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

945 ns

1 ms macropulse

Cur

rent

mini-pulse

Cur

rent

1ms

Front-End:Produce a 1-msec

long, chopped, H- beam

1 GeV LINAC

Accumulator Ring: Compress 1 msec

long pulse to 700 nsec

Chopper system makes gaps

Ion Source2.5 MeV 1000 MeV87 MeV

CCLCCL SRF, =0.61SRF, =0.61 SRF, =0.81SRF, =0.81

186 MeV 387 MeV

DTLDTLRFQRFQ

Accumulator Ring

RTBT

Target

HEBT

Injection Extraction

RF

Collimators

Liquid Hg Target

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

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;

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.

First Neutrons on April 28, 2006

An aggressive power ramp-up schedule has been adopted

30 kW operation30 kW operation

60 kW demonstration60 kW demonstration

60 kW operation60 kW operation

1 GeV demonstration1 GeV demonstration

Beam Power Ramp-Up – Timeline of Recent Events

90 kW demonstration90 kW demonstration

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

8.51013 ppp

High Intensity Beam Studies

For the nominal neutron production conditions, no instabilities have been observed so far. None were predicted.

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

Horizontal

Vertical

Intensity Scan of e-p Instability for Coasting Beam

4 C

8 C

16 C

Observations:

Turn-by-turn plot of frequency spectrum

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

0 100 200 300 400 500 600 7000.5

1

1.5

2

2.5

3

3.5

4

Turns

log(|

72n

d H

arm

on

ic|)

Evolution of 71st Harmonic

Re(Z)=168 K/m

0 50 100 1502

2.5

3

3.5

4

4.5

5

5.5

6

Turns

Log

(ma

gin

itu

de(7

5th

Ha

rmo

nic

))

Evolution of 75th Harmonic

Re(Z)=1.9 M/m

We can estimate the “effective impedance” of the electron cloud, at different intensities:

8 C beam:

16 C beam:

8 C

16 C

Evolution of the Dominant Harmonic

avetwiss

o

I

EZ

22

)Re(

* Formula works well above threshold, requires no beam distributions information.

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)

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.50

0.5

1

1.5

2

2.5

3

3.5x 10

4

Tune

Str

engt

h (A

U)

Tune Spectrum at Peak

Split Tunes Case

Nominal Tunes: (6.23, 6.20)

Split Tunes: (6.24, 6.16)

Tune splitting had only a small effect on the instability amplitude and frequency spectrum.

(6.23, 6.20)(6.23, 6.20)

(6.24, 6.16)(6.24, 6.16)

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

e-P instability is observed in both planes (vertical BPM signal shown at right):

Vertical BPM signal

Instability occurs at flat top, closer to front of the beam, and moves backwards.

Longitudinal Position of Instability

head tail

turn 50

turn 200

turn 300

turn 300

Real space turn-by-turn evolution of instability

Integrated signal for one electrode

Beam Loss in the Region of Instability

For same turn number, wall current monitor shows beam loss in region of high instability activity.

Integrated signal from BPM electrode

Wall current monitor signal

For the chopped beam with no RF, in frequency space the excitation bands drift downwards. The instability starts before end of beam accumulation.

Frequency Content of Chopped Beam Instability

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:

Computational statistics:- 6106 proton macroparticles- 10,000 – 30,000 electron macroparticles per electron node- 60 CPUs, 80 GFlops for 24 hours.

Computational statistics:- 6106 proton macroparticles- 10,000 – 30,000 electron macroparticles per electron node- 60 CPUs, 80 GFlops for 24 hours.

Stage 1Accumulate distribution.No ECloud nodes. Do it once only.

Stage 2Store 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)

Simulations by A. Shishlo

Simulations of chopped beam with No RF

Observations:

Instability seen right at the beginning of storage. Frequency content of instability is fairly narrow, possibly because of localized e-Cloud nodes.

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.

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

head tail head tail

Simulated Measured

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

Simulated

Measured

Simulated

Comparison of centroid oscillation~15 mm

Centroid oscillation is larger in experiment than simulation.

Experiment: ~15 mmSimulation: ~2.2 mm

We see huge beam loss in experiment, but almost no beam loss in simulation.

Experiment

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.

Summary

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.