The Electron Cloud And Secondary Electron Yield

Tom SchmitMentor: Bob Zwaska

The Electron Cloud• Fermilab and the intensity frontier – Accelerators are approaching a new regime

in intensity– Many difficulties are associated with

increased intensity

• Increased intensity results in electron cloud instabilities– Very hard problem to solve analytically– Not well understood

The Electron Cloud• A secondary effect of accelerating

particles

– Primary electrons collide with beam pipe and components and generate secondary electrons• In proton/antiproton accelerators, primary

electrons are formed mostly from residual gas ionization

• In electron/positron machines, primary electrons are generated by the photoelectric effect

– Secondary electron yield greater than unity for most materials => AMPLIFICATION!

Secondary Electron Yield

• Combat electron cloud formation by reducing secondary electron yield

• Commission a test stand to examine different materials for use in a beam line–Must be vacuum compatible– Durable– Have a low SEY

HV Supply

I

Electron Gun

Sample Bias

Picoammeter

Sample

The Setup

SEY Measurement Technique

• Bias the sample to +500 volts and measure beam current

• Bias sample to -50 volts and measure current• SEY current is the difference between the -50 V

bias reading and the +500 V bias reading• SEY is then given as the ratio of SEY current to

beam current

I

In Practice

-> Electron beam indicated by red arrow

•Sample is stationary and electrically isolated (mounted to SHV feedthrough)

•Pumped with Varian TPS-Compact pumping station (turbo pump backed with oil-less scroll pump)

•Inverted Magnetron Pirani vacuum gauge

SEY Measurement Technique• Why -50 volts?

• Convention suggests -20 volts

0 500 1000 1500 2000 2500

-2.5

-2

-1.5

-1

-0.5

0

Secondary Electron Yield as a Function of Beam Energy

-50 V Bias-20 V Bias-70 V Bias-100 V Bias-150 V Bias

Beam Energy (eV)

Yiel

d

0 500 1000 1500 2000 2500

-2

-1.8

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

Secondary Electron Yield as a Function of Beam Energy

+500 V / -50 V

Beam Energy (eV)

Seco

ndar

y El

ectr

on Y

ield

Initial Results• Copper sample• Qualitatively good data

Initial Results• Copper sample• Proof of conditioning

0 500 1000 1500 2000 2500

-2.5

-2

-1.5

-1

-0.5

0

Secondary Electron Yield as a Function of Beam Energy

July 20 (Unconditioned)

July 20 (Conditioned)

23-Jul

Beam Energy (eV)

SEY

Refining the Technique• Stray magnetic fields from gauge and ion pump

had an unknown effect on low energy beams• Mu-metal shielding

• SEY measurements appeared to be sensitive to gun parameters• Spot size changes as parameters change• Measurement drift due to self-conditioning

• Perform beam studies• Speed up measurements by automating process with

LabView

• SEY current from residual gas ionization (i.e. not all of the measured current was necessarily from sample)

• Improve vacuum

Refinements

• Shielded gauge using AD-MU-80; a high nickel alloy with a very large μ– Magnetic field reduced to a maximum of 3 gauss

outside the chamber and ~ 1 gauss in the chamber• Installed an ion pump– Vacuum improved to 4.1 E -7

-> Dirty sample (typical monolayer formation time ~ 25 seconds according to Building Scientific Apparatus by Moore, Davis and Coplan)

• Performed basic beam study; spot size measured through beam extinction technique

Mystery Magnetics

• Vacuum gauge produces a field of roughly 30 gauss at the KF flange

• After removal, 8-9 gauss still measured inside the chamber!

• Supports for the test stand were magnetic; fields on the order of 42 gauss!

• SOLUTION: Build degausser using a half torroid ferrite and a variac.

Degaussing a Wrench

Improved Test Stand

• Sample electrically isolated and stationary

• Aperture mounted on mechanical feedthrough for extinction measurements

• Ion pump installed

Mu-Metal shielding for gauge

Electron Gun

Mechanical feedthrough for aperture

Initial Results

0 500 1000 1500 2000 2500

-2.5

-2

-1.5

-1

-0.5

0

SEY as a Function of Beam Energy(new copper sample; beam current < 50 nA)

Average of Run 1 and Run 2

Beam Energy (eV)

SEY

Conditioning

• Made a critical mistake; first hour of conditioning the emission current indicated on the EGPS increased

• Estimated dose: ~ 137.63 μA-hr*Time

Curr

ent

18μA

30.99μA

1 hr 4 hr 40 min

Questions About Conditioning(Future Work)

• How does the conditioning beam energy effect sample conditioning?– Produce SEY comparisons of conditioned and

unconditioned samples for several different conditioning energies

• How does sample bias effect conditioning?– Conditioning current measured from EGPS “emission

current”, which does not always match actual beam current– Can the sample be effectively conditioned with a large

positive bias (allowing more accurate dose measurements)?

Results After Conditioning

0 500 1000 1500 2000 2500

-2.5

-2

-1.5

-1

-0.5

0

SEY as a Function of Beam Energy(conditioned copper sample; beam current < 70 nA)

Average of Run 1 and Run 2

Beam Energy (eV)

SEY

Investigate Spot Size

• Installed an aperture• Measurement technique:– Feed aperture into beam; record position when

beam current is reduced to 99% of the unobstructed beam current

– Continue feeding until beam current drops to 1% of unobstructed beam current and record position

• Double edge aperture used to gain a rough beam profile– Raster aperture back and forth, recording the

current as a function of position

Spot Size Measurements

Spot Size

Good Measurements Bad Measurements

0 2 4 6 8 10 120

2

4

6

8

10

12

1000 eV Beam

Aperture Displacement (mm)

Curr

ent (

μA)

0 5 10 15 20 250

5

10

15

20

25

30

35

40

45

400 eV Beam

Aperture Displacement

Curr

ent (

μA)

Spot Size

Conditioning Beam Bad Measurements

-4 -2 0 2 4 6 8 10 120

1

2

3

4

5

6

7

8

Conditioning Beam

Aperture Displacement (mm)

Curr

ent (

μA)

0 2 4 6 8 10 120

10

20

30

40

50

60

350 eV Beam

Aperture Displacement (mm)

Curr

ent (

μA)

What’s Going On?

Three Cases

• ~ 400 eV to 200 eV range => artificially high SEY from overlap condition

• ~600 eV to 400 eV range => appears as a plateau

• ~2000 eV to 600 eV range => qualitatively looks fine but still suffering from slight overlap

Ideal Case

New Parameters

Conditioning Beam:Energy 500

eVFocus 100 VGrid 1 V1st Anode 100.1 VSOURCE

Voltage 1.6 VCurrent 1.622 A

Emission 30.99μA

Double Sided Aperture Measurement Single Sided Aperture Comparison

0 2 4 6 8 10 12 140

0.5

1

1.5

2

2.5

3

Conditioning BeamMeasured with double sided

aperture

Conditioning Beam (Backward)

Aperture Displacement

Curr

ent (

μA)

-4 -2 0 2 4 6 8 10 12 140

1

2

3

4

5

6

7

8

Conditioning Beam Comparison

Conditioning Beam (old parameters)Conditioning Beam (new parameters)

Aperture Displacement (mm)

Curr

ent (

μA)

Conditioning Current

0 20 40 60 80 100 120 140 160 180 20023.5

24

24.5

25

25.5

26

26.5

Conditioning Current

Conditioning Current

Elapsed Time (minutes)

Emiss

ion

Curr

ent (

μA)

Dose approximately 0.041 C/cm^2

Initial Results with new Parameters

0 500 1000 1500 2000 2500

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

SEY as a Function of Beam Energy(conditioned copper sample; beam current < 40 nA)

Run 1

Beam Energy (eV)

SEY

Useful Comparison•New conditioning beam and new measurement beam (E=150 eV and F =

310 V in this example)

New conditioning beam and old measurement beam (E=150 eV and F = 150 V in this example)

0 2 4 6 8 10 12 140

0.5

1

1.5

2

2.5

3

Spot Size Comparison

Measurement Beam (nano-amps)Conditioning Beam (micro-amps)

Aperture Displacement (mm)

Curr

ent

0 2 4 6 8 10 12 140

0.5

1

1.5

2

2.5

3

Spot Size Comparison

Measurement Beam (nano-amps)Conditioning Beam (micro-amps)

Aperture Displacement (mm)

Curr

ent

CONCLUSION

• Successful proof of technique– Setup for biasing the sample is robust and easily

controlled, either manually or via computer– Low noise– Reasonable results

• Need to develop a solid set of gun parameters for future measurements– Raster the beam for a larger, more uniform conditioning?

• Future work should also include a formal study of conditioning

Acknowledgements

• Many thanks to Bob Zwaska for his continuedencouragement and support

• Thanks also to Eric Prebys and Carol Angarola along with the entire Lee Teng selection committee for this opportunity