Kerr Effectbased Measurement of the Electric Field

1
Kerr Effect-based Measurement of the Electric
Field

Recent Developments
Alex Sushkov Dima Budker Valeriy Yashchuk (UC
Berkeley) Neutron EDM Collaboration Meeting Los
Alamos National Laboratory June 2, 2003
2
(No Transcript)
3
E-field requirements

• Homogeneity over cell volume
• Stability over 500 s lt 1
• Reversibility

This reduces E-field-related systematics to lt
5?10-10 Hz, i.e. one tenth of the EDM shift for
dn10-28 e ?cm
Electric field monitoring 0.1 -1
4
The Kerr Effect

• Uniaxial E-field-induced anisotropy
• ?n n?-n? KE02
• For input light polarized at 45o to E, the
  induced ellipticity ? ?L?n/? (?L /?)
  KE02
• Circular analyzer

?

• Achievable sensitivity ?? ? 10-8 rad Hz-1/2

5
Electric Field Measurement

• Kerr constant for LHe estimated from experimental
  data for He at 300K K ?
  1.7?10-20(cm/V)2
• Electric field E0 50
  kV/cm
• Sample length L 10 cm
• Induced ellipticity ? ? 10-5
  rad
• A 1s measurement gives accuracy (?? ? 10-8 rad
  Hz-1/2)
• ?E0/E0 ? 5?10-4

? Kerr constant for superfluid He ? ?
Polarimeter sensitivity ?
6
Test set-up at Berkeley
Cryostat (T ? 1.4 K) with optical access
Laser
Thin-wall st. steel tube
Home-made cryogenic HV cable
Copper electrodes l38 mm gap6 mm
HV cable- connector
Graduate student A. Sushkov
Electrode Assembly
7
Results LN2 Kerr constant
Measurement K 4.2(1)?10-18 (cm/V)2 Literature
result K 4.0?10-18 (cm/V)2 K.Imai et. al.,
Proceedings of the 3rd Int. Conf. On Prop. and
App. Of Diel. Mat., 1991 Japan)
8
LHe Kerr Constant Measurements
9
Results LHe Kerr constant (T?1.3 K)
Measurement K 2.45(13)?10-20
(cm/V)2 Theoretical value K 2.0?10-20
(cm/V)2
(1s, 2s, 2p levels)
10
Modulation Polarimeter
(?,?)
Lock-in Amplifier

• Home-made diode laser battery current supply,
  780 nm
• Photo-Elastic Modulator frequency 50kHz,
• 
  phase modulation amplitude 0?A??
• Polarizers nearly crossed, opening angle ? ? 1
• Sample introduces rotation ?, ellipticity ?
  ?,? ? 1
• The polarimeter signal ?2 2 ?? J2(A) sin(2?t)
  (1?) ?? J1(A) sin(?t)

11
Polarimeter Performance

• Noise 3?10-7 rad/(Hz)1/2
• Drift 10-5 rad in 500 seconds

12
Possible improvements

• Light interference on optical elements due to
• laser frequency drift and temperature
  fluctuation
• (currently the suspected cause of the large
  drifts seen in the polarimeter)
• ? Anti-reflection coating
• Laser frequency stabilization
• Temperature stabilization
• Imperfections of windows
• (give an offset ellipticity of ? 2 degrees, which
  can fluctuate due to stresses)
• ? Find windows with small
  stress-optic coefficient
• Photo-elastic modulator imperfections
• (give an offset ellipticity of ? 1 degree
  drifting with temperature)
• ? Find
• Kerr effect modulation
• (modulate the angle ? of the incoming
  linear polarization with respect to the electric
  field ? ?/4 ? max Kerr effect
  ? 0 or ?/2 ? zero
  Kerr effect

13
Electric Field Measurement Sensitivity

• Kerr constant for LHe K 2.5?10-20(cm/V)2
• Electric field E0 50
  kV/cm
• Sample length L 10 cm
• Induced ellipticity ? 3?10-5 rad
• A 1s measurement gives accuracy (?? ? 3?10-7 rad
  Hz-1/2)
• ?E0/E0 ? 5?10-3

• NEXT STEPS
• Polarimeter drift and sensitivity improvement
• LHe Kerr-constant temperature dependence
  measurement