Beam scrubbing: specifications of beams and transverse stability considerations

Beam scrubbing: specifications of beams and transverse stability considerations Many thanks to: T. Argyropoulos, T. Bohl, S. Cettour Cave, K. Cornelis, H. Damerau, J. Esteban Muller, F. Follin, S. Hancock, W. Hofle, C. Lazaridis, L. Kopylov , H. Neupert, Y. Papaphilippou, B.Salvant, E. Shaposhnikova, M. Taborelli, C. Zannini and the SPS operator crew G. Iadarola, H. Bartosik, N. Mounet, G. Rumolo

Outline • Electron cloud and scrubbing at the SPS • A “doublet” scrubbing beam for the SPS • Simulation studies • First tests at the SPS • Stability considerations • First observations • e-cloud driven instabilities • Impedance driven instabilities

Electron cloud and scrubbing at the SPS 2000 (48 b. - 0.8x1011 ppb @inj.) • In the past e-cloud has been strongly limiting the performances with LHC beams with 25 ns spacing (detrimental effects both on vacuum and beam quality) • Scrubbing runs regularly performed over the years with evident beneficial effects on dynamic pressure rise and beam quality • Status in 2012: No degradation due to e-cloud for nominal beam parameters. Emittance blow up observed on trailing bunches of the last batches for larger bunch population 400% G. Arduini, K. Cornelis et al.

Electron cloud and scrubbing at the SPS 2000 (48 b. - 0.8x1011 ppb @inj.) • In the past e-cloud has been strongly limiting the performances with LHC beams with 25 ns spacing (detrimental effects both on vacuum and beam quality) • Scrubbing runs regularly performed over the years with evident beneficial effects on dynamic pressure rise and beam quality • Status in 2012: No degradation due to e-cloud for nominal beam parameters. • Emittanceblow up observed on trailing bunches of the last batches for larger bunch population 400% G. Arduini, K. Cornelis et al. 2012 (288 b. - 1.35x1011 ppb @inj.)

Electron cloud and scrubbing at the SPS 2000 (48 b. - 0.8x1011 ppb @inj.) • In the past e-cloud has been strongly limiting the performances with LHC beams with 25 ns spacing (detrimental effects both on vacuum and beam quality) • Scrubbing runs regularly performed over the years with evident beneficial effects on dynamic pressure rise and beam quality • Status in 2012: No degradation due to e-cloud for nominal beam parameters. Emittanceblow up observed on trailing bunches of the last batches for larger bunch population 400% G. Arduini, K. Cornelis et al. 2012 (288 b. - 1.35x1011 ppb @inj.) 2012 (288 b. - 1.45x1011 ppb @inj.)

Scrubbing runs at the SPS • ~1-2 weeks periods (typically once per year) devoted to condition the beam chambers (i.e. lower the Secondary Electron Yield) by means of the electron cloud itself • Scrubbing is performed at the injection energy for the LHC type beams (26 GeV) in cycling mode refilling the machine every ~40 s • The achievable dose rate is typically limited by heating and/or outgassing on some sensitive machine elements (e.g. kickers, septa, beam dumps)

Scrubbing runs at the SPS • ~1-2 weeks periods (typically once per year) devoted to condition the beam chambers (i.e. lower the Secondary Electron Yield) by means of the electron cloud itself • Scrubbing is performed at the injection energy for the LHC type beams (26 GeV) in cycling mode refilling the machine every ~40 s • The achievable dose rate is typically limited by heating and/or outgassing on some sensitive machine elements (e.g. kickers, septa, beam dumps) 43.2 s

Beam requirements for scrubbing • The beam parameters need to be chosen in order to maintain a strong e-cloud flux on the chamber’s wall • Integrated dose is more important than peak flux  need for a reliable operation, reduce stress on sensitive machine elements • Beam quality requirements less tight than for the LHC filling but only as long as it does not compromise the scrubbing efficiency

Beam requirements for scrubbing • The beam parameters need to be chosen in order to maintain a strong e-cloud flux on the chamber’s wall • Integrated dose is more important than peak flux  need for a reliable operation, reduce stress on sensitive machine elements • Beam quality requirements less tight than for the LHC filling but only as long as it does not compromise the scrubbing efficiency 2006 Scrubbing Run E. Benedetto et al.

Why do we need a dedicated “scrubbing beam”? What do we need?

Why do we need a dedicated “scrubbing beam”? What do we need? MBB – 25ns beam What do we have?

Why do we need a dedicated “scrubbing beam”? What do we need? Possible issue: • The beam is still degraded due to EC • The dose is not sufficient to continue scrubbing in a reasonable time Possible solution: • A “scrubbing beam” which exhibits a lower multipacting threshold • The 25 ns beam is the ideal scrubbing beam for the 50 ns beam • What could we used to scrub for the 25 ns beam? MBB – 25ns beam What do we have?

Why do we need a dedicated “scrubbing beam”? What do we need? Possible issue: • The beam is still degraded due to EC • The dose is not sufficient to continue scrubbing in a reasonable time Possible solution: • A “scrubbing beam” which exhibits a lower multipacting threshold • The 25 ns beam is the ideal scrubbing beam for the 50 ns beam • What could we used to scrub for the 25 ns beam? Scrubbing beam MBB – 25ns beam What do we have?

A “doublet” scrubbing beam for the SPS • Due to RF limitations in the PS, impossible to inject bunch to bucket with spacing shorter than 25 ns • A shorter “effective spacing” can be obtained injecting long bunches from the PS and capturing each bunch in two neighboring buckets  train of doublets

A “doublet” scrubbing beam for the SPS • Due to RF limitations in the PS, impossible to inject bunch to bucket with spacing shorter than 25 ns • A shorter “effective spacing” can be obtained injecting long bunches from the PS and capturing each bunch in two neighboring buckets

A “doublet” scrubbing beam for the SPS • Due to RF limitations in the PS, impossible to inject bunch to bucket with spacing shorter than 25 ns • A shorter “effective spacing” can be obtained injecting long bunches from the PS and capturing each bunch in two neighboring buckets  train of doublets 25 ns 25 ns

Why should it work? • Mechanism of e-cloud enhancement Std 25 ns beam Below the threshold all the electrons produced after a bunch passage are absorbed before the next one small accumulationover subsequent bunch passages PyECLOUD simulation

Why should it work? • Mechanism of e-cloud enhancement Std 25 ns beam Doublet beam More e- production and shorter e- decay  accumulation possible PyECLOUD simulation

Simulation study • The doublet beam shows a lower multipacting threshold compared to the standard 25 ns beam if the intensity is larger than 0.8e11ppb (1.6e11ppb from the PS) MBB - 26GeV • Intensity per bunch of the doublet (b.l. 4 ns) (b.l. 3 ns) PyECLOUD simulation

Simulation study • The scrubbed region is smaller  to be used, with radial steering, as a last stage of the scrubbing MBB - 26GeV PyECLOUD simulation • Intensity per bunch of the doublet (b.l. 4 ns) (b.l. 3 ns)

First tests at the SPS • First machine tests have been conducted at the SPS at the end of 2012-13 run in order to validate the production scheme and obtain first indications about the e-cloud enhancement • The production scheme has been successfully tested for a train of (2x)72 bunches with 1.7e11 p per doublet 4 3 1st inj. 200 MHz RF Voltage [MV] 2 1 0 4 -1 0 6 2 6 Time [ms] Thanks to T. Argyropoulosand J. Esteban Muller

First tests at the SPS • First machine tests have been conducted at the SPS at the end of 2012-13 run in order to validate the production scheme and obtain first indications about the e-cloud enhancement • The production scheme has been successfully tested for a train of (2x)72 bunches with 1.7e11 p per doublet 4 3 1st inj. 200 MHz RF Voltage [MV] 2 1 0 4 -1 0 6 2 6 Time [ms]

First tests at the SPS • First machine tests have been conducted at the SPS at the end of 2012-13 run in order to validate the production scheme and obtain first indications about the e-cloud enhancement • The production scheme has been successfully tested for a train of (2x)72 bunches with 1.7e11 p per doublet 4 3 1st inj. 200 MHz RF Voltage [MV] 2 1 0 4 -1 0 6 2 6 Time [ms] Thanks to T. Argyropoulosand J. Esteban Muller

First tests at the SPS • First machine tests have been conducted at the SPS at the end of 2012-13 run in order to validate the production scheme and obtain first indications about the e-cloud enhancement • The possibility of injecting a second batch without degrading the circulating been has also been shown 4 3 1st inj. 2nd inj. 200 MHz RF Voltage [MV] 2 1 0 4 -1 0 3590 3592 3594 3596 3598 3600 6 2 3602 3604 6 Time [ms]

First tests at the SPS • First machine tests have been conducted at the SPS at the end of 2012-13 run in order to validate the production scheme and obtain first indications about the e-cloud enhancement • The possibility of injecting a second batch without degrading the circulating been has also been shown 4 Profile of the first doublet 3 1st inj. 2nd inj. 200 MHz RF Voltage [MV] 2 1 0 4 -1 0 3590 3592 3594 3596 3598 3600 6 2 3602 3604 6 Time [ms]

First tests at the SPS • First machine tests have been conducted at the SPS at the end of 2012-13 run in order to validate the production scheme and obtain first indications about the e-cloud enhancement • Clear enhancement observed both on e-cloud detectors and pressure in the arcs MBA MBB

First tests at the SPS • First machine tests have been conducted at the SPS at the end of 2012-13 run in order to validate the production scheme and obtain first indications about the e-cloud enhancement • Clear enhancement observed both on e-cloud detectors and pressure in the arcs 25ns std. (1.6e11p/bunch) (1.7e11p/doublet) 25ns “doublet” MBA MBB

Stability considerations • Beam quality requirements: • For SPS scrubbing purposes the goal is to inject up to 4 batches (4 x 72 doublets) and store the beam at injection energy for ~30 s with limited losses and emittance blow-up • Studies are ongoing in order to assess if this beam can be accelerated to 450 GeV and delivered to the LHC for scrubbing purposes  tighter beam quality requirements • The present SPS damper: • After the LS1 consolidation it will be able to detect and damp the motion of the centroid of each doublet  Intra-bunch motion and “pi-mode” of the two bunchletsare not detected

Tests at the SPS: first instability observations • The tests were carried out without the transverse damper and with high chromaticity (ξx,y~0.4)  Under these conditions, instabilities could be observed even at low bunch intensities

Electron cloud driven instabilities • By definition a scrubbing beam works in a severe e-cloud environment and is therefore prone to e-cloud instabilities PyECLOUD simulation

Electron cloud driven instabilities • By definition a scrubbing beam works in a severe e-cloud environment and is therefore prone to e-cloud instabilities PyECLOUD simulation HEADTAIL simulation

Impedance driven instabilities • HEADTAIL simulation studies (considering a single doublet) have been recently started • Impedance model includes: wall impedance(6 different vacuum chambers, including the magnets iron) + low frequency trapped mode due to MKE kickers with serigraphy Thanks to C. Zannini

Impedance driven instabilities • HEADTAIL simulation studies (considering a single doublet) have been recently started • First simulations with zero chromaticity and no transverse damper show an instability with a “p” mode (bunches oscillating rigidly out-of-phase) 1.3 1011ppb

Impedance driven instabilities • HEADTAIL simulation studies (considering a single doublet) have been recently started • Dependence of the growth rate on chromaticity has also been investigated These instabilities have a rise time of at least several thousands of turns so might well be damped by Landau damping from (natural) non-linearities

Summary • Build up simulation studies have shown that that the “doublet” beam features and enhanced scrubbing efficiency with respect to the standard 25 ns beam • The production scheme has been successfully tested at the SPS at the end of 2012-13 run • First indications from the e-cloud detectors and dynamic pressure rise look very promising • During the tests (carried out without transverse damper) instabilities could be observed • After LS1 the present SPS damper will be able to detect (and damp) the center of mass motion of each doublet but will not be able to detect: • Intra-bunch motion driven by e-cloud • “pi-mode” driven by machine impedance • By fighting this kind of instabilities, the high bandwidth feedback would help preserving the beam quality and therefore increasing the scrubbing efficiency

Thank you for your attention!

First tests at the SPS

First tests at the SPS • MBA-like Stainless Steel liner • agdsgfsh 25ns “doublet” (1.7e11p/doublet) 25ns standard (1.6e11p/bunch)

First tests at the SPS • MBB-like Stainless Steel liner • agdsgfsh 25ns “doublet” (1.7e11p/doublet) 25ns standard (1.6e11p/bunch)

SPS tests???? • 72 “doublets” Arcs • 72 bunches Higher press. rise Arcs

Beam scrubbing: specifications of beams and transverse stability considerations