aCORN Collaboration Meeting

1
aCORN Collaboration Meeting

• Current Progress on Electrostatic Mirror Work
  Package
• by A. Laptev

IUCF, Bloomington, IN July 17-18, 2007
2
EM functions

• Gathers protons emitted in both directions
• Increases speed of slow proton group
• Makes even the velocities of two proton groups

• decreases the false asymmetry due to transverse
  magnetic field in p-collimator
• decreases the significance of energy-dependence
  of the p-detector efficiency

3
Three issues for the EM design

• Effect of the grids
• Electric field uniformity in the central region
• Electric field alignment

4
General Idea of the EM

• Both electrodes are thin wire grids
• Uniform electric field inside of EM volume
• Some supporting details

5
The carbon loaded PTFE will be used for electric
boundary condition shield of the EM
Properties Carbon loading
15 Electric resistivity 13 MOm Electric
current 0.5 µA _at_ HV
of 3 kV wall thickness of 1 mm
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Prototype of electric boundary condition shield
for the EM
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Prototype of electric boundary condition shield
for the EM
Properties Wall thickness 1
mm Diameter 4.5 cm Length
10 cm Has proper shape Fairly rigid
8
Neutron transmission measurement for 15 carbon
loaded PTFE

• Using of windowless electric boundary condition
  shield for the EM results in uniform electric
  field.
• The PTFE contains only C and F nuclei both of
  which have low neutron cross sections. But
  scattered neutrons can contribute to the
  background of aCORN.

9
Machined samples of carbon loaded PTFE
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Neutron transmission measurement

• The NG-6M of the NIST Center for Neutron Research
  (NCNR) was used for transmission measurement.
• The neutron wavelength is 5 Å. About the same
  neutron energy is expected to be used for the
  aCORN experiment.
• The neutron flux is 106 n/(cm2 s).

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Neutron transmission measurement at NG-6M beam
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Neutron transmission measurement at NG-6M beam
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Result of the neutron transmission measurement
 
 
 

• Detector is 235U fission ionization chamber.
• C1 collimator is 35 mm, C2 collimator is 10.3 mm.
• Transmission of the 1 mm sample of the 15 carbon
  loaded PTFE is (97?1).

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EM volume is ended by wire grids
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Grid frame of the electrodes

• Electrodes consist of two parts (copper).
• Grid of wires is mounted on the lower part.
• Grid replacement could be done without
  disassembling of EM
• Upper part has round slot for mounting of
  electric shielding cylinder.

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Final choice of grid parameters

• Wire spacing and diameter is based on MC
  calculations by Brian Collet, Hamilton College

• Wire spacing is 2 mm.
• Wire diameter is 40 µm.

• Wire material is tungsten covered by gold.
  Manufacturer is the California Fine Wire Company.

17
New attempt of spot welding

• Gold covering of copper grid frame was done.
  Thickness of covering is 1.5 µm.
• The gold plated W wire was used for test grid
  manufacturing.
• The used welding power was increased up to 8.2
  W/s.
• Welding process was much easier than that for
  clear materials.
• Resulted welding is pretty fragile bond could
  easily be destroyed, but the wire was not broken.

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Gold plated grid frame
There are some visible spots at welding places.
Gold plating either was burnt or striped by W
wire.
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An idea of mechanical wire attachment for grid
manufacturing

• Using very tiny bolts (0-80 3/32)
• Bond is very strong
• Good electric contact of wire and frame
• Easy to repair broken wires
• Method can be applied for 20 µm and 40 µm wire.

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20 µm wire attachment
Gold plated W wire
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40 µm wire attachment
Clear W wire
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40 µm wire attachment (cntd.)
Bond of single wire can hold a weight of copper
gasket
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Results

• The hollow PTFE cylinder for electric boundary
  condition shield of the EM can be machined. It is
  expected to be in proper shape and pretty rigid.
• This shielding will scatter about 6 of neutron
  beam (upstream and downstream).
• Spot welding results in very fragile bond of W
  wire in case of copper frame.
• Grids can be manufactured using tiny bolts for 40
  µm wire attachment.