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Experimental Observations and Simulations of Electron-Proton Instabilities in the Spallation Neutron Source Ring S. Cousineau, A. Shishlo, A. Aleksandrov, S. Assadi, V. Danilov, C. Deibele, M. Plum ECLOUD 07, Daegu, S. Korea
SNS Accelerator Complex Accumulator Ring Collimators Front-End: Produce a 1-msec long, chopped, H- beam Accumulator Ring: Compress 1 msec long pulse to 700 nsec 1 GeV LINAC Injection Extraction RF Liquid Hg Target RTBT 2.5 MeV 87 MeV 186 MeV 387 MeV 1000 MeV Ion Source HEBT DTL RFQ SRF,b=0.61 SRF,b=0.81 CCL Target Chopper system makes gaps 945 ns mini-pulse Current Current 1 ms macropulse 1ms
For eP instability mitigation: a) All pieces of vacuum chamber coated with TiN; b) Solenoids near the regions with high loss; (collimation?) c) Clearing electrode near the stripper foil; SNS Ring Parameters • Design ring parameters: • 1 GeV beam • Intensity: 1.41014 ppp • Power on target – 1.4 MW • Working point (6.23,6.20) • Ring circumference – 248 m • Space charge tune shift – 0.15
First Neutrons on April 28, 2006 • Beam and Neutronics Project Completion goals were met • 1013 protons delivered to the target • The SNS Construction Project was formally Completed in June 2006 • We have officially started SNS operations, and are in the power ramp-up phase.
Beam Power Ramp-Up – Timeline of Recent Events An aggressive power ramp-up schedule has been adopted 90 kW demonstration 60 kW demonstration 60 kW operation 1 GeV demonstration 30 kW operation
High Intensity Beam Studies For the nominal neutron production conditions, no instabilities have been observed so far. None were predicted. 8.51013 ppp • Dedicated, high intensity beam experiments have been performed. • 8.41013 ppp (13.5 uC) of bunched beam have been accumulated. • 9.51013 ppp (16 uC) of coasting beam have been accumulated. • For these experiments, we have varied parameters: • Chopped or coasting • RF on or off • beam intensity • chromaticity (natural or corrected) • lattice tunes
Vertical Horizontal Electron-proton instability for Coasting Beam • Most of the experiments performed for coasting beam configuration • Instability observed beginning at >21013 ppp. • Instability is observed in both planes – vertical stronger. • No instability is observed at natural chromaticity. BPM trace for a 16 C (11014 ppp) beam
4 C 8 C 16 C Intensity Scan of e-p Instability for Coasting Beam Turn-by-turn plot of frequency spectrum Observations: • Instability gets faster with increasing intensity (~40 turns for 16 uC case). • Frequency spectrum is more sharply peaked at higher intensity. • At highest intensity, frequency = 79 MHz.
Calculation of Effective e-p Impedance Evolution of the Dominant Harmonic We can estimate the “effective impedance” of the electron cloud, at different intensities: 8 C * Formula works well above threshold, requires no beam distributions information. 16 C 8 C beam: Re(Z)=168 K/m 16 C beam: Re(Z)=1.9 M/m We have seen 3 types of instabilities in SNS ring. e-P has largest impedance, by far (over 3 times larger for 16 uC case).
Coupling observed between transverse planes • Instability is observed in both planes. • Coupling is observed between the planes. Vertical BPM signal Horizontal BPM signal Both fractional tunes observed in the betatron spectrum of the horizontal data (Qx=0.23, Qy=0.2)
Split Tunes Case (6.23, 6.20) (6.23, 6.20) Nominal Tunes: (6.23, 6.20) (6.24, 6.16) (6.24, 6.16) Split Tunes: (6.24, 6.16) Tune splitting had only a small effect on the instability amplitude and frequency spectrum.
Instability for a chopped beam with no RF In the latest set of high intensity experiments, 8.51013 ppp of chopped beam accumulated with no RF on. Gap is mostly full by extraction. Some structure remains. BCM signal at extraction Vertical BPM signal e-P instability is observed in both planes (vertical BPM signal shown at right):
Longitudinal Position of Instability Instability occurs at flat top, closer to front of the beam, and moves backwards. Integrated signal for one electrode turn 300 Real space turn-by-turn evolution of instability turn 300 turn 200 head tail turn 50
Integrated signal from BPM electrode Wall current monitor signal Beam Loss in the Region of Instability For same turn number, wall current monitor shows beam loss in region of high instability activity.
Frequency Content of Chopped Beam Instability For the chopped beam with no RF, in frequency space the excitation bands drift downwards. The instability starts before end of beam accumulation.
e-P Simulations with ORBIT code • The parallel ORBIT electron-cloud: • Includes the interactions of electron cloud and proton beam in both directions (electrons act on protons and protons act on electrons). • Describes the electron cloud build up and includes a secondary emission surface model (Furman and Pivi model). • Uses PIC method for space charge for both proton and electron beam. • Uses 3D space charge for the proton beam • Allows an arbitrary number of localized electron cloud in the ring, up to the limiting computational ability of the parallel system. • Allows e-cloud nodes in magnets. • Model has been benchmarked with analytic model (Y. Sato) and PSR experimental data (A. Shishlo)
Simulation of Chopped Beam Case We performed simulations of the chopped beam case. Simulations done in two stages: Simulations by A. Shishlo Stage 1 Accumulate distribution. No ECloud nodes. Do it once only. Stage 2 Store distribution, insert ECloud nodes. Do multiple runs, varying e-node parameters. • Stage 2 parameters varied: • Number of e-Cloud nodes in the ring (<= 4 for computational expense). • Location of e-Cloud nodes • Type of initial electron cloud (surface or volume) • Proton loss rate (how electrons are ejected) Computational statistics: - 6106 proton macroparticles - 10,000 – 30,000 electron macroparticles per electron node - 60 CPUs, 80 GFlops for 24 hours.
Simulations of chopped beam with No RF • Case parameters: • 2 electron cloud nodes: one in drift, one in dipole. • Proton loss rate: 110-4 protons per meter. • SEM parameters: TiN, 100 electrons per proton. • Did not have good knowledge of energy distribution in beam, or evolution of beam in gap. • Observations: • Instability seen right at the beginning of storage. • Frequency content of instability is fairly narrow, possibly because of localized e-Cloud nodes.
Comparison of simulation and measurement – real space The longitudinal location of strongest instability is roughly the same between simulation and measurement. Both show migration toward tails. Turn-by-turn evolution of beam centroid Simulated Measured head tail head tail
Frequency comparison of simulation with measurement We see narrower excitation frequency in the simulation: 20 – 65 MHz. Excitation frequency content and extent is likely due the position and localization of the two ECloud nodes. We see the same drift of excitation bands to lower frequency in both simulation and experiment. Measured Measured Simulated Simulated
Comparison of centroid oscillation ~15 mm Experiment Centroid oscillation is larger in experiment than simulation. Experiment: ~15 mm Simulation: ~2.2 mm We see huge beam loss in experiment, but almost no beam loss in simulation. Simulation
Electron cloud density • In the simulation, complete neutralization of the beam in the gap occurs. • Unfortunately, no electron collectors were available in the experiment to measure e-cloud.
We have accumulated up to 8.41013 ppp of bunched beam, and 9.51013 ppp of coasting beam. No instabilities are seen for bunched beam, or for natural chromaticity beam. We see e-p instability for coasting beam and chopped beam cases with no RF Simulations of the chopped beam show reasonable agreement with experiment, though some differences exist. More accuracy in simulation might be gained by adding more e-cloud nodes, and having better knowledge of beam energy spread. Summary