Study of the magnets used in a mobile isocenter gantry · [email protected] 18 Magnetic line...
Transcript of 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
Introduction
The aim of the WP21 was to participate in the research line already pursued at CNAO concerning the carbon ion gantry design
It means to work together with the ULICE
WP-6 researchers
in developing a conceptual
design of a carbon ion gantry
Introduction
+
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
Why a mobile isocenter gantry
90° bending magnet
Scanning magnets
Quadrupoles
Corrector magnets
The layout shows:
•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
Why a mobile isocenter gantry
Why a mobile isocenter gantry
Preliminary conceptual layout for a mobile isocenter gantry
Why a mobile isocenter gantry
Synchrotron Phase shifter stepper
Match 1
Match 2
Rotator
Gantry
Magnetic line for a mobile isocenter gantry
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
Magnetic line for a mobile isocenter gantry
Magnetic line for a mobile isocenter gantry
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)
Magnetic line for a mobile isocenter gantry
Additional features
To characterize the influence of the rotator structure on the magnetic field of quadrupoles and correctors
Magnetic line for a mobile isocenter gantry
Additional studies
Time dependent analysis of magnet features
Magnetic line for a mobile isocenter gantry
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 line for a mobile isocenter gantry Bending dipole (/8)
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
Magnetic line for a mobile isocenter gantry Bending dipole (/8)
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
Magnetic line for a mobile isocenter gantry Bending dipole (/8)
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
Magnetic line for a mobile isocenter gantry Bending dipole (/8)
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
Magnetic line for a mobile isocenter gantry
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
Magnetic line for a mobile isocenter gantry 90° bending magnet
Static Simulations
Magnetic flux density
Contribution of the tension bars to the magnetic field in the GFR
Magnetic line for a mobile isocenter gantry 90° bending magnet
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
Magnetic line for a mobile isocenter gantry 90° bending magnet
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
Magnetic line for a mobile isocenter gantry 90° bending magnet
Dynamic simulations
Current density inducted in the structure in the Case of all its parts were iron
Magnetic line for a mobile isocenter gantry 90° bending magnet
Dynamic simulations
Magnetic flux lines and eddy current distribution in the tension bars
Magnetic line for a mobile isocenter gantry 90° bending magnet
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
Magnetic line for a mobile isocenter gantry 90° bending magnet
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
Magnetic line for a mobile isocenter gantry 90° bending magnet
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
Magnetic line for a mobile isocenter gantry 90° bending magnet
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
Magnetic line for a mobile isocenter gantry
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
Quadrupole Magnetic line for a mobile isocenter gantry Static Simulations
Tension bars
Rotator structure
Rotator
CAD design of the rotator structure
2D geometry implemented in the FEM code
Quadrupole Magnetic line for a mobile isocenter gantry Static Simulations
2D
Gradient in the center of the magnet
[T/m]
Delta (%)
Only iron yoke 19.481 0
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
Quadrupole Magnetic line for a mobile isocenter gantry Static Simulations
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
Quadrupole Magnetic line for a mobile isocenter gantry
Dynamic simulations
B(t) using a linear current ramp of dI/dt = (2xImax )/t = 572.5 A/s
NO rotator
Quadrupole Magnetic line for a mobile isocenter gantry
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
Magnetic line for a mobile isocenter gantry
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
Magnetic line for a mobile isocenter gantry
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
Magnetic line for a mobile isocenter gantry
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
Magnetic line for a mobile isocenter gantry
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
Magnetic line for a mobile isocenter gantry
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 line for a mobile isocenter gantry
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 (%)
Only iron yoke 0.060614 0
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
Magnetic line for a mobile isocenter gantry
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
Magnetic line for a mobile isocenter gantry
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
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