Electron Cloud Studies in the Fermilab Main Injector using Microwave Transmission

Transcription

1 Electron Cloud Studies in the Fermilab Main Injector using Microwave Transmission J. Charles Thangaraj on behalf of E-cloud Fermilab (B. Zwaska, C. Tan, N. Eddy,..)

2 p ω c ω ω Microwave measurement principle The wind and the waves are always on the side of the ablest navigators ; 4 ; c r c k e e p p c ρ π ω ω ω ω = = The phase shift depends on the plasma frequency, which depends on the density. The phase shift per unit length is given by ) ( c p c L ω ω ω φ

3 Measurement setup and method Three different methods Direct phase shift Sideband spectrum Zero span

4 Measurement Techniques Sideband spectrum Send a carrier wave (1.5 GHz) Any phase modulation, then would show up as a sideband. Sideband dbc = 0 log (A/), where A is the amplitude of phase modulation Zero span Set the spectrum analyzer to the expected sideband frequency Collect data over the full injector cycle. The signal amplitude would show a increase in case of phase modulation Direct phase shift Microwave interferometry Convert the signal to baseband and record and average out the beam signals over a single machine turn.

5 Measurement Pickups Pickups are optimized to couple to TE 11 mode Cutoff for the beam pipe just below 1.5 GHz Straight (15m) Dipole (13m) db GHz

6 Measurement Location MI 5 BEND REGION Quad.5m Dipole 4m 1.9 m Dipole 4m Quad.5m MI 40 BEND REGION Quad.5m 15.1m Quad.5m Standard elliptical beam pipe At MI40 no magnetic field Heliax cable We get reflections

7 Sideband spectrum measurements Microwave Carrier -40dBc Beam Harmonics -65dBc 39mRad PM.mRad PM

8 Zero span sideband MI 5 BEND Transition Extract Beam 39.8e1 No Carrier -60 Signal (d dbm) Time (s) start 0.8s after $3

9 Zero span sideband MI-40 STRAIGHT Transition Beam 39.8e1 No Carrier -50 Signal (d dbm) Time (s) start 0.8s after $3

10 Direct Phase Shift Measurement V olts (83m Rad /V ) usec Down convert the transmitted microwave to baseband Phase shift by 90 0 using delay to cancel input. Turn-by-turn carrier frequency & phase should remain constant Beam harmonics average out after many turns Though the direct beam harmonics are large, the are not correlated with the microwave carrier and so average in time domain

11 Direct Phase Shift Results MI 60 MI 40

12 New e-cloud test lattice MI 51 TEST REGION A-B 51A-C B 1m A 1m C db RFAs RFAs Absorber 6 Round Pipe Absorber GHz Round beam pipe Short distance (1 m) for propagation Comparison with other diagnostics (RFAs) Cutoff frequency 1. GHz The ends of the beam pipe are elliptical and so reflection. a CAVITY!

13 Resonant BPM Immune to reflections Carrier frequency just below the cutoff for the trapped mode

14 Resonant BPM response

15 Resonant BPM measurements MI 51 dbm Spectrum No phase modulation observed Frequency (GHz) Amplitude Demodulation db(amrms* **) db(radrms**) Frequency (khz) Phase Demodulation Frequency (khz)

16 FNAL reflection method Immune to reflections (C. Tan Tech Note FERMILAB)

17 Transmission with and without absorbers No absorber (Sept 010) Absorber (Sept 009) Nodes

18 Sideband spectrum measurements MI 51 0 Spectrum dbm db(amrms**) db(radrms**) Frequency (GHz) Amplitude Demodulation Frequency (khz) Phase Demodulation Frequency (khz) e- e- e- e- e-

19 Zero span sideband MI Ramp Zero Span Sideband $9 Cycle 8.4e1 Transition 51 Transmission 51 No Carrier Transmission Extract dbm Time(s)

20 Laser detection of e-cloud: My two cents Can we use a laser to measure e-cloud? Challenge: very small phase shift Solution : Cavity type interferometry Could be sent through a dipole for measuring polarization changes? Could also be used to construct spatial profile

21 Summary Evidence of e-cloud at different locations established by three different methods Challenges remain: Non-homogenous e-cloud, magnetic fields, reflections We plan to redo the resonant BPM technique and start on a small scale testing new detection of e-clouds

22 Thank you

23 Status of Microwave Measurements in the Main Injector Nathan Eddy EC Meeting Oct, 009

24 Microwave Transmission Theory May 6, 009 Nathan Eddy 4 ) ( )] ( ) ( [ π ω ω ω φ ω ω ω ω ω φ c p p c c c L c L = c p c l ω ω ω φ = From plasma physics, expect a microwave travelling down a waveguide to experience a phase shift due to a homogeneous plasma From the microwave dispersion relation For an electron cloud is proportional to e density 4 c p πρ e r e ω =

25 Measurement Setup Made three different measurements of the phase shift Measure sideband spectrum of 1.5GHz carrier with SA For phase modulation of amplitude β, sideband dbc = 0log(β /) Measure 1 st sideband over a full MI ramp (800ms) in zero span mode with SA Mix down to baseband and record IF with deep memory scope (10MHz BW) Pickup connections to optimize coupling to TE 11 mode Measure -0db transmission for two pickups and 15m of beam pipe Cutoff for beam pipe is just below 1.5GHz May 6, 009 Nathan Eddy 5

26 Previous Measurement Locations MI60 Bend Region Quad.5m Dipole 4m 1.6m Dipole 4m Quad.5m MI40 Straight Region 17.6m Quad Quad.5m.5m At MI60 Bend Region able to use spare Heliax cable At MI40 Straight Region have to use RG8 bpm cable See an addition 0db of attenuation on transmitted signal Appear to get coupling between the cables Put the 40db drive amplifier in the tunnel at this location May 6, 009 Nathan Eddy 6

27 New Dedicated Pickups MI5 Bend Region 1.9m Quad.5m Dipole 4m Dipole 4m Quad.5m Straight (15m) Dipole (13m) MI40 Straight Region 15.1m Quad Quad m.5m Dedicated BPMs installed at MI5 Bend and MI40 Straight Standard elliptical beam pipe and pickups Completely field free at MI40 straight Good quality heliax cable pulled for each BPM Expect improved sensitivity for MI40 Straight db GHz May 6, 009 Nathan Eddy 7

28 Zero Span Sideband in Dipole at MI5 10//09 Nathan Eddy 8

29 Zero Span Sideband in Dipole MI60 Bend MI5 Bend? Beam 39.8e1 No Carrier Sig gnal (dbm) Time (s) start 0.8s after $3 MI60 was measured from 53 to 601, MI5 from 518 to 519 Drastically different response is not understood Beam Conditions or Location Dependency (Lattice) Propose remeasuring MI60 if possible May 6, 009 Nathan Eddy 9

30 Zero Span Sideband in Straight MI40 Before MI40 Now Beam Only Carrier -7.9dbm Carrier, No Beam Beam 39.8e1 No Carrier dbm Signa al (dbm) Time from $3 (sec) Time (s) start 0.8s after $3 Difference due to improved signal from better cables See roughly same behavior 10//09 Nathan Eddy 30

31 Direct Phase Shift Measurement Volts (83m Rad /V ) usec Mix the transmitted microwave signal to baseband Use the delay to effect 90 phase shift (zero DC offset) Theoritically, should only see PM modulation as AM cancels Scope aquires from ms to 0ms sampling at either 500MS/s or 100MS/s respectively Expect ecloud induced phase shift to be the same each turn The beam harmonics behave as noise which averages away Use 100 turn average at MI60 and 1700 turns at MI40 Size of the beam harmonics impacts the dynamic range May 6, 009 Nathan Eddy 31

32 Direct Phase Shift Results MI40 Before MI40 Now Need to calibrate response for current measurements mv/mrad good to ~3 Again see reasonable agreement May 6, 009 Nathan Eddy 3

33 Coated Pipe/RFA Comparison Setup at MI A-B 51A-C B 1m A 1m C Absorber RFAs RFAs 6 Round Pipe Absorber db GHz Three Large Aperture BPMs have been installed around 1m long test pipes at MI-51 Cutoff for the 6 round pipe is just below 1.GHz As measured phase shift is proportional to length -3db sensitivity Will provide direct comparison with RFA Expect to be very useful in understanding φ e density May 6, 009 Nathan Eddy 33

34 Initial Response at 51 Initially saw response during first beam (~0e10) Have not seen any signal since > 30e10 Resonant measurement? 10//09 Nathan Eddy 34

35 Summary Demodulate transmitted signal to separate PM & AM Verified we are observing Phase Modulation Verified expectation of no Amplitude Modulation Current response in MI5 dipole is not understood Would be nice to remeasure at MI60 Results from MI40 straight are as expected Calibrate phase shift to electron density Comparison with RFA measurements Comparison with simulation results 10//09 Nathan Eddy 35

36 Sideband Spectrum Measurement Microwave Carrier -40dBc Beam Harmonics -65dBc 39mRad PM.mRad PM 10//09 Nathan Eddy 36

37 Direct Phase Shift Results MI60 MI40 MI60 MI60 10//09 Nathan Eddy 37

38 Simple Model for Transmission From plasma physics, expect a microwave travelling down a waveguide to experience a phase shift due to a homogeneous plasma From the microwave dispersion relation k ω ω c ω p φ p = = c l c ω ωc ω For an electron cloud p 4πρ e r e c ω = is proportional to e density

39 e e Phase Modulation Measurement ( t) = Acos( ω t + β sin( ω t + φ )) β c ( t) A cos( ( ω ω ) t φ ) + cosω t + cos( ( ω + ω ) t + φ ) c m m m m c β c m m => Dbc db wrt carrier is proportional to pm amplitude

40 MI40 S1 Measurements Beam measurements on 9/4/09 show opposite change! see +3db change on carrier transmission at GHz Found N-connector upstairs misthreaded fixing did not change response perhaps was effecting earlier measurements? Any changes to beamline in this region? Perhaps beam valve was closed in August? -> remeasure with valve closed Do not see any change at MI5

41 Are Reflections an Issue? Attempted to look at step/pulse response but very difficult to interpret without very fast/short pulse Structure of S1 measurement suggests complicated system Do not expect observed structure from simple waveguide and bpm response Idea to look at PM measurements at different frequencies

42 PM Measurements -5dbc -65dbc Change of 13db or about a factor of 4 in phase shift Concrete evidence we do not have simple transmission

43 Oversimplified Examples No Interference Constructive Destructive

44 Measurement of ecloud Development in the Fermilab MI using Microwave Transmission Nathan Eddy, Jim Crisp, Ioanis Kourbanis, Kiyomi Seiya, Bob Zwaska, FNAL Stefano De Santis, LBNL May 6,

45 Fermilab Main Injector Pbar Production Project X Upgrade.1MW beam power, ~3e11 prot/bunch Predict 10 5 increase in ecloud density NuMI Batch 1 Batch 6 Batch 5 Main Injector 8GeV to 10GeV 6 Batches ~80 bunch/batch <1e11 prot/bunch 300KW beam power Batch Batch 3 Batch 4 May 6, 009 Nathan Eddy 45

46 Microwave Transmission From plasma physics, expect a microwave travelling down a waveguide to experience a phase shift due to a homogeneous plasma From the microwave dispersion relation k ω ω c ω p φ p = = c l c ω ωc p 4πρ e r e c ω = For an electron cloud is proportional to e density ω May 6, 009 Nathan Eddy 46

47 Measurement Setup Made three different measurements of the phase shift Measure sideband spectrum of 1.5GHz carrier with SA, for Phase Modulation β β e( t) A cosωct + cos( ( ωc + ωm ) t + φm ) cos( ( ωc ωm ) t φm ) Where β is the phase modulation amplitude, sideband dbc = 0log(β /) Measure 1 st sideband over a full MI ramp (800ms) in zero span mode with SA Mix down to baseband and record IF with deep memory scope (10MHz BW) Pickup connections to optimize coupling to TE 11 mode Measure -0db transmission for two pickups and 15m of beam pipe Cutoff for beam pipe is just below 1.5GHz May 6, 009 Nathan Eddy 47

48 Measurement Locations MI60 Bend Region 1.6m Quad.5m Dipole 4m Dipole 4m Quad.5m MI40 Straight Region 17.6m Quad Quad.5m.5m Necessary to access MI Tunnel to reconfigure bpms Bpms no longer available for operation Can be months between MI access opportunities Severely limits which bpms are available At MI60 Bend Region able to use spare Heliax cable At MI40 Straight Region have to use RG8 bpm cable See an addition 0db of attenuation on transmitted signal Appear to get coupling between the cables Put the 40db drive amplifier in the tunnel at this location May 6, 009 Nathan Eddy 48

49 Sideband Spectrum Microwave Carrier -40dBc Beam Harmonics -65dBc 39mRad PM.mRad PM May 6, 009 Nathan Eddy 49

50 Zero Span Sideband MI60 Bend Transition Bunch Rotation Ramp Start May 6, 009 Nathan Eddy 50

51 Zero Span Sideband MI40 Straight Transition Bunch Rotation Ramp Start May 6, 009 Nathan Eddy 51

52 Direct Phase Shift Technique Volts (83m Rad /V ) usec Mix the transmitted microwave signal to baseband Use the delay to effect 90 phase shift (zero DC offset) Theoritically, should only see PM modulation as AM cancels Scope aquires from ms to 0ms sampling at either 500MS/s or 100MS/s respectively Expect ecloud induced phase shift to be the same each turn The beam harmonics behave as noise which averages away Use 100 turn average at MI60 and 1700 turns at MI40 Size of the beam harmonics impacts the dynamic range May 6, 009 Nathan Eddy 5

53 Direct Phase Shift Results MI60 MI40 MI60 MI60 May 6, 009 Nathan Eddy 53

54 Summary and Plans To calculate the ecloud density is difficult Non-homogeneous plasma, magnetic fields, possible reflections Efforts underway to simulate the microwave transmission See TH5PFP019 and FR5PFP089 Right now, have very interesting measurements of microwave phase shifts under a variety of beam intensities Strong evidence that these are ecloud induced Use demodulation to uniquely identify PM and AM The end goal is to see good agreement between measurements and simulation for current MI intensities Must rely upon simulation to predict what measures are needed to mitigate the ecloud for Project X The direct phase shift in the time domain can be directly compared with the simulation of a single machine turn See TH5PFP03 During the upcoming summer shutdown, a dedicated system will be installed pickups in dipole bend, 3 pickups in ~m straight where two 1m coated beam pipes are being installed along with absorbers Facilitate ease of measurements Implement dedicated digital receiver measure only PM, improve S/N May 6, 009 Nathan Eddy 54

55 Backup Slides May 6, 009 Nathan Eddy 55

56 Mixing Phase Modulated Signal e e ( t) = Acos( ω t + β sin( ω t + φ )) c ( t) A cosω t + cos( ( ω + ω ) t + φ ) cos( ( ω ω ) t φ ) RF = neglect A e( t) c ω terms β LO = cos IF = RF x LO = = ( ω t + φ ) [ cos( φ ) + β sin( φ ) sin( ω t + φ )] L c L c A cos m c L m m m β ( φ ) + cos( ω t + φ φ ) cos( ω t + φ + φ ) L m β m m m L c β m m m m L May 6, 009 Nathan Eddy 56

57 Full Transmission Response May 6, 009 Nathan Eddy 57