Magnetic fields R&D update

Magnetic fields R&D update

B. PlasternEDM November 2007 Collaboration Meeting

Results from prototype studies of a 1/6-scale B0 coil with

Results from 4-Kelvin test of a prototype systemcos θ coil + ferromagnetic shield + Pb superconducting

shield

(∂Bx/∂x)/B0 ~ 3 x 10−5 cm−1 over scaled fiducial volumes

Last collaboration meeting

B. Plaster

In process of fabricating (~90% complete) a 17%-scale prototype with optimized N=34, ℓ/r = 6.4 [ r = 10.9 cm, ℓ = 69.7 cm ]

Nℓ / r

global minimu

m

Factor of ~20 more uniform along x-axis than

previous prototypes

nEDM November 2007 Collaboration Meeting

cell cell

~ 2 10−6 cm −1

Completed Summer SURF

Model Prediction

N=34, ℓ/r = 6.4 field profile


First-iteration map



Mispositioned groove predicted to generate gradient of wrong sign

First-iteration mapSecond-iteration map

Correction should exacerbate gradient !!



Instead of trying to achieve “perfection”, instead decided to correct with a gradient coil

Unwanted gradient ~linear

Gradient coil

N=2 cos θ coil winding, but as an “anti-Helmholtz” pair

1.0 A in N=3412 mA in N=2

Current ratios

Will no doubt face similar problems in “real” experiment



uniformity ~ 4 10−5

(∂Bx/∂x)/B0 ~ 3 x 10−5 cm−1

(∂Bx/∂x)/B0 ~ 5 x 10−6 cm−1

scale-up to full-size desired (∂Bx/∂x)/B0

1 10−6 cm−1



Test at 77 K



With ferromagnetic shield at 300 K

Known for some time that N=34 uniformity worsens in presence of ferromagnetic shield

Hence, reason for design of “modified” cos θ coils with wire positions offset from nominal

ASU, S. Balascuta TOSCA

Caltech, M. Mendenhall

Pb superconducting shield tests


Pb superconducting shield

Radius = 9.5 cm

Length = 100.0 cm

Accommodates N=40 prototype cos θ coil + ferromagnetic shield

4K dewar from J. Eisenstein group (Caltech condensed-matter)Baffled heat-shield neck (5 layers of Cu-coated PCB)

Feedthroughs for probe (z-motion only), leads, sensors, etc.

Pb Tc = 7.2 K

1/16”-thick, on Al frame



First set of tests, August 2007

Initial cooldown of Pb shield from ~100K required ~60 ℓ of LHeOnly one temperature sensor mounted at top of shield

One initial test, (only) Earth’s field locked-in during transition through Tc

80 Gauss axial field (Helmholtz coil) applied after T < Tc

Probe oriented for axial measurements, Bz(z)

Lower limit on axial shielding factor of ~ 200 (limited by power supply)

top of shield

consistent with

residual background

80 G



Second set of tests, early-September 2007Setup to test N=40 cos θ coil +

ferromagnetic shield mounted within Pb shield

Received 2 new temperature sensors, mounted near bottom/middle of PbTest failed, unable to cool entire assembly to 4 K with 120 ℓ LHe

Later hypothesized that an Al plate that coupled cos θ coil onto its support stand obstructed flow of LHe to bottom of dewar

Idea was to map Bx(z) for coil+ferro+Pb, compare with TOSCA



Third set of tests, mid-September 2007

Identical setup of N=40 + ferro + Pb

Coupler support plate reamed out

Leads for 3 temperature sensors, cos θ coil DC leads, degaussing coil AC leads, magnetometer probe, etc.

Successfully cooled entire assembly from ~100 K to 4 K with 120 ℓ LHeT < Tc for ~30 hours

No LHe level sensor



Map of Bx(z) for N=40 cos θ coil + Metglas + Pb at 4 K

Thanks to S. Balascuta (ASU) for TOSCA calculations

Precision limited by “hand mapping”



During warmup, field at center recorded every ~3 seconds

Demonstration of effectiveness of superconducting shield at screening time-varying backgrounds

Power supply stability better than ~5 × 10−5; fluctuations at T > Tc from background fluctuations leaking through ferromagnetic shield

Future R&D work


Attention now focused on construction of a 1/2-scale prototype modified cos θ coil and ferromagnetic shield

acrylic tube

24” O.D.

6’ + 1.5’ (glued)

3-ring set of acrylic “rings”

Not “true” cos θ coil; wire positions optimized for uniformity in presence of ferromagnetic shield

Future R&D work


Status

Acrylic materials on hand since August 2007

Machining to be done at the Caltech HEP shop

Plagued by delays at machine shop (i.e., were considered low priority)Recently (last week, after 2-month delay) HEP shop arranged for outside shop to start machining on the acrylic rings

Also in process of upgrading our automated mapping system to accommodate the 1/2-scale coil and the 1/2-scale dewar

Goal is first results (perhaps not best !!) by April/May/June time-scale

25’-tall structure, Caltech HEP high-bay area

Magnetic fields R&D update

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