Study of the magnets used in a mobile isocenter gantry · [email protected] 18 Magnetic line...

ESR

Jhonnatan Osorio Moreno

Supervisor

Marco Pullia

Study of the magnets used in a mobile isocenter gantry

Pavia– 14th September 2012

General overview

1. Introduction 2. Why a mobile isocenter gantry 3. Magnetic line for a mobile isocenter gantry 4. Conclusions

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Introduction

The aim of the WP21 was to participate in the research line already pursued at CNAO concerning the carbon ion gantry design

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It means to work together with the ULICE

WP-6 researchers

in developing a conceptual

design of a carbon ion gantry

Introduction

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+

WP-21 WP-6

=

The job was distributed in:

•Magnet simulations • Beam line simulations (beam optics) • Shielding aspects • Mechanical structure •....

+ = Conceptual design

Mobile isocenter gantry

Introduction

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Why a mobile isocenter gantry

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90° bending magnet

Scanning magnets

Quadrupoles

Corrector magnets

The layout shows:

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•Less magnets in the rotating structure. Only 90° bending magnet

contributes to the system torque

•Approximate cost: 15.5 M€

• Approximate total gantry

weight 400 tons



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Preliminary conceptual layout for a mobile isocenter gantry

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Synchrotron Phase shifter stepper

Match 1

Match 2

Rotator

Gantry

Magnetic line for a mobile isocenter gantry

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Development of technical aspects of magnets

These magnets are: • Bending dipole

(/8) • 90° last bending magnet • Quadrupoles • Correctors


to be used in the conceptual gantry design




Additional features: the rotator

What is the rotator? It is a set of quadrupoles placed in a rotating structure which allows to match the fix part of the line with the rotating part (gantry)



Additional features

To characterize the influence of the rotator structure on the magnetic field of quadrupoles and correctors



Additional studies

Time dependent analysis of magnet features



Bending dipole (/8)

In the gantry line there are three bending dipoles (/8)

They are used to control and match the beam dispersion along the line.


Magnetic line for a mobile isocenter gantry Bending dipole (/8)

Magnetic flux density

Static Simulations

The static simulations are useful to verify: 1. The iron field saturation level 2. The magnetic field intensity in the Good Field Region (GFR) 3. The magnetic field homogeneity

GFR

Iron yoke coil



Magnetic flux density Magnetic field quality

Magnetic field profile in the GFR

Static Simulations

Magnetic length 1.696 m

B=1.53T

-2x10-4≤ ΔB/B≤2x10-4



To validate the Finite Element Method calculations, a comparison between dynamic measurements

and simulations has been done

Time dependent calculations

The energy beam variations during the treatment requires variations in the magnetic field strength of every magnetic element in the gantry line.

Magnetic flux density in the center of GFR dynamic and static

calculation

dI(t)/dt = 5630 A/s dB(t)/dt= 3 T/s

Tension bars



Time dependent calculations

The energy beam variation during the treatment, requires time variations in the magnetic Field strength of every magnetic line element.

The results show a good agreement between the calculated and the measured data

0

0.05

0.1

0.15

0.2

0.25

0.3

0.48 0.68 0.88 1.08 1.28 1.48 1.68 1.88 2.08

(Bst

ati

c-B

dy

na

mic

s)

t(sec)

simulation measurement

Ramp end 0.5 sec



Remarks about bending dipole (/8)

• This magnet model has been used as validation model in the dynamic regime

Magnetic field[T] 1.539

(Bx-B0)/B0 at GFR [-0.03x10-4,1.67 x10-4]

Stored Energy[J] 48418.4

Inductance[mH] 12.22

Dissipated DC power[kW] 36.88

DC voltage in the coil[V] 13.10

Total dipole resistance[mΩ] 4.65

Integrated Magnetic field[T*m] 2.5461

Magnetic length[m] 1.696

Main features calculated for /8 bending dipole



90° bending magnet

Its function is to transport the ions from the scanning magnets to the patient


Magnetic line for a mobile isocenter gantry 90° bending magnet

Static Simulations

This magnet have particular features:

• Tension bars • Ferromagnetic stiffening frame • Magnet weight: 82 tons • GFR: 20 x 20 cm2



Static Simulations

Magnetic flux density

Contribution of the tension bars to the magnetic field in the GFR



Static Simulations

B in the center of the magnet [T]

Delta (%)

Only iron yoke 1.8753 0

Yoke+tension bars 1.888 0.67

Yoke+ tension bars + stiffening frame

1.890 0.78

Values of the magnetic field strength in the middle of the GFR

Magnetic Field profile in the GFR

1.874

1.876

1.878

1.88

1.882

1.884

1.886

1.888

1.89

1.892

-150 -100 -50 0 50 100 150

|B|(

T)

Xgfr(mm)

Yoke

Yoke+tension bars

yoke+tensionbars+stiffening frame

Magnetic field homogeneity

25


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Static Simulations

-2.00E-05

0.00E+00

2.00E-05

4.00E-05

6.00E-05

8.00E-05

1.00E-04

1.20E-04

1.40E-04

1.60E-04

-110 -60 -10 40 90

LBM

LBM+Tension bars

LBM+tb+Stf

∆𝐵/𝐵

Xgfr(mm)

Magnetic field homogeneity contribution of the tension bars and of the tension bars + the stiffening frame, respectively

dB/dt = 0.4 T/s dI / dt = 506.6 A/s



Dynamic simulations

Current density inducted in the structure in the Case of all its parts were iron



Dynamic simulations

Magnetic flux lines and eddy current distribution in the tension bars



Dynamic simulations

Difference between static field (Bs) and dynamic field (Bd) at the end of the feeding current ramp t = 4.5 s

Slow ramp

The eddy currents generated in the tension bars give a time constat (τ) of 1.13 s



A similar magnet with a reduced GFR (15 x 15 cm2) was also studied in order to evaluate the possibility to minimize weight and/or power consumption

An alternative: reducing the GFR

2D magnet profile of reduced GFR

Conceptual view of reduced GFR 90 bending magnet

Magnetic flux lines Magnetic field homogeneity



GFR ( 15 x 15 cm2)

GFR ( 20 x 20 cm2)

Magnetic field [T] 1.814 1.87

ΔB/B0 at GFR [-0.58x10-4, 1.375x10-4] [-0.8x10-4, 1.03x10-4]

Stored Energy [J] 882227.24 1213924.48

Inductance [H] 0.26 0.47

Dissipated DC power [kW] 426.64 613.65

DC voltage [V] 164.1 269.16

Inducted Voltage [V] 150.6 236.8

Excitation current[A] 2600 2800

Ampere-turns 156000 182400

Magnet Weight [tons] 70 82

An alternative: reducing the GFR

Comparison table between the reduced 90° bending magnet (GRF 15 x 15 cm2) and the reference 90° CNAO bending magnet (GFR 20 x 20 cm2)

The magnet with 15 x 15 cm2 reduced GFR gap allows to save approximately 10 tons in weight and to save 30% of power consumption



Remarks about 90° bending magnet

• Static simulations of the CNAO 90° large gap dipole have shown that the 2D field homogeneity is acceptable and that the influence of the stiffening frame structure and of the tension bars cannot be neglected

• The variation in the absolute value of the field in the gap is not significant for the requirement on the excitation current and the effect on the field homogeneity is unimportant

• A reduced GFR 90° bending magnet has been evaluated. The corresponding layout allows saving about the 30% of power consumption and 10 tons on the overall weight



Quadrupole

Effect of the rotator on the quadrupole field

It is a four poles magnet that provides the essential focusing force to the particles in the transversal plane.


Quadrupole Magnetic line for a mobile isocenter gantry Static Simulations

Gradient field homogeneity at Good Field Region GFR

Quadrupole without rotator

Magnetic flux lines

Gradient field 19.481 T/m

Boundary GFR

Magnetic field vs Radial distance

Specification= Δg/g ≤ 1x10-3



Tension bars

Rotator structure

Rotator

CAD design of the rotator structure

2D geometry implemented in the FEM code



2D

Gradient in the center of the magnet

[T/m]

Delta (%)


Yoke + tension bars 19.503 0.11

Yoke + tension bars + rotator structure

19.505 0.12

Values of the magnetic field gradient in the GFR boundary

(r = 30 mm)

Rotator model

Rotator



Values of the magnetic field gradient homogeneity in the GFR boundary (r = 30 mm)

Quadrupole+Rotator

-6.00E-04

-5.00E-04

-4.00E-04

-3.00E-04

-2.00E-04

-1.00E-04

0.00E+00

1.00E-04

2.00E-04

3.00E-04

0 5 10 15 20 25 30 35 40 45 50

Ra

dia

l Ho

mo

ge

ne

ity

Angle(deg)

HomoBt (alone)

HomoBr (alone)

HomoBt (quad+rot)

HomoBr (quad+rot)

Specification= Δg/g ≤ 1x10-3


Quadrupole Magnetic line for a mobile isocenter gantry

Dynamic simulations Tension bar

Sketch of quadrupoles lamination with the tension bars welded in the external part of

the yoke

Eddy currents generated in the tension bars of the quadrupole

The evaluation point is in the GFR boundary

r= 30 mm



Dynamic simulations

B(t) using a linear current ramp of dI/dt = (2xImax )/t = 572.5 A/s

NO rotator



Dynamic simulations

0.00E+00

5.00E-05

1.00E-04

1.50E-04

2.00E-04

2.50E-04

3.00E-04

0 0.5 1 1.5 2 2.5 3 3.5 4

Bst

atic -

Bd

ynam

ics

t(sec)

difference

The eddy currents generated in the tension bars give a time (τ) of 0.48 s

Errors on the gradient of the order of 1x10-3 are considered acceptable


Quadrupole + rotator


Dynamic simulations

0.00E+00

2.00E-07

4.00E-07

6.00E-07

8.00E-07

1.00E-06

1.20E-06

1.40E-06

0 0.5 1 1.5 2 2.5 3 3.5 4

Difference between quadrupole+ tension bars and the quad+tension bars+ferromagnetic rotator sructure

diff No Fe diff Yes Fe and alone

Bd

ynam

icqu

ad –

Bd

ynam

icro

tato

r

t(s)

Two lines overlapped



Remarks about the quadrupole magnet

• Differently from the 90° bending magnet, the dynamic effects introduced by additional elements, like the tension bars and rotator structure, can be neglected

• The static field contribution of the previously mentioned structures is almost negligible; it amounts to approximately 0.1% and it is mainly due to the ferromagnetic tension bars welded to the iron


Effect of the rotator on the corrector B field


Corrector

Its main function is to correct the orbit distortions.

Transverse field profile in the GFR ( field request=0.06T)


corrector Magnetic line for a mobile isocenter gantry Static Simulations

Magnetic field homogeneity Magnetic flux lines

Specification ΔB/B≤1x10-2


Corrector + rotator

Magnetic line for a mobile isocenter gantry Static Simulations

rotator

2D geometry implementation In the FEM code

3D geometry implementation in the FEM Code

The magnetic

field has been measured in the center of

the GFR



Static simulations

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0 100 200 300 400 500 600 700

|B

|

z(mm)

no rotator with rotator

Lmag (No rotator) = 539.9 mm Lmag (with rotator) = 499.3 mm

Corrector + rotator

Longitudinal magnetic field profile in the center of the GFR. Blue line corrector without rotator Red line corrector inside the rotator



Static simulations Integrated Field homogeneity

0.00E+00

1.00E-03

2.00E-03

3.00E-03

4.00E-03

5.00E-03

6.00E-03

7.00E-03

8.00E-03

9.00E-03

1.00E-02

0 5 10 15 20 25 30 35

Inte

grat

ed

ho

mo

gen

eit

y

Xgfr(mm)

Corrector Y=0

corrector+rotator Y=0

Integrated homogeneity requested 1x10-2

Corrector + rotator



Magnetic field profile different contributions

0.060605

0.06061

0.060615

0.06062

0.060625

0.06063

0.060635

0.06064

0 5 10 15 20 25 30 35

|B|

(T)

Xgfr(mm)

Alone

Partially fully

corrector fullFe

Static simulations

2D

B field in the center

of the magnet

[T]

Delta (%)


Corrector + partially full of Fe

0.060606 0.012

Corrector+ Totally full of Fe

0.060606

0.012

Values of the magnetic field in the center of the GFR

Corrector + rotator



Dynamic simulations

Corrector + rotator (full Fe)

No tension bars

Corrector + rotator


corrector Magnetic line for a mobile isocenter gantry

Dynamic simulations

Linear current ramp used

dI/dt=280.1 A/s

0.00E+00

5.00E-04

1.00E-03

1.50E-03

2.00E-03

2.50E-03

3.00E-03

3.50E-03

4.00E-03

4.50E-03

5.00E-03

0.5 0.6 0.7 0.8 0.9 1

Bst

atic

- B

dyn

amic

t(s)

Bs-Bd

The eddy currents generated in the coils and rotator structure provides a time (τ) of 0.068 s

Corrector



Corrector + rotator

Remarks about the corrector magnet

• The effect of the rotator on the corrector field strength is very low (0.012%) and it can be neglected.

• 3D and dynamic simulations of the magnet in the rotator are in progress

51 [email protected]

Conclusions

• The magnets of the gantry line have been characterized.

• In general, the dynamic effects in the transfer line magnets can be neglected because of the low ramp rate needed to follow the energy change.

• Although there is no strong impact in the dynamics of quadrupoles and correctors due to rotator structure, the magneto static 3D simulations suggest an optimization phase of magnet and rotator design to compensate the (small) effect on the field homogeneity.

Thank you for your attention


Additional slides


Among normal conducting or superconducting magnets [email protected] 54


Superferric iron dominated 90° dipole

This magnet is the superconducting version of the conventional 90° dipole presently employed in the CNAO vertical line. The use of a superconducting cable allows reducing the dimension by a factor two. The superconducting coil is placed in the cryostat, which is included into the ferromagnetic yoke


Superferric iron dominated 90° dipole

Pros Cons

Low weight wrt normal conducting magnet

Complex winding and cryostat

Low required power at 300K wrt normal conducting magnet

AC losses at 4.2K limiting ramp rate at 0.1 T/s (0.2 T/s )

The winding is a limited part of the magnet wrt full SC magnet

Low temperature margin using NbTi (but much higher than full SC magnet)

Low AC losses wrt full SC magnet Very difficult coil using Nb3Sn

Magnetic forces curbing/withstanding


Double helical coil 90° dipole

number of double layers 1

dipole field 3 T

bending radius 2.2 m

current 6112 A

turns per layer 838

good field region diameter 120 mm

inductance 0.12 H

stored energy 2.23 MJ


Double helical coil 90° dipole

number of double layers 1

dipole field 3 T

bending radius 2.2 m

current 6112 A

turns per layer 838

good field region diameter 120 mm

inductance 0.12 H

stored energy 2.23 MJ

Detailed mechanical analyses showed that in order to hold the magnetic forces, a massive structure is required. The difficulties in finding a reasonable overall technical solution led to abandon this option in favor of a standard normal conducting 90 degree magnet

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Study of the magnets used in a mobile isocenter gantry · [email protected] 18 Magnetic line...

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