Preliminary proposal for SEYPreliminary proposal for SEYPreliminary proposal for SEY Preliminary proposal for SEY studies on cold surfacesstudies on cold surfacesstudies on cold surfacesstudies on cold surfaces

AsenaAsena KuzucanKuzucanAsenaAsena KuzucanKuzucan

The ElectronThe Electron Cloud EffectCloud EffectThe ElectronThe Electron--Cloud EffectCloud Effect• Electron cloud (EC) effects involve the interaction between high-energy beams and

low-energy electrons produced in the vacuum chamber.

• There are 3 different sources of EC:1) A major contribution to the electron cloud are photoemitted electrons (PE) produced through the collision of synchrotron radiation photons and the vacuumproduced through the collision of synchrotron radiation photons and the vacuum chamber walls.The photoemitted electrons can be accelerated by a charged particle beam, acquiring sufficient energy to produce secondary electrons by getting kicked to the q g gy p y y g gopposite walls of the vacuum chamber.2) Ionization of the residual gas3) Secondary electrons, emitted by primary electrons from the walls. This can lead ) y y p yto an exponential increase in the electron density during the passage of a bunch train.

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TThe Secondary Electron Yield (SEY)he Secondary Electron Yield (SEY)TThe Secondary Electron Yield (SEY)he Secondary Electron Yield (SEY)

• The electron cloud density depends on characteristics ofthe circulating beam (bunch length, magnetic field, chargeand spacing) and the secondary electron yield (SEY) of theand spacing) and the secondary electron yield (SEY) of thewall from which the electrons are generated.

• If the secondary electron yield (SEY) coefficient of the wallmaterial is greater than one, the runaway condition ofbeam-induced multipacting can develop.

• Typically a peak SEY value is δ ≈ 3 for an as received• Typically a peak SEY value is δmax ≈ 3 for an as receivedaluminium at the room temperature.

• The space charge from the cloud, if sufficiently large, canl d b i bili d l l i l ilead to beam instability and losses ultimately causing areduction in the collider luminosity.

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The Secondary Electron Yield (SEY)The Secondary Electron Yield (SEY)The Secondary Electron Yield (SEY)The Secondary Electron Yield (SEY)

The SEY (δ) definition is determined from equation:e S (δ) de o s de e ed o equa o

IIII

P

T

P

TP

P

SE

II

III

II −=−== 1δ

IP is the primary current( the current leaving the electron gun and impingingon the surface of the sample) and IT is the total current measured on theTsample (IT=IP-ISE). ISE is the secondary electron current leaving the target.

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Experiment description for cold measurementsExperiment description for cold measurementsExperiment description for cold measurementsExperiment description for cold measurements

• Th t d t SEY i d f UHV h b• The system used to measure SEY is composed of one UHV chamber where the pressure is in the low 10-10 mbar. We use a bakeable(300°C) UHV chamber. The UHV system is pumped by a 260 l/s turbo molecular pump A Bayard Alpert gauge and a electron gunturbo molecular pump. A Bayard Alpert gauge and a electron gun are already installed.

• The system will be equipped with a residual gas analyser too.T th t ili di d d l ti i t• Two thermometers , one silicon diode and one platinum resistance thermometer, will be installed on the cold head before we mount the cold head on the system.F h h l ill b b b d d b h• For the measurements the sample will be bombarded by the primary electrons with a variable energy between 50 and 3000 eV.

• We are going to use an electron gun, which is able to focus a beam 10below 2 mm at 200 mm working distance with a beam current 10-10

to 10-9A.

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Experimental setupExperimental setupExperimental setupExperimental setup

Cold SystemCo d Syste

Vacuum chamber

The place, where the cold head will be replaced soon

Electron gun

Turbo pump

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Experimental SetupExperimental SetupExperimental SetupExperimental Setup

• The path from electron gun to sample

-y

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Experimental SetupExperimental SetupCold Head• The liquid helium coming from a transfer line arrives into a phase separator.• Gas produced during the transfer feeds a heat exchanger which cools down a• Gas produced during the transfer feeds a heat exchanger which cools down athermal screen at about 30K.

• Liquid from the phase separator is transferred through an expansion valve to aexchanger The helium flow and a heater on the cold source permit to regulate theexchanger. The helium flow and a heater on the cold source permit to regulate thetemperature of the sample.• To combine good thermal contact (lowest attainable temperature) between thesample and the cold source with electrical insulation of the sample the cold headsample and the cold source with electrical insulation of the sample, the cold headhas a indium welded copper-sapphire-copper sample holder head.

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What is still to be done for cold What is still to be done for cold measurements?measurements?

• Th ld h d h b l d i th f l i L b A• The cold head has been cleaned in the surface cleaning Lab. A temperature sensor (DT-470-SD from Lakeshore) is to be installed on the cold head. H t d fi iti d i t ll ti ( d t h t b hi d• Heater definition and installation (we need two heaters, one behind the sample and one on the cold head)

• A shield around the cold head for a differential pumping is being dmade.

• Accordingly the cold head will be mounted and the first measurements will start.

• Bake out to remove the water vapor adsorbed on the surface.• We want to do measurements with gas injection to investigate the

effect of the residual gas in LHC ( How does the adsorption of a gas g ( p gchange the SEY?) For this measurements, a gas injection table has to be designed and installed.

• Electron dose effectElectron dose effect

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Measurements at room temperatureMeasurements at room temperatureMeasurements at room temperatureMeasurements at room temperature• First Measurements for checking of the system 1

not baked out P=6E-8 mbar

2.5

3

1.5

2

SEY Al

Cu

0.5

1 Au/Cu

SS

0

0 500 1000 1500 2000 2500 3000

Energy (eV)

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Measurements at room temperatureMeasurements at room temperatureMeasurements at room temperatureMeasurements at room temperature

• Drilled Cu

2 3

Drilled Cu vs. a.r. Cu

1 7

1.9

2.1

2.3

Drilled Cu

1 1

1.3

1.5

1.7

SEY

Drilled Cu

Ref. Cu

0.7

0.9

1.1

0 500 1000 1500 2000 2500 3000

Diameter of one hole=1mm, 92 holes/cm2

0 500 1000 1500 2000 2500 3000Energy (eV)

The microstructure can be a factor which can change the SEY. The commonlyaccepted hypothesis is that for a given chemical surface the surface withaccepted hypothesis is that, for a given chemical surface, the surface withmicrostructures has a lower SEY than a smoother one.

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Measurements at room temperatureMeasurements at room temperatureMeasurements at room temperatureMeasurements at room temperature• Distance effect

4 stainless steel samples has been used for the distance effect measurements. All the samples were mounted on a rotating sample holder with different distances from sample to collector. The distances of each samples are like following: p p gSample 1=145mm, Sample 2=152mm, Sample 3=157mm, Sample 4=160mm.

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Measurements at room temperatureMeasurements at room temperatureMeasurements at room temperatureMeasurements at room temperature

2.6

2.8

Distance Effect

2.6

2.8

Distance Effect after the change of the samples

2.0

2.2

2.4

145mm

152mm2.0

2.2

2.4

145mm

152mm

1.4

1.6

1.8

SEY

152mm

157mm

160mm

1.4

1.6

1.8

SEY

152mm

157mm

160mm

1.0

1.2

0 1,000 2,000 3,0001.0

1.2

0 1,000 2,000 3,000

Energy (eV) Energy (eV)

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Measurements at room temperatureMeasurements at room temperatureMeasurements at room temperatureMeasurements at room temperature

• Measured SEY decreases with an increasing distance After the first• Measured SEY decreases with an increasing distance. After the first measurements the results show, that our samples do not follow this thesis. The second nearest sample (sample 2) has the lowest SEY value. For the second measurements we changed the sample 1 with sample 2 to investigate, if an other effect dominates the distance effect After the second measurements we assumed thatdistance effect. After the second measurements we assumed, that other effects (like the roughness of the samples) could play a role.

• The samples have been given to the metrology lab for the surface examinations . They used a 3D VEECO NT3300 surface profiler for

i t h i ht d f t t Th lt f thmeasuring step heights and surface texture. The results from the metrology lab cannot give a satisfactory answer to our problem.

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Measurements at room temperatureMeasurements at room temperatureSample Roughness

parameter (µm)

1 Ra= 0.10Rz= 0.72

2 Ra = 0.10Rz= 0 69Rz= 0.69

3 Ra = 0.009Rz= 0.67

4 Ra 0 114 Ra = 0.11Rz= 0.82

Ra is the arithmetic mean of the absolute departures of the roughness profile from the p g pmean line.

Rz is the maximum peak to valley height of the profile within a sampling length.

∫=l

dxxzl

Ra0

)(1

The roughness parameter of the samples are quit similar so we cannot define the results just with the effect of roughness. Actually, several parameters (like chemical composition, treatment, history...) are known to affect SEY. Because of that, a full characterization of samples is important.

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