Heterodyne measurement of Coherent Transition Radiation CTR from

Heterodyne measurement of Coherent Transition Radiation (CTR) from Seeded Self-Modulation (SSM) in AWAKE Falk Braunmueller, P. Muggli, M. Martyanov, F. Batsch, K. Rieger, A. Caldwell & AWAKE team 27 September 2017 3 rd European Advanced Accelerator Concepts Workshop Elba, Italy

Outline - Setup of heterodyne CTR-measurements - Measurement principle - Measurement processing - Main result: f. CTR = fplasma(n. Rb) - Further results: Dependence of SSM on Rbdensity gradient 2

SSM-Diagnostics via CTR Coherent transition radiation @ fmodulation (90 -280 GHz) preliminary Modulated p+-bunch OTR OAPmirror CTR F. Batsch, Poster session 19: 30 Courtesy: T. Haubold preliminary Coupled into WR 90 waveguide 15 m transmission

SSM-Diagnostics via CTR Coherent transition radiation @ fmodulation (90 -280 GHz) Modulated p+-bunch OTR Frequency: Heterodyne mixing OAPmirror 15 m transmission Courtesy: T. Haubold Amplitude: Schottky diodes Laser-based Waveguidebased

Diagnostic setup End of transmission line • 3 Heterodyne receivers for CTR: - Laser-based mixing presentation) (last Mirror - WR 8 / 90 -140 GHz: Radiometer. Beam splitter systemnew - WR 3. 4 / 255 -270 GHz: VDIsystem from EPFL | replaced by WR 4. 3/170260 GHz system August June • Can detect 2 nd harmonics of fmodulation 5

Measurement principle Signal: Intermediate frequency: f. RF ~260 GHz f. IF ~10 -20 GHz Reference: fref~270 GHz • Expected signal (f. IF=10 GHz) Mixer fref from frequency-multiplication of tunable local oscillator fref = nharm f. LO è Also mixing with weaker parasitic reference frequencies fref = nharm, 1 f. LO. (nharm, 1= nharm +/-1 , …) è Confirm that signal on oscilloscope is from mixing with correct reference frequency: fix • f. IF= | f. RF – nharm f. LO| measured setting to be determined nharm = ∆f. IF/∆f. LO 6

CTR-signal from mixer • Short signal, close to expected length • Very precise • Strong single-frequencycomponent (find via spectrogram) f. IF 7

Data-selection Choice of useful data: • Signal level large enough, 40 m. V e. g. > • Use only ‘prominent peaks’: Significantly higher than other IFpeaks Previously: selection ‘by eye’ (shot-to-shot variation of parameters) 8

CTR-analysis Fit f. IF vs. f. LO to check nharm In general, expected nharm =8 / 12 / 24 is confirmed (sometimes ambiguous) f. RF = nharm f. LO +/– f. IF Average & standard deviation of f. RF (here: 255. 9 GHz +/- 1. 4 GHz) Unclear if from change of CTR-freq. 9

Results of CTR-analysis Result: f. CTR vs. n. RB f. CTR = fplasma(n. Rb) Error bars: Std + 1. 5 GHz vapour è SSM with f. CTR=fplasma as predicted è Rb fully ionized • Good match between fundamental & 2 nd harmonics proof that correct nharm(f. LO) was chosen • Excellent fit result: parameters within 0. 3% • Error analysis incomplete Preliminary result (95% confidence of fit on mean) 10

CTR-amplitude (WR 4 -system) (Standard dev. ) • Preliminary result Amplitude increasing with beam-charge ECTR~q, but: - SSM-amplitude affected in nontrivial way - Emission angle & coupling may be affected è Promising for future analysis 11

f. CTR-dependence on n. Rb-gradient 10% gradient 138 GHz 10 m Preliminary result Negative Gradient: f. CTR=129 GH z~const. 12 131 GHz 12 5% gradient 134. 5 GHz 10 m 131 GHz Moving average Evolving interaction over several meters! • Frequency increasing with positive gradient, but basically constant with negative gradient Explanation from SSM?

SSM-Dependence on n. Rb-gradient No/Small gradient microbunches reach less far f. RF~fplasma(end) Preliminary result & possibly varying parameters ξ along bunch [a. u. ] Negative Gradient: f. CTR=129 GHz≈const. 10% gradient Check with simulations ? ! fplasma Gradient >5%: - Microbunches longer visible after seeding - f. RF corresponds more to fplasma(end) entry 131 GHz 13 138 GHz 10 m

Summary • Several successful upgrades of heterodyne CTR setup • Consistent results after data down-selection • Very successful measurement of f. CTR=fplasma(n. Rb), confirming full ionization + character of SSM • Clear correlation between beam charge & signal amplitude • Investigation of self-modulation physics: - f. CTR=fplasma(n. Rb, downstream) for positive n. Rb-gradient - Longer persisting microbunches • Analysis to be continued Preliminary results longer interaction? 14

Thanks for your attention! Acknowledgement (255 -275 GHz-system): Work supported by Requip, Sinergia and (No: 200020 -120503/1), grants of the Swiss National Science Foundation, by the Ecole Polytechnique Federale de Lausanne (EPFL) and by Faculty of Basic Sciences of EPFL.

Additional slides 22. 10. 2016

Analysis/Measurement To-Do-List - Apply criterion of prominent peak to all points - Analysis of signal amplitude: need to correlate with ‘good shots’ from streak camera & two-screen halo. BTV • Frequency-variations correlated with alignment/ angle of p+-defocusing/… ? - Ratio of signal amplitudes V(2 nd harmonics)/V(fundamental) vs. p+ charge • idea: more non-linear stronger 2 nd harmonics? 17

Measurement principle • f. RF = nharm f. LO +/– f. IF to be determined known measured f. RF=100 GHz Find nharm by scanning f. LO: nharm = ∆f. IF / ∆f. LO f. RF=260 GHz

Heterodyne Measurement • Measure intermediate frequency (IF) between CTRsignal (RF) and known reference • Reference signal from frequency-multiplied tunable local oscillator (LO) • Waveguide Transmission of RF over 15 m VDI heterodyne receiver from • Small measurement bandwidth Swiss Plasma • Good signal efficiency Center (SPC) at EPFL (Lausanne) ~10 -20 GHz out RF in Reference

Waveguide Transmission Line - Detector behind shielding wall - 15 m of overmoded waveguide WR 90 (fundamental mode 812 GHz) E-field polarization

Measurement principle • f. RF = nharm f. LO +/– f. IF to be determined known measured fixed f. LO f. RF=100 GHz Single f. RF with fixed f. LO can give several f. IF-signals Signal frequency must be constant to within 1 -2 GHz! f. RF=260 GHz nharm f. LO = f. RF