XXIII European Synchrotron Light Source Workshop, November 2015 Eirini Koukovini-Platia Diamond Light Source Collective effects at Diamond Experimental and theoretical studies Acknowledgements: R. Bartolini, L. Bobb, R. Fielder, A. Morgan, S. Pande, G. Rehm, V. Smalyuk (BNL)

Multi-particle effects are important for the performance of an accelerator Its performance is usually limited by one multi-particle effect Beam loss or degradation can occur when the intensity is pushed above a certain threshold Collective effects (coherent) – Transverse Coherent tune shift Fast beam loss – Longitudinal Bunch lengthening, synchrotron tune shift, energy loss Beam degradation Multi-particle effects XXIII European Synchrotron Light Source Workshop, November 2015

Short-lived wakes (broad-band impedance) Coherent tune shift with increasing bunch current Study the transverse instability thresholds for – zero (Transverse Mode Coupling Instability - TMCI) and positive chromaticity (head-tail instability) – different RF voltages – closed and open Insertion Devices (IDs) Monitor the bunch lengthening with current using the streak camera (analysis to follow) Infer the machine impedance Single bunch instabilities XXIII European Synchrotron Light Source Workshop, November 2015

Diamond parameters Energy [GeV]3 Geom. emitt. x [nm.rad] (*)2.66 Coupling (%)0.3 Natural bunch length [mm] (*)3.7 Momentum spread (*)1.07 x Circumference [m]561.6 Mom. compaction factor1.6 x Tunes (x,y,s(*))27.21/13.364/ Energy loss per turn [MeV] (*)1.225 RF voltage [MV]2.5 Average beta functions (x,y)[m]10.65/12.84 (*) values when wigglers are on XXIII European Synchrotron Light Source Workshop, November 2015

Vrf=2.5 MV TMCI at (1.47±0.06) mA XXIII European Synchrotron Light Source Workshop, November 2015 Transverse mode coupling instability Horizontal plane (Q’x=0, Q’y=3)

XXIII European Synchrotron Light Source Workshop, November 2015 Simulation using sbtrack Horizontal plane (Q’x=0, Q’y=3) Horizontal impedance model Broad band resonator (BBR) impedance in x – Q = 1, R = 0.10 MΩ/m, f r = 8.3 GHz Resistive wall impedance – σ StSt =1.37x10 6 S/m, a=40 mm, b=12.1 mm Measured TMCI (1.47±0.06) mA sbtrack: 1.6 mA

Vrf=2.5 MV XXIII European Synchrotron Light Source Workshop, November 2015 Transverse mode coupling instability Vertical plane (Q’y=0, Q’x=3) TMCI at (0.58 ± 0.05) mA Fast transverse instability limits the bunch intensity to less than 7x10 9 p/b

XXIII European Synchrotron Light Source Workshop, November 2015 Simulation using sbtrack Vertical plane (Q’y=0, Q’x=3) Vertical impedance model in sbtrack Broad band resonator impedance in y – Q = 1, R = 0.25 MΩ/m, f r = 8.3 GHz Resistive wall impedance – σ StSt =1.37x10 6 S/m, a=40 mm, b=12.1 mm Measured TMCI (0.58±0.05) mA sbtrack: 0.65 mA

(for ξ=0 and l=0) XXIII European Synchrotron Light Source Workshop, November 2015 Imaginary effective vertical impedance Im(Z eff )=(334.2±0.5) kΩ/m

Lower RF voltage  lower synchrotron tune and a longer bunch length For lower RF voltage, the TMCI threshold is lower For higher RF voltage, the TMCI threshold is higher RF voltage [MV]TMCI threshold in y [mA] Vertical TMCI thresholds for different V RF XXIII European Synchrotron Light Source Workshop, November 2015

Vrf=1.7 MV XXIII European Synchrotron Light Source Workshop, November 2015 Transverse mode coupling instability Vertical plane (Q’y=0, Q’x=3) Measured TMCI 0.51 mA sbtrack: 0.5 mA

TMCI threshold for closed IDs (gap 5 mm): 0.58 mA TMCI threshold for open IDs (gap 30 mm): 0.74 mA TMCI threshold for open IDs (gap 30 mm): 0.74 mA XXIII European Synchrotron Light Source Workshop, November 2015 Effect of closed and open IDs in the vertical plane

XXIII European Synchrotron Light Source Workshop, November 2015 Simulation using sbtrack for open IDs Vertical plane (Q’y=0, Q’x=3) Broad band resonator (BBR) impedance in y – Q = 1, R = 0.25 MΩ/m, f r = 8.3 GHz Resistive wall impedance – σ StSt =1.37x10 6 S/m, a=40 mm, b=13 mm Measured TMCI 0.74 mA sbtrack: 0.75 mA

Head-tail instability with Q’y=3.5 XXIII European Synchrotron Light Source Workshop, November 2015 m=+1 m=0 m=-1 m=-2 Measurements sbtrack

XXIII European Synchrotron Light Source Workshop, November 2015 We need sufficiently high positive chromaticity to move the threshold to higher currents Summary of transverse thresholds

Multi-bunch instabilities XXIII European Synchrotron Light Source Workshop, November 2015 The phase advance between the betatron position of one bunch to the next, is given by the mode number μ

Grow damp experiment at Diamond Artificially excite mode  by using a stripline kicker for 250 turns at the frequency (pM +  )  0 +   Stop the excitation and measure free oscillations for 250 turns Run feedback to damp any unstable mode or any residual oscillation for 250 turns Repeat for all modes 936 bunches, 2 ns (500 MHz) spacing (full fill: M=936) 530 kHz revolution frequency, current up to 300 mA bunch length ps rms – with current XXIII European Synchrotron Light Source Workshop, November 2015

Grow damp experiment at Diamond amplitude 250 turns fit slope here Example of mode that is naturally damped Recording the complex amplitude on a turn-by-turn basis only of the mode previously excited XXIII European Synchrotron Light Source Workshop, November 2015 see G. Rehm et al IBIC turns

Growth rates of vertical coupled bunch modes Resonator Data suggest resistive wall and few high Q resonators Vertical TMF data full fill - zero chromaticity - ID gap open Resistive Wall β = m, b = 13.5 mm, ρ = 7.3·10 -7 Ω·m -measured -fit Courtesy V. Smalyuk, R. Bartolini

Effect of closing the IDs Closing the gap of all IDs changes the geometric and RW impedance ID res. walls geometry Forest of spikes at modes has been associated to IDs 3 4

Repeat measurements Large resonant peak moved in the last months from μ = 21 at μ = 81 Same conditions: full fill 936 bunches – 0 chromaticity – IDs open Collimator blade at 2.5 mm Collimator blade at 3.5 mm Collimator blades do not correctly return to set position when moved away and back again (using collimator to dump low current beam in MD had resulted in large drift during the last run) Courtesy R. Fielder

Status of the DDBA impedance model XXIII European Synchrotron Light Source Workshop, November 2015 See I. Martin talk

Elements included (so far) in the model ID transition taper ID full structure Pumping ports Upstream/downstream tapers Dipole radiation shading bumps U/s transition Dipole vessel

Kick and loss factors from DDBA impedance model XXIII European Synchrotron Light Source Workshop, November 2015 Loss factor (V/pC)Kick factor hor. (V/pC/mm) Kick factor vert. (V/pC/mm) Pumping Port Transition block Transition block&step Upstream end taper ID straight (5 mm gap) ID straight (15 mm gap) * ID straight (30 mm gap) Shading bump (symm.) Shading bump (asym.) U/s transition Dipole Vessel

Imported STL file in CST Particle Studio XXIII European Synchrotron Light Source Workshop, November 2015 Just by importing the dipole vessel in CST, the beam path follows the x- rays path rather than its own X-rays path Beam’s normal path

Rotating the geometry around the beam path XXIII European Synchrotron Light Source Workshop, November 2015 Faced a similar problem?

XXIII European Synchrotron Light Source Workshop, November 2015 Summary and future steps We found an impedance model that can predict well the transverse measured tune shift in all cases – zero/positive chromaticity – different RF voltages – closed and open ID’s TMBF system was used to study MBI and peaks were identified from the machine impedance database DDBA impedance database is in progress Next steps Fit several broad-band resonators to the total transverse and longitudinal impedance (database build from CST, analytical formulas etc) Compare measurements and simulation Continue the work on the DDBA

XXIII European Synchrotron Light Source Workshop, November 2015 Thank you for your attention!

XXIII European Synchrotron Light Source Workshop, November 2015 Back up slides

BPM buttons Resonance 1, mode 22 f r = GHz, Q = 2000, Δf = 61.7 MHz Resonance 2, mode 64 f r = GHz, Q = 20000, Δf = 51.4 MHz Resonance 3, mode 119 f r = GHz; Q = 1000; Δf = MHz XXIII European Synchrotron Light Source Workshop, November 2015 Courtesy V. Smalyuk, R. Fielder

Grow-damp - Vertical Collimator Scan of vertical collimator gap

Further analysis on IDs Occasional jumps in the mode spectrum with ID gap These are however “better understood” as they are clearly correlated to the ID gap changes – jumps of 10 or more modes ! XXIII European Synchrotron Light Source Workshop, November 2015