent inspection revealed significant HOM damage

26-October-2006 PEP-II MAC Session Melted Ti

26-October-2006 PEP-II MAC Session Spoiler “trapped” mode GHz Q factor 1097  r =30, loss tangent 0.11,  r =1 absorbs E fields Copper chamber and Ti spoiler 2mm wide spoiler 476 MHz/4*5=.595 GHz -> close to by4 Electric field

26-October-2006 PEP-II MAC Session Magnetic field near base of spoiler and tiles Magnetic field Vs/M 2 1.4e-7 6.7e-7

26-October-2006 PEP-II MAC Session Surface currents

26-October-2006 PEP-II MAC Session Power loss density W/M 2 High power loss density on the edge

26-October-2006 PEP-II MAC Session Power loss density VA/M 2 Spoiler edge effect Magnetic field

26-October-2006 PEP-II MAC Session Shorter spoiler: 5 cm, GHz More field near absorber. Q lower by factor of 2 Shorter spoiler 7cm -> 5 cm Electric field

26-October-2006 PEP-II MAC Session LER spoiler Summary Trapped mode has observed power loss profile 50 MHz from by 4 frequency. 1 cm Mainly magnetic field at absorber location (lossy Ferrite?) Current density high on outer edges. Change profile. Length effects: 7 cm=.54 GHz, 5 cm=.62 GHz. Factor of 2 lower Q Increasing resistance ~factor of 2 Q decrease. (A. Kulikov) Increasing conductance raises Q dangerously. Change length to avert resonance Stay away from by4 Graphite spoiler

26-October-2006 PEP-II MAC Session Existing Q2 bellows absorption

26-October-2006 PEP-II MAC Session New proposal