Accelerator Magnet Technology Division

Magnet system for Indus-2

2.5 GeV Indus-2 (SRS) lattice, a Double Bend Achromat, consists of eight super periods each having two dipole magnets, four focusing and five defocusing quadrupole and two focusing and two defocusing sextupole and seven corrector dipole magnets(CDM). All these magnets have been designed using 2-D POISSION/PANDIRA and 3-D TOSCA codes for the energy range of 0.6 GeV (injection energy) to 2.5 GeV (nominal beam energy). At the design stage special care has been taken to avoid the effect of core saturation on the field quality and reduce the higher multipole components. After careful measurements of all the three different magnets, it was observed that the measured field quality satisfy the beam optics requirement. Close resemblance between the design and measured field quality is observed. Entry-exit tapers are optimized for all the magnets for better field quality. In quadrupole, optimization was done to reduce the 12th pole component, which is the first systematic multipole that comes due to design.

A computer controlled 3-D Hall scanner is developed locally for Dipole magnet measurements. It can scan in the volume of 3000x800x600 mm cube. Periodic recalibrations of hall probes is done using NMR probe. The Quadrupole and Sextupole magnets are characterized by using a rotating coil system made by DANFYSIK model no. 692 (obtained on loan basis from ESRF, France). It is capable of measuring the main strength and harmonic content of the multipole magnet, magnetic centering and precise positioning of alignment targets on the magnet. The coil of 1.2 m. length and 32.5 mm radius is used for the measurement. Sensitivity of the main harmonic field is 3 x10-4 and lateral position of the magnetic center with respect to the rotation axis is with in 30 micron.

Dipole Magnets

In Indus-2 ring 16 Dipole Magnets are present with a bending angle of 22.5 provided by each magnet. The magnets are designed for a maximum field of 1.5 T at nominal beam energy (2.5 GeV). The magnet parameters are listed in table 1. In addition to the main coils, a pair of secondary coils powered with current source of ±10 A will also be used to correct magnet to magnet field mismatch. Figure 1 shows the photo of dipole magnet.

Table 1: Dipole Magnet Parameters

Magnetic Field

1.5 T at 2.5GeV,
0.36T at 0.6GeV

Bending radius

5.55m

Magnetic Arc Length

2.17948m

Pole Gap

0.05m

 NI (main coil)

66,880 at 2.5GeV



Figure 1: Full view of Indus-2 Dipole Magnet

Magnetic Field Quality

A computer controlled 3-D CMM is used for the movement of the Hall probe  in the mid plane of the pole gap of Dipole Magnet with precise positioning. Group3 make DTM151 Hall probe is used with accuracy ± 0.01 % of reading ±0.006% of full scale max. at 25 degree C  verified against NMR. For a particular magnet overall integrated field measurement shows a repeatability of ±1 ´10-4.  Measured integrated field uniformity in the good field region (± 32mm) at various energy levels is shown in figure 2. Figure 3 shows the measured integrated magnet to magnet variation.



Figure 2: Integrated field uniformity in the good field region (±32 mm) at different energy levels.



Figure 3: Magnet to magnet integrated field variation.

The correction margin provided by the secondary coils at the low energy is very high   (6-8 x 10-2) and goes down to 6 x 10-3 at 2.5 GeV which is twice the correction required at this level. The measured field qualities of the magnets are within the limit. The magnet to magnet integrated field variation is bit higher but it can be easily corrected using secondary coils. Figure 4 shows the maximum correction which can be carried out by excitation of secondary coils incorporated in dipole magnets.



Figure 4: Correction margin using secondary coils of Dipole Magnet (typical).

Quadrupole Magnets

Storage ring consists of total 72-quadrupole magnets, out of which 40 are closed type magnets and 32 are open-type quadrupole magnets. Open type quadrupoles are required to facilitate the path for Synchrotron Radiation and they are placed before and after the main dipole magnet in the ring. The magnet parameters are listed in table 2. Figure 5 and 6 shows the photos of closed type and open type quadrupole magnets.

Table 2: Quadrupole magnet parameters

 

Q1

Q2

Q5

Q3 and Q4

 

Close Type

Open Type

Aperture radius (mm)

42.5

42.5

Maximum Gradient (T/m)

16

16

Magnetic length (m)

0.3

0.55

0.4

0.4

Power dissipation (kW)

5.4

7.4

6.1

5.8

No. of magnets

16

16

8

16+16


Figure 5: Full view of closed type quadrupole  magnet.
 
Figure 6: Open type quadrupole magnet for Indus-2 is being tested on Harmonic Bench.

Magnetic Field Quality

Quadrupole magnets have been tested using the rotating coil bench model 692 constructed by Danfysik (obtained on loan basis from ESRF, France). Figure 7 and 8 shows the measured higher order harmonics in Q2 and Q3 type magnets respectively. Integrated gradient errors for Q5 and Q4 type magnets are shown in fig. 9 and 10 respectively.

 
Figure 7: Higher order multipoles in Q2 type magnet   Figure 8: Higher order multipoles in Q3 type magnet
 

Figure 9: Integrated gradient errors of Q5 type quadrupole magnet (Close Type).

 

Figure 10: Integrated gradient errors of Q4 type quadrupole magnet (Open Type)

Higher order multipoles are low and are meeting the beam dynamics requirements. The higher order multipoles are not varying with excitation level. Measured non-linearity in excitation curve is 6.46% (Q1), 2.3% (Q2,Q3 and Q4) and 2.6% (Q5) which are low, it would be easy to track the magnets during the ramping and will also help in tune excursion during ramping.

Sextupole Magnets

There are 32 sextupole magnets in the ring. The bore radius of the magnet is 0.046 m. The width of the pole and return yoke are 0.036 and 0.044 m, respectively. Expected good field region is ?0.032m. The required field gradient at nominal energy (2.5 GeV) is 400 T/m2 and the ideal effective length of the sextupole magnet is 0.2 m. Figure 11 and 12 shows the field errors and variation of effective length.

 

Figure 11: Field errors normalized with respect to the sextupole component at 1.5 GeV and at 0.025 m radius for all the 32 Sextupole magnets.

 

Figure 12: Variation of effective length with current at different radii.

Interferences between various magnets

The distances between all the magnets cannot be made large to keep the size of the ring modest. This may cause interference of the magnetic fields between the adjacent magnets. All the individual magnets are characterized in the absence of any adjacent magnet and the results are shown above. Therefore, it is important to find out the effect of mutual coupling between magnets in the actual condition in the ring and find out the ways to overcome this. Table 3 gives the % change in the integrated strength of Q1 type quadrupole magnet when corrector dipole magnet (CDM) is placed in vicinity of quadrupole magnet. Figure 13 shows the higher order harmonics normalized with respect to the main quadrupole component when CDM is placed at 201 mm away from the edge of the QPM and the other QPM stands alone.

Table 3: Change in the integrated strength of Q1 type quadrupole magnet when CDM is placed in its vicinity

Distance of CDM (mm) from the edge of QPM

% change of the integrated QP  field strength of Q1 from the nominal value

Experiment

Simulation

135
201
260
410

0.317
0.095
0.039
No

0.312
0.093
0.035
0.0058




Figure 13: Higher order harmonics of QPM in presence/absence of CDM

Figure 14 shows the experimental set up for the measurement of magnetic field interference between quadrupole and sextupole magnet using Danfysik harmonic bench model 692. Sextupole magnet is placed on the special base for additional 3D alignment. Figure 15 shows the plot of the ratio of the measured integrated quadrupole field gradient, to the nominal gradient against sextupole excitation for different currents in the QPM (Iqp). Edge to edge distance between the quadrupole and sextupole magnet is 265mm. No interference observed as all the results are within experimental error.

 

Figure 14: Experimental set up for the measurement of magnetic field interference between quadrupole and sextupole magnet.

  Figure 15: Higher order harmonics of QPM in presence of sextupole magnet.

Best viewed in 1024x768 resolution