Electron Cloud Studies - mgt-lhc-machine-advisory...

Electron Cloud StudiesElectron Cloud Studies

Tom Kroyer, Edgar Tom Kroyer, Edgar MahnerMahner, Fritz Caspers, CERN, Fritz Caspers, CERN

LHC MAC, 7. December 2007 LHC MAC, 7. December 2007

T. Kroyer, E. Mahner, F. Caspers, CERN 2LHC MAC, 7.12.2007

AgendaAgenda

Introduction to electron cloud effectsOverview over possible remedies

surface coatingsrough surfacesclearing electrodes

Tests planned in the SPSClearing electrodes

impedanceexperiments in the PS

Conclusion


IntroductionIntroduction

Since the first observation of the electron cloud (EC) in the sixties and seventies this effect has been found in many high intensity machines.In the past years a large effect has been put into understanding the EC by numerical simulations, machine studies and dedicated EC diagnostics. The goal is to mitigate or better completely suppress the effect.By tuning the beam and machine parameters the EC build-up can be reduced, but this is usually undesired, as it makes operation more difficult and affects the machine performance.


Remedies 1Remedies 1

There are two main approaches for suppressing the EC effects:

Modify the chamber surface such that the secondary emission yield (SEY) is minimized. This can be done by

conditioning the chambers by beam scrubbing, which costs machine timesurface coatings with low SEY with or without baking, such as TiN, nonevaporable getters (NEG), carbon, ...artificial surface roughness to increase the probability of a secondary electron to be absorbed by the surface. This roughness can be macroscopically (mm range) or microscopically (micron range).


Remedies 2Remedies 2

The second approach is to change the dynamics of the EC by externally applied electric, magnetic or EM fields

Solenoids have been shown to efficiently suppress the EC by confining the electrons close the chamber walls. However, solenoids can only be employed in straight sectionsClearing electrodes allow to apply an electric field that changes the trajectories of the electrons and may disturb the EC build-upElectromagnetic fields were also proposed for electron cloud clearing

Ideally we would prefer a passive means to suppress the EC that works even when the vacuum pipe has been vented and does not need baking


Studies for the SPSStudies for the SPSIn the framework of the SPS Upgrade Study Team convened by E. Shaposhnikova various approaches to suppress the electron cloud effect in the SPS are being evaluated. Several test chambers aregoing to be installed in the machine before the 2008 run, including surface coatings, rough surfaces and clearing electrodes. Dedicated EC diagnostics, including strip detectors, shielded pick-ups and the microwave transmission method are going to be used to evaluate the achievable EC suppression with magnetic field

MBA chamber

Copper strips

Holes (transparency 7%)

Beam

B field

courtesy: J.M. Jimenez

Strip detector Beam screen with electron collecting holes


Surface CoatingsSurface CoatingsThe SEY of a given surface is affected by

the properties of the clean surfacean oxide layer andadsorbed molecules

In general adsorbed molecules increase the SEY, while some oxides may have a lower SEY than the base metal (e.g. Cu2O)Coatings for SEY reduction are being actively researched at CERN, in particular sputter-deposited NEG, TiN and carbonThe LHC straight sections have been NEG coated, so there is a considerable body of experience with this techniqueVery low SEY have also been obtained with TiN, however there is a large scatter in the data reported in the literature. The challenge about TiN is to get a layer with low SEY with good reproducibility for large scale production. In addition to that is has to be examined if the obtained SEY degrades when the layer is exposed to airOther surfaces, such as carbon-like surfaces are also being considered

courtesy: S. Calatroni, M. Taborelli, P. Chiggiato, presented at Beam’07: http://indico.cern.ch/materialDisplay.py?contribId=75&sessionId=17&materialId=slides&confId=20082


Rough SurfacesRough SurfacesIn order to limit the increase in beam coupling impedance, macroscopic surface roughness should be in the shape of longitudinal grooves. Microscopic structures can have any shape as long as they are not much larger than the skin depth in the relevant frequency range. They should be mechanically stable and comply with vacuum requirementsThe challenge for rough surfaces is to produce such structures with a sufficient aspect ratio economically on an industrial scaleExample below: rough Ti surface produced by chemical etching

courtesy: S. Calatroni, M. Taborelli, P. Chiggiato


Clearing electrodesClearing electrodesIn several simulations and experiments is has been shown that localized clearing electrodes can effectively suppress electron multipacting close to the electrodeDistributed clearing electrodes could be used to fight the electron cloud effect over longer regions of an accelerator. However, issues such as impedance, aperture and manufacturing get more important for larger lengthsIn the past, electrodes for electron or ion clearing have been employed in several machines:

Ion clearing in the CERN AA (antiproton accumulator) machine where large stacks of p-bars had to be kept. The AA clearing system started off with 20 metallic electrodes and towards the end we had 50 ceramic ones with high resistive coating for beam coupling impedance issuesCeramic electrodes with highly resistive coating were applied in the CERN Electron Positron Accumulator (EPA)For the SNS floating-ground BPMs have been designed where a DC voltage can be applied for electron cloud clearing

EPA clearing electrodes


Electrodes implemented as aElectrodes implemented as a resistive resistive coating coating can be used can be used to minimize the to minimize the impedanceimpedance. They are basically . They are basically ““invisibleinvisible””electrodes in the sense that they are electrodes in the sense that they are much thinner than the penetration depth much thinner than the penetration depth and thus do not interact strongly with and thus do not interact strongly with the beam fields. Another condition for a the beam fields. Another condition for a low coupling to the beam field is that the low coupling to the beam field is that the coatingcoating’’s surface resistance is much s surface resistance is much larger than the free space impedance of larger than the free space impedance of 377 377 ΩΩ..

LowLow--impedance electrodesimpedance electrodes

resistive layer

dielectric

beam

Such an electrode behaves almost like a dielectric layer. Such an electrode behaves almost like a dielectric layer.

In practice, a solution could consist of a In practice, a solution could consist of a dielectric layer dielectric layer made of made of enamel or alumina for the dielectric isolator. A enamel or alumina for the dielectric isolator. A resistive coating resistive coating as the as the actual electrode is deposited actual electrode is deposited on top on top of the dielectric.of the dielectric.Such a structure in particular has good mechanic stability and gSuch a structure in particular has good mechanic stability and good ood thermal contact to the beam pipe; for a thin dielectric the aperthermal contact to the beam pipe; for a thin dielectric the aperture ture reduction is small.reduction is small.


There are two conflicting requirements for the resistive layer: There are two conflicting requirements for the resistive layer: In order to minimize the beam coupling impedance the resistivityIn order to minimize the beam coupling impedance the resistivity should should be high be high However, to minimize the voltage drop on the electrode for a givHowever, to minimize the voltage drop on the electrode for a given en clearing current the resistivity should be lowclearing current the resistivity should be low

Considering the clearing currents measured in the PS and the Considering the clearing currents measured in the PS and the impedance calculations, the surface resistance should be in the impedance calculations, the surface resistance should be in the range range of Rof R□□ = 1 to 100 k= 1 to 100 kΩΩThe standard coating technology uses thick film paste (mainly The standard coating technology uses thick film paste (mainly Ruthenium oxides) which is fired at 800 deg C on alumina. ApplicRuthenium oxides) which is fired at 800 deg C on alumina. Application ation of the thickof the thick--film paste onto enamel is being researched.film paste onto enamel is being researched.

Resistive layersResistive layers


The longitudinal impedance of resistive layer clearing The longitudinal impedance of resistive layer clearing electrodes was estimated analytically and using electrodes was estimated analytically and using numercialnumercialsimulations (CST Microwave Studio and HFSS) simulations (CST Microwave Studio and HFSS) For thin dielectric layers Z/n is essentially imaginary andFor thin dielectric layers Z/n is essentially imaginary and

Im(Z/nIm(Z/n) is proportional to the dielectric cross) is proportional to the dielectric cross--sectionsectionIm(Z/nIm(Z/n) increases slowly with ) increases slowly with εεIm(Z/nIm(Z/n) is rather flat up to very high frequencies) is rather flat up to very high frequencies

Estimation for one 0.1 mm thick electrode with Estimation for one 0.1 mm thick electrode with εεrr = 5 all = 5 all around the SPS (pipe radius 25 mm, 20 mm dielectric around the SPS (pipe radius 25 mm, 20 mm dielectric width): width): Im(Z/nIm(Z/n)= 0.3 )= 0.3 ΩΩ (entire machine today: Z/n~10 (entire machine today: Z/n~10 ΩΩ))

Longitudinal impedanceLongitudinal impedanceWire simulation of two thin clearing electrodes

Real part of long. impedance Imag part of long. impedance

plots for rotationally symmetric structure, courtesy: B. Salvant


The transverse impedance was estimated analytically for structurThe transverse impedance was estimated analytically for structures es with rotational symmetry using the with rotational symmetry using the BurovBurov--LebedevLebedev formula and formula and simulated using CST Microwaves Studio and HFSS [1]simulated using CST Microwaves Studio and HFSS [1]To first order the increase in ZTo first order the increase in ZTRTR is purely imaginary and frequencyis purely imaginary and frequency--independent; independent; Preliminary results scaled to one 0.1 mm thick centered electrodPreliminary results scaled to one 0.1 mm thick centered electrode e with with εε = 5 all along the SPS (pipe radius 25 mm, electrode width 15 = 5 all along the SPS (pipe radius 25 mm, electrode width 15 mm): mm): Im(ZIm(ZTR,yTR,y) = 4 M) = 4 MΩΩ/m (entire machine today: Z/m (entire machine today: ZTRTR ~ 20 M~ 20 MΩΩ/m)/m)

Transverse impedanceTransverse impedance

[1] T. Kroyer, F. Caspers, E. Metral, F. Zimmermann, Distributed electron cloud clearing electrodes, Proceedings of the ECL2 Workshop, CERN, Geneva, 2007

plot courtesy: E. Metral


During the shutdown 2006/2007 a During the shutdown 2006/2007 a ““simplesimple”” experiment was installed experiment was installed in the CERN PS [1,2]in the CERN PS [1,2]The goal was to research whether there is an electron cloud builThe goal was to research whether there is an electron cloud buildd--up up during the last few ms to during the last few ms to μμs, when the bunches get shortened before s, when the bunches get shortened before transfer to the SPStransfer to the SPSThe experiment comprised a shielded button pickThe experiment comprised a shielded button pick--up, vacuum up, vacuum diagnostics and a small dipole magnet. A diagnostics and a small dipole magnet. A striplinestripline electrode was added electrode was added to examine the properties of clearing electrodes.to examine the properties of clearing electrodes.

Recent experiments in the PSRecent experiments in the PS

[1] http://ab-div.web.cern.ch/ab-div/Meetings/APC/2007/apc070706/EM-APC-06-07-2007.pdf

[2] http://ab-div.web.cern.ch/ab-div/Meetings/APC/2007/apc070803/TK-APC-03-08-2007s.pdf


Components of the PS electron cloud setupComponents of the PS electron cloud setup

Shielded button pickupsShielded button pickupsPenning gaugePenning gauge

Clearing electrodeClearing electrode

HighHigh--voltage voltage feedthroughfeedthrough


The PS electron cloud experiment in SS98The PS electron cloud experiment in SS98

BPU 1 BPU 2Vacuum

gauge

Stripline upstream Stripline downstream

BeamB

BPU

Stripline

PS elliptical vacuum chamber with PS elliptical vacuum chamber with dimensions 1050 x 146 x 70 mm.dimensions 1050 x 146 x 70 mm.Special antechamber for clearing Special antechamber for clearing electrode without aperture electrode without aperture reduction.reduction.Material: stainless steel 316 LN Material: stainless steel 316 LN


A clear electron cloud effect was found during the last ~50 msA clear electron cloud effect was found during the last ~50 msIndications:Indications:

Vacuum pressure rise (see below)Vacuum pressure rise (see below)Current on the shielded pickCurrent on the shielded pick--upsupsClearing current on the Clearing current on the striplinestripline electrodeelectrodeThe effect can be switched off with an appropriate clearing voltThe effect can be switched off with an appropriate clearing voltageage

ResultsResults

26180 26185 26190 26195 2620010-9

10-8

10-7

12.6.2007

P [m

bar]

t [s]

SS 984xLHC25 (72 bunches)4xLHC25 (72 bunches)

~ 3.6 s~ 3.6 s

~ 0.8 s~ 0.8 s transition

ejection

Very fast pressure peaks with ~30 ms rise time, ~20 ms after onset of electron signal on the shielded buttons


Bias voltage on pickup: Bias voltage on pickup: +60 VNo magnetic fieldNo magnetic fieldLast three turns before ejection Last three turns before ejection plottedplotted

With a With a --300 V on the 300 V on the striplinestripline no no more electron cloud buildmore electron cloud build--up is up is visible above the noise levelvisible above the noise levelThe EC can be suppressed with The EC can be suppressed with positive clearing voltages, as wellpositive clearing voltages, as well

The clearing current on the The clearing current on the striplinestripline is large for positive is large for positive clearing voltages (~500 clearing voltages (~500 μμA/mA/m) and ) and small for negative clearing voltages small for negative clearing voltages (~2 (~2 μμA/mA/m))

Shielded PU signalsShielded PU signals

−6 −5 −4 −3 −2 −1 0 1−0.2

−0.15

−0.1

−0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

Time [μs]

Stripline signal [100 V]Shielded PU2 [V], no clearing voltageShielded PU2 [V], −300 V clearing voltage


The magnetic field B and the clearing The magnetic field B and the clearing voltage Uvoltage USLSL were varied; the points were varied; the points where measurements were taken are where measurements were taken are marked on the axis of the plotmarked on the axis of the plotThe The maximum EC signalmaximum EC signal on a shielded on a shielded pickpick--up over one turn is plotted up over one turn is plotted as a as a function of B and Ufunction of B and USLSL

The The RF gymnasticsRF gymnastics in the PS in the PS decreases the bunch lengthdecreases the bunch length, which , which allows to characterize the EC buildallows to characterize the EC build--up up as a function of bunch length (and as a function of bunch length (and long. profile) in one single shotlong. profile) in one single shotThe third bunch splitting takes place The third bunch splitting takes place 27 to ~5 ms before ejection, giving 72 27 to ~5 ms before ejection, giving 72 bunches with a 4bunches with a 4σσ bunch length of 14 bunch length of 14 ns. ns. 5 ms to ~300 5 ms to ~300 μμs before ejection: s before ejection: Adiabatic bunch compression, bunch Adiabatic bunch compression, bunch length decreased to 11 nslength decreased to 11 nsLast ~300 Last ~300 μμs: bunch rotation, bunch s: bunch rotation, bunch length reduced to 4 ns.length reduced to 4 ns.

Effect of the magnetic fieldEffect of the magnetic field

Maximum EC signal on shielded PU1 [V], 20 ms before ejection, as a function of the magnetic field B and the clearing voltage USL

[V]


EC signal from shielded PU1 plotted at different times before ejEC signal from shielded PU1 plotted at different times before ejectionectionBuildBuild--up starts earlier with magnetic field; Islands with large EC appup starts earlier with magnetic field; Islands with large EC appear in the ear in the parameter space.parameter space.For large enough clearing voltages (|UFor large enough clearing voltages (|USLSL|> 1 kV) EC suppression was found in all cases|> 1 kV) EC suppression was found in all cases

Islands with surviving ECIslands with surviving EC

t=-45 ms t=-20 ms t=-10 ms

t=-2 μst=-100 μst=-1 ms

[1e-3]


ConclusionConclusionThe electron cloud effect is a serious issue for the CERN SPS and LHC and potentially for the PS with intensities beyond nominal LHC beamThere are various means for combating the EC; at the end of the day we would like an economic and reliable solution that can be installed over large sections of the machine. Ideally, itshould work even after venting the vacuum chamber and without bakingDevelopment work is ongoing on surface coatings, rough surfaces and clearing electrodes. These concepts will be tested in the SPS in 2008. In parallel, EC studies in the PS are going to be continuedThe concept of low-impedance enamel clearing electrodes was outlined and the efficiency of clearing electrodes demonstrated in the PS

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