the electron cloud and secondary electron yield
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The Electron Cloud And Secondary Electron Yield. Tom Schmit Mentor: 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 - PowerPoint PPT PresentationTRANSCRIPT
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 SizeGood 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 SizeConditioning 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 ParametersConditioning 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