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