1 / 17 author cern – geneva – ch [email protected] 14th international magnetic measurement...

AuthorCERN – Geneva – [email protected]

14th International Magnetic Measurement Workshop26-29 September 2005, Geneva, Switzerland 1 / 17

TitleField maps of the U17 PS magnet

Alexander ASKLÖVLinköping University

Régis Chritin CERN



PS extraction region adaptation needed for the LHC New current operation → ejection unit must be better known Effects of the metallic shims on ejected beam and circulating beam Access limitation → new bench developed

Alternated Gradient principle Combined function units (Focusing and Defocusing) C shaped magnets 10 blocks assembled, laminated total geometrical length = 4260 mm

U17 PS magnet



U17 magnet different coil schematics

Main coil

Pole face windings (pfw)

Figure-of-eight loop

Used to adjust the Tunes and Chromaticities (sextupolar component) modified by saturation

Used to change the Tune (quadrupolar component) without affecting the curvature (integrated bending field)

Combined function magnet

Main coil

Defocusing (5 blocks) Focusing (5 blocks)

Figure of eight loop

Pole face windings



PFW configuration

)(2 0

0

00 xx

xxWGxy

Gni

2

0

20

22

0

0

0

32

0 )(12)(4)

3(

2 xx

xWx

xx

WxSyx

ySni

To create pure quadrupole on hyperbolic profile:

To create pure sextupole on hyperbolic profile:

• Windings configuration and field component:

)1(2

1

0xx

Wy

gaptotalW

)1(2

1

0xx

Wy

gaptotalW

)(xni per pole = equation of equipotential

)(xni per pole = equation of equipotential



CurrentsIp I8 IpfwF IpfwD

Cycle Test 2500 A - - -

Cycle E 669.2 A - - -

Cycle C 5413.15 A 1257.9 A 200.7 A 99.75 A

Cycle LHC 5400.56 A 1452.8 A 206.7 A 86.9 A



Measurement techniques

2D Field mapping

Hall probes KSY44 (Siemens) Semiconductor mono-crystalline GaAs Low temperature dependency β = -0.036 %/°C Hall current 3mA. High sensitivity 200 V/A/T gives 600 mV/T Low noise, less than 0.2 gauss (time span 2-20min) Tiny active surface, 0.1225 mm2

Resistance - Each probe: about 900 ohm at 1 T - 11 probes connected in series- Vary with induction and thus also the Hall current

746.1

)0()( 9.1006.01

Bii HallBHall



- 11 Hall probes measured once during a power cycle → 481 ms for a scanning- 600 ms Flat-top → magnetic field not constant due to Eddy Current - 1 Hall probe measured 3 times → Eddy Current effects correction

11 Hall probes (calibrated in their fixed position)

116 mm

3 thermistors

19 mm

Hall probes assembly



Magnetic distances measurements

With the PS magnet we can measure easily the individual magnetic distances between the probes with good precision.

- Magnet gradient = 5.2 T/m @ 5400 A

→ small probe displacement = significant field change

- Heidenhain ruler → precise measurement head displacement (micron resolution)



Wanted precision

Better than 1 per mille (for the induction B) Absolute error smaller than 10 Gauss (Nominal field = 1.2 tesla (26 GeV))

Gradient 5.2 tesla/m Position precision of 0.1 mm

Other parameters Hall current, 4 pulsed power currents, temperature, time evolution, measurement equipment

(positioning errors)

The whole installation

7 m

Magnetic measurement stand which may be used to map ANY long magnet (up to 6.5 m) with closed gap (34 mm min).



Bench synoptic diagram



Kevlar thread

Encoder

Maxon motorRubber wheel Sandpaper

Molybdenum wheel

Displacement system

- Conceived to be as temperature independent as possible

- Calibrated with respect to the molybdenum wheel radius

- System based on a 7 meters long straight plastic arm

- Movement regulated by the position encoder



Procedure

Array of 11 Hall probes Scan (HP34970A + module 34901A multiplexer) 37 ms between each Hall probe measurement 3 thermistors for temperature compensation

Steps 1 cm in longitudinal direction Displacement precision. Kevlar® (-2 ppm/K) thread (diam. 0.6 mm) and

Molybdenum wheel (+5 ppm/K). ROD 450 Encoder. Maxon motor DC. 500 steps + 4 more transversal positions (5 x 10 cm) Total map with around 27500 points

Relative changes with and without magnetic shims to know their influences

® Dupont de Nemours



Temperature corrections for the probes Using β = -0.036 %/°C (linear)

Hall current corrections Using linear correction.

Hall probe calibration 15 degree polynomial as a function of the Hall tension. Real magnetic distance between each Hall probe

Position correction of the guiding rail Using laser tracker measurements

Time evolution corrections (mainly due to eddy currents) Using a second order approximation function for correction One probe measured three times on the flat top: in the beginning, the middle and

the end. For each longitudinal position

Corrections applied



Position measurements

Position corrections approximating by linear lines the non rectitude of the guiding rail.

After shims installation

Made with a Laser Tracker system

Rail movementsPosition reproducibility



Field integral under new LHC conditionsFor consecutive measurements: 6*10-5

No movement of the rail No calibration deviation

Mean absolute relative difference between first and final measurements are 5*10 -4

Rail movements during mechanical manipulations on the magnet Long term calibration deviation

Rem: the two longitudinal extremities, where the field drops quickly are neglected. The precision of the longitudinal coordinate (z) is too bad here (slipping checked with telescope < 0.4 mm).

New PFW validationMaximum position difference between old and new PFW < 0.5 mm

Magnetic shims (each block gap = 34 mm)Different radial positions between simulations and the physical installation:198 gauss field drop instead of 105 gauss estimated.

Results



With shims

Magnetic shims homogenize the field by shielding the ejected beam from a non linear fringe field different radial position of the five different shims

Shims

Magnet blocks

← Focusing side with

and without shims



Future

New field maps of U17 with 4 new PFW power supplies instead of 2, when narrow sides and wide sides will be powered independently.

Use the bench for magnets with no lateral access (6.2 meter long SPS dipoles – magnets with small gaps)

Verify measured Btrain in the PS Strong Eddy currents at the end blocks, which are not taken into account in the actual B train coils located

near the central blocks of the reference magnet.

Coils measuring the full integral, seeing the whole eddy current picture. Not very sensitive to transversal displacement (F and D halves compensate each other), but very sensitive to any yaw-angle though

Thesis report on the web (https://edms.cern.ch) EDMS document 609417 EDMS seminaire technique 609283

Measurement data G:\Divisions\AT\Groups\MTM\MTM-RM\Technical_Students\AlexanderAsklov\Measure Data

_____________________

P+ P+ ejected

1 / 17 author cern – geneva – ch [email protected] 14th international magnetic measurement...

Documents

author cern geneva ch

magnetic field

hall current slide

hall probes assembly

u17 ps magnet slide

c hall current

field component

switzerland pfw configuration