Conceptual Design of the Neutron Guide Holding Field

Christopher Crawford, Yunchang Shin University of Kentucky nEDM Collaboration Meeting 2009-06-19

Outline

 • • • Issues: constraints adiabaticity / abruptness field gradients  • • • Design: DSCTC steel flux return taper in DSCTC 2m DSCTC  -metal ext.

1010 steel flux return

Constraints

     • • preserve neutron polarization (holding field) Larmor precession adiabatic abrupt – field uniformity – field smallness Majorana transitions ?

• • • avoid gradients in measurement cell from: holding field coils (left on) magnetized Metglas (HF off) magnetic material – field fringes – field fringes – no magnetic material spin dressing field uniformity neutron guide construction – no conductors in B 0 region – no current sheets in guide SM polarizer – 300 G – 100 mG field taper

Issue – adiabaticity / abruptness

either or both conditions will preserve polarization:  • • • 100 mG doldrums too large for abrupt changes too low for adiabatic rotation in cryostat could try and ‘steer’ spins into fringe with exit fringe

Field and neutron spin direction – 100 mG

Field and neutron spin direction – 70 mG

Field and neutron spin direction – 40 mG

Polarization vs Field (corner of guide)

Field lines in double-cos-theta coil

                                                                          require: B=0 outside B=B 0 inside solve  M B r with boundary conditions calculate j from B t boundary using  M 1” flux return

Current windings on end face

   B t =0 on ends so solution is axially symmetric equipotentials  M =c form winding traces for current on face n £(H=r  M ) end plates connect along inside/outside

Issue – gradients



Design – DSCTC

  • • guide field ~ dipole directly affects B 0 field if left on magnetizes Metglas if turned on and off repeatedly • • flux return ~ quadrupole magnetic material in cryostat distorts the field currents – DSCTC similar to dressing coil design arbitrary geometry inner coils – guide field outer coils – flux return end-caps – contain B-field current sheet omitted

Integration of DSCTC and steel flux return

Issues – current sheet / spin dressing coils

   guide field should terminate at beginning of B 0  field: conductors inside spin-dressing coils perturb RF field  to match up and cancel out fringes don’t want current-sheet on end-cap • • of the DSCTC complicates neutron guide need to cancel B 0 fringe • • quadrupole residual direct – gradient indirect – magnetization + =

Stray fields from DSCTC

B(15cm, 15cm, 25cm) = (456,15.3, 149) x 10 -8 G

No Shields

dB x dB y dB z /dx 3.1 1.0 10.

/dy 1.0 1.0 0.5 x10 -8 G/cm /dz 10. 0.5 4.1

Shield & B 0 (40 mG)

dB x dB y dB z /dx 0.4 128? 0.8

/dy 0.1 0.1 0.2 x10 -8 G/cm /dz 0.9 0.2 0.7

Septimiu Balascuta

triple-axis fluxgate magnetometer Nested Metglas shields: 1) 25.5” O.D., 67.5” long, 1.6 mils (2 layers) 2) 17.25” O.D., 48.5” long, 2.4 mils (3 layers) H. Yan, B. Hona, B. Plaster

Lab Setup

deguassing coils “quadrupole loops”

B x (z) Step #1: Quadrupoles off (baseline) Step #2: Quadrupoles on (impact) Step #3: Quadrupoles off (hysteresis) B y (z) B z (z) quadrupole at this end Note: x = vertical, y = horizontal

B x (x) B y (x) Step #1: Quadrupoles off (baseline) Step #2: Quadrupoles on (impact) Step #3: Quadrupoles off (hysteresis) B z (x) Shapes (  gradients) similar Probably should be repeated for higher precision, test repeatability

Results along y-axis are similar

Holding field downstream of bender

 • • • horizontal vs. longitudinal field double-cos-theta-coil transition need same current as solenoid only on top and bottom each side can mount separately      5 G holding field in 10 m of guide downstream of bender external 1010 steel yoke, 1/16” x 42.5 cm x 42.5 cm • • 40 cm x 1 mm Al winding 160 A-turns top and bottom 92  /m, 4.7 W/m • • • coil vs. permanent magnets: allows use of steel on all four sides of guide for both internal and external shielding can be turned off during measurement cycle low power • lightweight – 31 kg/m mount on guide housing

External shielding

   factor of 10 shielding of Earth’s magnetic field B y /B x = 50 mG / 5 G 0.57

± perturbation of • holding field angle only matters at interface with double-cos-theta coil

Issue – field taper

Top view y   z B x  J  Side view z y x   B  J      beam right      beam left guide top guide bottom

Calculation – optimal taper

Results – optimal taper

Design – DSCTC taper

0 m – 100 mG 1 m – 189 mG 2 m – 460 mG 3 m – 2.4 G 4 m ~ 10 G j max =152 A/m P max =11.3W/m 2 P ~ 100 W 1.16 A 50 windings

Design – DSCTC taper

flux return lines

Extra Slides: B 0 field alone / with DSCTC

at x=12.5, y=12.5 cm (worst case)

B 0 =100mG+DSCTC x=12.5 cm

B 0 +DSCTC, x=12.5 cm

B 0 =100mG+DSCTC, x=6.25 cm

B 0 =DSCTC, x=6.25 cm

B 0 +DSCTC – 100 mG

B 0 +DSCTC – 40 mG