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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
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
ARIES 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)
Present work in magnet community for IFE
HCX cylindrical
HCXPlate racetrack
IRE design of array
For IFE with HI driver, cost minimization of magnet system is paramount
Cost minimization of final optics by
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
Implications of technology program for HU IFE final optics magnets
IFE magnet construction Epitaxial YBCO films
STRUCTURAL PLATE/ SHELL
HIGH TEMPERATURE SUPERCONDUCTOR
YBCO (123) material
Ni-based structure
CeO2
Silver coating
LTS magnet designPlate 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
Plate magnets, with superconductor on the surface of structural plates
Two different arrangements are possible (a = 0 or b = 0)
~ r2 (a sin (2) + b cos (2)
sin (2)cos (2)
Current density sin (2) and cos (2) quad arrays
cos (2) sin (2)
cos (2 ) plate design optionsHTS
sin (2) 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 (2):
|Kwall| = 4 A x /o (on the x-wall), -a < x < a |Kwall| = 4 A y /o (on the y-wall), -a < y < a
For the cos (2): |Kwall| = 4 A a /o
In both cases, the magnetic field is:| | = 2 A r and the gradient is
| |’ = 2 A
Current distribution in quad arraysHTS vs LTS supercondutors
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
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 implicationsHTS and LTS
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
For sin (2) arrays, field is perpendicular to SC For cos (2) arrays, field is parallel to SC
sin (2) requires non-uniform current density, which means that for similar average current density, peak is higher (for comparable thicknesses, current density is higher)
cos (2) requires uniform current density
LTS is isotropic
The conductor thickness determined by required surface current density K, conductor current Jcond, insulation thickness tins, and number of layers Nlayer
For HTS:
For LTS
Conductor thicknessHTS and LTS
€
tcond=1
0.9J condK
+2tinsNlayer
€
tcond=2
0.7J condK
+2tinsNlayer
Forces in Epitaxial quad arraysHTS and LTS
Because of symmetry, forces are mostly along the substrate (i.e., tension or compression, with almost no shear)
For both sin (2) and cos (2) arrays: F = 4 A2 a3/ o
= 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
100
1000
10000
100000
0 5 10 15 20 25
"Un-Critical" Critical Current
Density, A/mm 2
Applied Field, T
Nb3Al
Bi2212Tape
Bi2223Round 4.2K
ITER-Nb 3Sn
Low AC Loss
Bi2223Round 75 K
YBCO Tape 75 K
NbTi
YBCO Tape
At 4.2 K UnlessOtherwise Stated
1.8 KNb-Ti-Ta
Nb3Sn
High AC Loss
ARIES RS
HTS Superconductor options and comparison with LTS(YBCO tape at 4 K and at 75 K)
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 & temperaturesvs NbTi and Nb3Sn at 8 T and 4.2 K. (M. Suenaga, :The Coated Conductor Issues”, 98HTS/LTS Workshop for High Energy Physics, Napa, CA, Mar, 98)
Typical Quad magnet design
HTS and LTS
HTS LTS
Structure stress Pa 5.0E+08 5.0E+08Conductor current density A/m^2 9.0E+09 7.0E+08Thickness of beam tube m 0.001 0.001
0.00001 0.00025Thickness of MLI m 0.002 0.002
Bmax T 6 6a m 0.02 0.05Field gradient T/m 300 120A T/m 150 60
Structure thickness m 0.0011 0.0029
SuperconductorCurrent density (K)
sin 2 theta A/m 9.5E+06 9.5E+06cos 2 theta A/m 1.9E+07 1.9E+07
Thickness for 1.e6 A/cm^2sin 2 theta m 0.0011 0.0136cos 2 theta m 0.0021 0.0273
Fraction of cell for magnet, insulation, beam tubesin 2 theta 0.31 0.53cos 2 theta 0.37 0.66
Current, cos 2 thetanumber of turns per layer 10 10number of plates, 10 microns YBCO 212 27Current per turn A 360 7000
Current, sin 2 thetanumber of turns per layer 10 10number of plates, 10 microns YBCO 106 1364Current per turn A 360 7000
SC for IFE magnetsYBCO for HTSNbTi for LTS
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
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
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
200 250 300 350 400 450 500 550
Field gradient (T/m)
Magnet, insulation, tube fraction
sin 2 theta
cos 2 theta
Fraction of cell not allocated to beamField gradient = 300 T/m
0
0.5
1
1.5
2
2.5
0 2000 4000 6000 8000 10000 12000
Current density (MA/m^2)
Magnet, insulation, tube fraction
sin 2 theta
cos 2 theta
Fraction of cell not allocated to beamField gradient = 300 T/m
0
0.1
0.2
0.3
0.4
0.5
0.6
0 100 200 300 400 500 600
Stress in structure (MPa)
Magnet, insulation, tube fraction
sin 2 theta
cos 2 theta
Magnet quenchHTS and LTS
Energy in magnet is small (for both sin 2 theta and cos 2 theta):
E = 16 A2 a4 L / 3 o (L is the quad length)
For typical magnets (L ~ 1 m), E ~ 20 kJ/quad
If 100 quads are in parallel, 2 MJ per supply
If single power supply, @400 A and 5 kV discharge, dump ~ 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 arrayCurrent density of outermost quad array boundary modified to control
field profile
Magnet assembly toleranceHTS and LTS
For HTS with Epitaxial techniques
manufacturing of the YBCO plates with high precision (~ m’ 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
CoilCooling:
HTS
“Dry” cooling
Assuming that quads are only cooled at each end If only Ni-based material, large temperature raise to midplane of
quad (T ~ 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
0
5
10
15
20
25
30
35
40
0 0.02 0.04 0.06 0.08 0.1
Fraction of Cu
Delta T (K)
1 mW/cm^3
Structure
Copper (for condution cooling)
Superconductor
Coil Cooling:LTS
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 issuesHTS vs LTS
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
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 considerationsHTS vs LTS
Final focusing magnets will experience high dose:
40 MGray/year from ’s, 8 MGray/year from neutrons (Latkowski)
Very high peaking, ~105 – 106 (Sawan, 1981)
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
Design options for plate magnets for IFE quad final optics have been investigated Use High temperature superconductor (YBCO) sin (2) 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