Conceptual Design of the Neutron Guide Holding Field

Christopher Crawford, Yunchang ShinUniversity of Kentucky

nEDM Collaboration Meeting2009-06-19

Outline

Issues:• constraints• adiabaticity / abruptness• field gradients

Design:• DSCTC• steel flux return• taper in DSCTC

DSCTC

2m

1010 steel flux return

-metal ext.

Constraints

preserve neutron polarization (holding field)• Larmor precession

- adiabatic – field uniformity- abrupt – field smallness

• Majorana transitions ?

avoid gradients in measurement cell from:• holding field coils (left on) – field fringes• magnetized Metglas (HF off) – field fringes• magnetic material – no magnetic material

spin dressing field uniformity – no conductors in B0 region

neutron guide construction – no current sheets in guide

SM polarizer – 300 G – 100 mG field taper

Issue – adiabaticity / abruptness

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

either or both conditions will preserve polarization:

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=B0 inside

solve M with Br boundary conditions

calculate j from Bt boundaryusing M

1” flux return

Current windings on end face

Bt=0 on ends so solution is axially symmetric

equipotentials M=c

form winding traces for current on face n£(H=rM)

end plates connect along inside/outside

Issue – gradients Design – DSCTC

guide field ~ dipole• directly affects B0 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 designarbitrary geometryinner coils – guide fieldouter coils – flux returnend-caps – contain B-fieldcurrent sheet omitted

Integration of DSCTC and steel flux return

Issues – current sheet / spin dressing coils

guide field should terminate at beginning of B0 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 B0 fringe

quadrupole residual• direct – gradient• indirect – magnetization

=+

Stray fields from DSCTC

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

dBx dBy dBz

/dx 3.1 1.0 10./dy 1.0 1.0 0.5 x10-8G/cm/dz 10. 0.5 4.1

No Shields

dBx dBy dBz

/dx 0.4 128? 0.8/dy 0.1 0.1 0.2 x10-8G/cm/dz 0.9 0.2 0.7

Shield & B0 (40 mG)

Septimiu Balascuta

Lab Setup

“quadrupole loops”

triple-axis fluxgate magnetometer

deguassing coils

H. Yan, B. Hona, B. Plaster

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)

Nested Metglas shields:

Bx(z)

By(z)Bz(z)

quadrupole at this end

Step #1: Quadrupoles off (baseline)Step #2: Quadrupoles on (impact) Step #3: Quadrupoles off (hysteresis)

Note: x = vertical, y = horizontal

Bx(x)

By(x)

Bz(x)

Results along y-axis are similar

Shapes ( gradients) similarProbably should be repeated for higher precision, test repeatability

Step #1: Quadrupoles off (baseline)Step #2: Quadrupoles on (impact) Step #3: Quadrupoles off (hysteresis)

Holding field downstream of bender

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

horizontal vs. longitudinal field• double-cos-theta-coil transition• need same current as solenoid

only on top and bottom• each side can mount separately

External shielding

factor of 10 shielding of Earth’s magnetic field

By/Bx = 50 mG / 5 G

0.57± perturbation of holding field angle• only matters at interface

with double-cos-theta coil

x

z

y

z

Top view

Side view

B

J

BJ

beam left

beam right

guide bottom

guide top

y

x

Issue – field taper

Calculation – optimal taper

Results – optimal taper

Design – DSCTC taper

1.16 A50 windings

0 m – 100 mG1 m – 189 mG2 m – 460 mG

3 m – 2.4 G4 m ~ 10 G

jmax =152 A/mPmax =11.3W/m2

P ~ 100 W

Design – DSCTC taper

flux return lines

Extra Slides: B0 field alone / with DSCTC at x=12.5, y=12.5 cm (worst case)

B0=100mG+DSCTC x=12.5 cm

B0+DSCTC, x=12.5 cm

B0=100mG+DSCTC, x=6.25 cm

B0=DSCTC, x=6.25 cm

B0+DSCTC – 100 mG

B0+DSCTC – 40 mG