Transcript Magnet considerations for ARIES IFE HTS/LTS algorithms and design options

Magnet considerations for ARIES IFE
HTS/LTS
algorithms and design options
L. Bromberg
With contributions from J.H. Schultz
MIT Plasma Science and Fusion Center
ARIES Meeting
UCSD
January 10, 2002
ARIES IFE
 Status of near term magnet development for IFE
 High current transport experiment (low energy)
 Acceleration section (medium energy acceleration)
 Superconductor magnet options for IFE
 High temperature (last meeting)
 Low temperature
 Insulation considerations
Present work in magnet community for IFE
 HCX (High Current Transport Experiment):
 800 m A x 1.6-2 MeV, front-end of:
 IRE (Integrated Research Experiment):
 200 A x 200 MeV
 Possible front-end of HIFSA commercial reactor:
 10 kA x 10 GeV)
HCX cylindrical
HCX
Plate racetrack
IRE design of array
Implications of technology program for HU
IFE final optics magnets
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For IFE with HI driver, cost minimization of magnet system is
paramount
Cost minimization of final optics by
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Epitaxial techniques
High Tc to minimize quench requirements and cost of the
cryogenic system (dry system)
Optimize structural requirements
LTS magnets require winding, a more costly proposition
IFE magnet construction
Epitaxial YBCO films
LTS magnet design
Plate array
Cryostat over magnet assembly; shown over individual plate array for illustration purposes
BSCCO 2212 layered pancakes on silver
(L. Bromberg, MIT, 1997)
Magnet configuration options
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Plate magnets, with superconductor on the surface of structural plates
Two different arrangements are possible (a = 0 or b = 0)
f ~ r2 (a sin (2q ) + b cos (2q )
cos (2q )
sin (2q )
Current density
sin (2q ) and cos (2q) quad arrays
cos (2q )
sin (2q )
cos (2 q) plate design options
HTS
sin (2q) magnet section
(not planar due to current return!)
Current distribution in plate quad arrays

The current density for the two cases is different
 For the sin (2q):
 |Kwall| = 4 A x /mo (on the x-wall), -a < x < a
 |Kwall| = 4 A y /mo (on the y-wall), -a < y < a
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For the cos (2q):
 |Kwall| = 4 A a /mo
In both cases, the magnetic field is:
|B|=2A r
 and the gradient is
| B |’ = 2 A
Current distribution in quad arrays
HTS vs LTS supercondutors
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HTS superconductor is deposited
directly on structural plates
Epitaxial techniques can be used to
achieve desired current density for
optimal quadrupole field generation
 Deposition of superconducting
material directly on structural
substrate minimized motion of
SC and heat generation
Insulation is achieved by depositing
thin insulating layer between
superconductor and substrate
Conductor surface fraction ~ 0.9
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LTS superconductor is wound on
structural plates
Epitaxial techniques can not be used
due to superconductor stability
 Stability, quench protection and
manufacturability requires
substantial amounts of copper
 For NbTi, assume Cu/SC ratio of
1. (Decreases strand current
density by a factor of 2).
 Stability requires operation at 0.7
of critical
Insulation around each turn; minimal
thickness 1 mm.
 Due to winding difficulties,
circular cross-section conductors
preferred.
 Due to winding, conductor surface
filling fraction ~ 0.5
Superconductor implications
HTS and LTS
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YBCO HTS material is very sensitive to field orientation (anisotropic):
 preferred orientation of field is parallel to YBCO tape, (B || ab)
 Preliminary critical current density: 1010 A/m2
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For sin (2q) arrays, field is perpendicular to SC
For cos (2q) arrays, field is parallel to SC
sin (2q) requires non-uniform current density, which means that for
similar average current density, peak is higher (for comparable
thicknesses, current density is higher)
cos (2q) requires uniform current density
LTS is isotropic
Conductor thickness
HTS and LTS
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The conductor thickness determined by required surface current
density K, conductor current Jcond, insulation thickness tins, and number
of layers Nlayer
For HTS:
tcond 

1 J cond
 2t insN layer
0.9 K
For LTS
tcond
2 J cond

 2t insN layer
0.7 K
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Forces in Epitaxial quad arrays
HTS and LTS
Because of symmetry, forces are mostly along the substrate (i.e., tension or
compression, with almost no shear)
For both sin (2q) and cos (2q) arrays:
 F = 4 A2 a3/ mo
 s = F/t (t : thickness of structural plate)
To limit strain in SC to 0.2% (comparable to LTS)
 Since superconductor material is integrated with structure, equal strains
 Not additional constraint, since 0.2% corresponds to yield limit in
structure.
 High modulus structural material is desirable
For LTS design with increased conductor thickness due to lower SC current
density, need for substantial copper and thicker insulation thickness, the
structural performance (percentage of structure in magnet cross-section is
decreased compare with HTS
HTS Superconductor options and comparison with LTS
(YBCO tape at 4 K and at 75 K)
"Un-Critical" Critical Current
Density, A/mm2
100000
At 4.2 K Unless
Otherwise Stated
YBCO Tape
10000
YBCO Tape 75 K
Bi2212
Tape
ARIES RS
Bi2223
Round 4.2K
1000
NbTi
Nb3Sn
High AC Loss
1.8 K
ITER-Nb3Sn
Nb-Ti-Ta
Nb3Al
Low AC Loss
Bi2223
Round 75 K
100
0
5
10
15 T
Applied Field,
20
25
Courtesy of Lee, UW Madison
YBCO Current density with field in the “bad” direction (B||c)
as a function of temperature
Figure 2 Comparison of Representative Data for YBCO for various fields & temperatures
vs NbTi and Nb3Sn at 8 T and 4.2 K. (M. Suenaga, :The Coated Conductor Issues”, 98
HTS/LTS Workshop for High Energy Physics, Napa, CA, Mar, 98)
Typical Quad
magnet design
HTS and LTS
HT S
LT S
5.0E+08
9.0E+09
0.001
0.00001
0.002
5.0E+08
7.0E+08
0.001
0.00025
0.002
6
0.02
300
150
6
0.05
120
60
0.0011
0.0029
9.5E+06
1.9E+07
9.5E+06
1.9E+07
0.0011
0.0021
0.0136
0.0273
Fraction of cell for magnet, insulation, beam tube
sin 2 theta
cos 2 theta
0.31
0.37
0.53
0.66
Current, cos 2 theta
number of turns per layer
number of plates, 10 microns YBCO
Current per turn
A
10
212
360
10
27
7000
A
10
106
360
10
1364
7000
Structure stress
Conductor current density
Thickness of beam tube
Pa
A/m^2
m
Thickness of MLI
m
Bmax
a
Field gradient
A
T
m
T/m
T/m
Structure thickness
m
Superconductor
Current density (K)
sin 2 theta
cos 2 theta
Thickness for 1.e6 A/cm^2
sin 2 theta
cos 2 theta
Current, sin 2 theta
number of turns per layer
number of plates, 10 microns YBCO
Current per turn
A/m
A/m
m
m
SC for IFE magnets
YBCO for HTS
NbTi for LTS
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Current density in conductor can be as high as 1010 A/m2 (1 MA/cm2)
for fields as high as 9 T, a temperatures between 20 K and 77 K
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At 9T (B || c), T < 20K
Temperatures between 4 K and 77 K can be achieved by using dry
system (no liquid coolant)
Cooling is provided by conduction cooling to a cryocooler head.
Conductor current density of NbTi is 7 108 A/m2 (4 109*(0.7/2/2)
A/m2)
Fraction of cell not allocated to beam
(linear ratio, not area ratio)
HTS
0.5
Magnet, insulation, tube fraction
0.45
0.4
0.35
0.3
0.25
sin 2 theta
cos 2 theta
0.2
0.15
0.1
0.05
0
200
250
300
350
400
Field gradient (T/m)
450
500
550
Magnet, insulation, tube fraction
Fraction of cell not allocated to beam
Field gradient = 300 T/m
2.5
2
1.5
sin 2 theta
cos 2 theta
1
0.5
0
0
2000
4000
6000
8000
Current density (MA/m^2)
10000
12000
Fraction of cell not allocated to beam
Field gradient = 300 T/m
Magnet, insulation, tube fraction
0.6
0.5
0.4
0.3
0.2
sin 2 theta
cos 2 theta
0.1
0
0
100
200
300
400
Stress in structure (MPa)
500
600
Magnet quench
HTS and LTS
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Energy in magnet is small (for both sin 2 theta and cos 2 theta):
E = 16 A2 a4 L / 3 mo
(L is the quad length)
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For typical magnets (L ~ 1 m), E ~ 20 kJ/quad
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If 100 quads are in parallel, 2 MJ per supply
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If single power supply, @400 A and 5 kV discharge, tdump ~ 1 s
 However, at HTS very high current density, magnet will self destroy at this very
high current density
Quench protection for HTS: passive, quench prevention by large thermal mass to avoid
flux jumping, large heat release.
Quench protection for LTS: provide enough copper so that the current flowing in the
copper during quench does not result in temperature greater than ~150-200 K.
Edge conditions of the quad array
Current density of outermost quad array boundary modified to control
field profile
Magnet assembly tolerance
HTS and LTS
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For HTS with Epitaxial techniques
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manufacturing of the YBCO plates with high precision (~ mm’ s)
Plates can be assembled to tolerances of 5 mils (0.12 mm)
Plate supports can be assembled with 10 mil accuracy
Magnet racetrack can be placed with 10 mils accuracy (0.25 mm) along
the entire way of the plate (~ 1 m across, 1 m apart).
For LTS with winding techniques:
20 mil gap for insertion of conductor into grove, with random location
within groove.
Similar tolerances in assembling plates as HTS
Coil
Cooling:
HTS
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“Dry” cooling
Assuming that quads are only cooled at each end
 If only Ni-based material, large temperature raise to midplane
of quad (DT ~ 100 K)
 Cu placed in parallel to structural plates (bonded at the
structural plate edges, to prevent warping)
 Required cooling cross-section: ~ 5% of plate cross section
1 mW/cm^3
Coil Cooling:
LTS
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NbTi superconductor cooled by pool boiling.
 Liquid He required for conductor stability (only material with any
thermal heat capacity at 4 K)
 NbTi conductor will require a cryostat vessel around the magnets
Average additional cross section requirement: 20% of magnet cross
section
NbTi magnet require more sophisticated plumbing and manifolding
than HTS magnet
Cryogenic cooling issues
HTS vs LTS
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Due to good design (as in the IRE design shown above), operation at 77 K only slightly
reduces the required thermal insulation gap
 Little radiation from 77 K to 4 K
 Support can be long without implication to radial build
 Required distance determined by requirement of no contact between 4 K and 77 K
Therefore, the use of HTS is not driven by cryogenic requirements.
Cryogenic requirements substantially reduced by operation at ~ 77 K (by about a factor
of 8, scaling from IRE design)
Operation of HTS below 77 K by the use of cryocooler (dry operation)
LTS requires cooling with as low a temperature as possible (4.4-4.8 K), because of large
sensitivity of operating current density to temperature
Advantages of plate quad arrays
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The forces are only in tension/compression in the plates
 There are no loads that are perpendicular to the plates that need to be reacted in
shear.
 Very important consideration for insulator
These forces are easy to react, and do not require the need of large inter-quad structure
 The only structure is needed to control off-normal loads, gravity and for accurate
positioning of the plates.
The field is a perfect quadrupole field, with total absence (in the ideal conditions) of
higher order components
 Errors due to
 misalignment
 errors in plate manufacture
 end-fields
Possibility for cheap manufacturing!
Irradiation considerations
HTS vs LTS
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Final focusing magnets will experience high dose:
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40 MGray/year from g’s, 8 MGray/year from neutrons (Latkowski)
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Very high peaking, ~105 – 106 (Sawan, 1981)
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Very high instantaneous dose rate: 105 – 106 Gray/s
LTS materials will require organic insulation (at least hybrid insulation, i.e.,
ceramic/organic composites), with limited lifetime (107-108 Gray)
HTS design uses mainly inorganic insulation (mainly ceramic), with longer
lifetime (maybe 1011-1012 Gray)
Irradiation testing of insulators at M.I.T.
• Irradiation damage to organics mainly due to radiochemistry, driven
mainly by electrons
• Radiation facility consisting of:
• E-beam unit (200 kV, 4 mA)
• Capable of large dose rates (up to Grad/s)
• Large sample capability (5cm x 5 cm)
• Capable of cryogenic testing
Summary
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Design options for plate magnets for IFE quad final optics have been
investigated
 Use High temperature superconductor (YBCO)
 sin (2q) option requires angle elements (non-planar)
 Epitaxial technique on flat plates, angle elements
 Large number of elements, easily manufactured.
Design algorithms were developed
 HTS designs
 LTS materials limit substantially design space
Irradiation effect need to be determined
 Lack of large shear between elements allows the possibility of all
inorganic insulation
 Very large dose rates