Recirculation: looking from inside

1. Recirculation: looking from inside Igor, Alessandro and Alexey Special thanks to Erik

2. 12 GHz PETS testing at CLEX, CERN (as it was shown in 2008) To the Load Variable Splitter (coupling: 01) Variable phase shifter PETS output CTF3 PETS input #1 Drive beam DL CR DBA CTF2 TBTS CLEX #2 #3 • Different scenarios of the drive beam generation in the CTF3 • In order to demonstrate the nominal CLIC power level and pulse length, it was decided to implement a different PETS configuration – PETS with external re-circulation. Round trip efficiency: 76% Round trip delay: 25 ns <30A 14 A Calculated output RF pulse envelops in PETS with re-circulation. Circles – mode 2, diamonds – mode 3, boxes – the CLIC pulse by design. Solid line – PETS output, dashed line – to the load. 4 A • To compensate for the lack of current, the active TBTS PETS length was significantly increased: from the original 0.215 m to 1 m. #1. The coupling and pulse length optimized to provide pulsed parameters comparable to the CLIC nominal values. #2. Full re-circulation (coupling=1) and full pulse length for the mode 3.

3. Rec. loop RF check configuration Measured transmission and reflection for the case of the full recirculation Measured transmission and reflection for the case with NO recirculation PETS #2 #1 #3 Network analyzer Transmissions: S12 Pistons position S13 Pistons position

4. PETS with recirculation operational analysis Measured spectrum of the recycling loop transmission Distance between bunches (~RF phase) PETS single bunch response (GDFIDL) Number of round trips Number of bunches Bunch amplitude (~q and F) artificial RF phase delay The complete system single bunch response spectrum Multi-bunch part • The method allows for the detailed reconstruction of re-circulation including: • The tuning of the loop phase length (if needed) • Manipulation of the current amplitude and RF phase along the bunch train • Provides direct calculation of the reflection in the loop and the power extraction to accelerating structure

5. Example of the full recirculation & rect. 200 ns DB current pulse Direct power production Power build up in 49 bunches Analyzing the spectrum of the signal, we can add artificially an extra RF phase delay () to the original measurements and tune the total recirculation phase: TBTS PETS single bunch response GDFIDL simulation Resonant extraction =0 single bunch signal circulating in a loop (full re-circulation) Destructive extraction =180 Power build up for the full (200 ns) beam and full recirculation forward reflected

6. Full recirculation and RF phase errors vs. simple model (used by Erik Adli) g=sgrt(0.752) t=25 ns As measured =0 =/2 Excellent agreement! = Off regime for the On/Off mechanism

7. NO recirculation single bunch signal circulating in a loop When we switch-off recirculation, the RF phase is needed to be shifted by /2 compared to the best phase advance used for the full recirculation. If not then residual recirculation will occur: =0 Back to the PETS To the structure From the PETS Losses in attenuator ~ 10% =/2 I=10A

8. Measured: Loop=0.76 Att=0.9 g2 = S Loop =TP Loop/ Att TP=S  Att Reference plane Power split=(1-S)Att In 2010 we have measured Tp (blue). In 2011 we measured g2 and Power split (red). Power balance (2011) zoom (1+S)-S 1-S = Power split / 0.9 S = g2 / 0.76

9. Following theory, both curves can be nicely fitted with cosine/sine functions: g2, Power split

10. The attenuator settings s=16.55 mm (g22011 = 0.334). The recirculation is tuned in phase with rectangular dB pulse Vs. simple model PETS output DB: 10 A x 250ns Att: 16.6 mm To structure R/Q= 2290 Ohm Beta= 0.453 C Q= 7200 L= 1 m With measured DB current pulse (FF2=1) Power production in the PETS (normalization): 1 A drive beam produces 0.306 MW RF power

11. The whole system signals analysis. The measured spectra of the 2nd attenuator and accelerating structure were used Simulated power (s=16.55 mm) scaled to the measured (s=16.60mm): F2=0.415 =3.73 mm From PETS (#707) From PETS (#707) Fitted phase error for recirculation is 1.80 Power split (#701) Reflected from 2nd attenuator All signals tracking F2=0.56 =3.03 mm (#710) F2=1.8 F2=0.415 =3.73 mm 707 701 F2=0.415 F2=0.415 702 708 These simulations allow to conclude on the relative calibration errors. For example, if one believe that #707 is properly calibrated, then power level in #701 is overestimated by 35% and in #710 by factor 4.3.

12. Spectral analysis vs. Adli’s model g2 = 0.334 measured g2 = 0.355 fitted 10% =1.80 F=1 The two approaches show a good agreement. The difference in recirculation gain is ~3%

13. Analysis stage by stage Drive beam amplitude The drive beam BPM bandwidth issues Rise time 20 ns Fall time 30 ns

14. Drive beam phase and frequency 20 degrees RF phase jump + frequency detuning by 1.1 MHz • The residual discrepancy of the pulses shapes can came from: • Variation of the bunches form factor • linearity of the detector

15. Relative single bunch form factor ‘simple’ sine-type modulation 0.19

16. All signals tracking Fitted form-factor 0.906 707 701 710 X 3.9 706 702 X 0.6

17. -50dB Directional coupler re-calibration