Engineering of electromagnetic systems for controlled thermonuclear fusion Scuola di Dottorato in...

Engineering of electromagnetic systemsfor controlled thermonuclear fusion

Scuola di Dottorato inIngegneria Industriale

Università degli Studi di Bologna

22,24 giugno 2009

2

INDEX

Introduction to controlled thermonuclear fusion Superconductivity NbTi e Nb3Sn superconducting cables ITER (International Tokamak Experimental Reactor) experiment Wendelstein experiment

Introduction toControlled Thermonuclear Fusion

4

Fission and Fusion nuclear reactions

5

Fusion reactions

)MeV.(n)MeV.(HeTD 11453 10

42

31

21

)MeV(H)MeV(T

)MeV.(n)MeV.(HeDD

31

4282011

31

10

322

121

)MeV.(H)MeV.(HeHeD 71473 11

42

32

21

)MeV.(HeBH 783 42

115

11

With neutron emission (activation of materials)

Without neutron emission

6

Fusion reactions

In order for the fusion reaction to take place, the kinetic energy of the reacting nuclei must be high enough to overcome the repulsive force due to their positive electric charge.

Distance ( r )

Potential Energy potenziale

R0 5 10-15 nuclear radius

Z1 Z2 e2 / (4 0 r)

0.28 Z1 Z2 MeV

Potential energy vs. distance between nuclei

7

Thermonuclear fusion

The higher is the temperature of the the nuclear fuel (a gas mixture of deuterium and tritium for the D + T reaction), the higher is the kinetic energy of the nuclei.

E

f(E)

Maxwell velocity distribution

kT

Eexp

kTEnEf;

kT

mvexp

kT

mnf

2

322

3

12

22 v

k = Boltzmann constant = 1.3805 10-23 J K-1

8

Thermonuclear fusion

The D -T gas mixture should reach a temperature higher than 1 keV = 11 600 000 K.

The gas is in the plasma state: fully ionized but macroscopically neutral (for distances larger than the Debye length).

R = reaction rate = cross section

10 1 100 1000 10-26

10-25

10-24

10-23

10-22

<v> (m3 s-1)

T (keV)

D - D D - T

121221 vEnnR

9

Plasma confinement

The plasma can be confined by means of:

High magnetic fields (magnetic confinement)

Due to the high value of the required magnetic field the winding producing it must be realized with superconducting materials.

High power LASER pulse (inertial confinement)

10

Magnetic Confinement

An electric charged particle (q = electric charge) moving in a uniform magnetic field region, follows an helical trajectory around a field line. The velocity component parallel to the field (vp) is constant. In the plane orthogonal to the field the motion is of the uniform circular type with a radius rL which is called Larmor radius and an angular

velocity () which is called cyclotron frequency.

B B

q > 0 q < 0

Bq

vmr

m

Bqtetancosv

rmBvq

rvdt

dv

qdt

dm

nL

p

Ln

Ln

p

2

0

Bvv

Particles are completely confined in the directions normal to the field but no confinement is present in the direction parallel to the field

11

Magnetic confinement

A magnetic field with closed toroidal field line can be utilized.

The magnetic field is larger in the inner region than in the outer one. As a consequence a charge separation takes place which produces a vertical electric field.

B

B larger

q < 0

q > 0

B smaller

E B

12


Due to the electric field a drift velocity of the particles vD in the

radial direction is present which is independent from the charge of the particle and produces a motion of the entire plasma

In order to confine the plasma one more component of the magnetic field is necessary, normal to the toroidal one. Thus should be simultaneously present:

2

0

0

B

.tcos

m

q

dt

dm

q

dt

d

qdt

dm

D

D,nn

pp

nnn

p

p

BEv

vvvvE

BvEv

Ev

BvEv

A toroidal magnetic field

A poloidal magnetic field

And the field lines should be of helical type

13


The poloidal magnetic field can be generated by:

A toroidal plasma current (TOKAMAK TOroidalnaya KAmera and MAgnitnaya Katushka (toroidal chamber and magnetic coil) )

External windings (STELLARATOR)

14

TOKAMAK - STELLARATOR

TOKAMAK STELLARATOR

15

TOKAMAK

BJ p

Radial profiles of pressure (p),toroidal magnetic flux density (B) and

poloidal magnetic flux density (B)

Equilibrium equation

The plasma is the secondary winding of a transformer; the primary winding of the transformer is the central solenoid external coil.

z

r

Central solenoid =primary winding of a transformer

plasma =secondary winding of a transformer z

p

B

Br

16

TOKAMAK

17

TOKAMAK

18

STELLARATOR

Winding system to produce poloidal magnetic field

19

Reactor

)MeV.(HeTnLi 8442

31

10

63

)5.2(10

42

31

10

73 MeVnHeTnLi

Natural Litium is a mixture of Litium-6 (7.4 %) and Litium-7 (92.6 % )

Ignition is reached when the energy produced by the fusion reactions and transported by the charged particles which are confined in the plasma equals the energy which is lost by the plasma due to thermal conduction and radiation.

At ignition, the energy which is transported by the neutrons, which are not confined in the plasma, can be used to produce heat and then electric energy by means of a standard turbine plant.

20

Reactor: plasma energy balance

LauxOH PPPPdt

dE

E = Plasma energy (n = density of D and T nuclei)

POH = Power loss due to Joule effect

pV

dVTnkE 3

pV

ppOH dVjP 2

P = Power generation due to fusion reactions: the fraction which is released to the plasma is that transported by alfa particles which are confined in the plasma

pV

dVvnEQP 2

PL = Power loss due to heat conduction, convection and radiation (E = energy confinement time) E

L

EP

LauxOH PPPP At ignition: 0dt

dE

Paux = Power input by additional heating system

21

Reactor

22

Reactor

Research and development ………..

23

International Thermonuclear Experimental Reactor ITER

To demonstrate the scientific and technological feasibility of electric energy production by means of controlled thermonuclear fusion: ignition conditions should be reached and the energy produced by fusion reaction should be much larger than that utilized to heat the plasma

The goal is:

24

Fusion power : 500 MW

Q ( ) : 10

Average neutronic flux :0.57 MW/m2

Maior radius : 6.2 m

Minor radius : 2.0 m

Plasma current : 15 MA

Magnetc flux density on axis : 5.3 T

Plasma volume (m3): 837 m3

International Thermonuclear Experimental Reactor ITER

energyInput

energyFusion

25

ITER superconducting magnets

18 coils to generate toroidal field: stored magnetic energy 41 GJ, maximum field 11.8 T, centripetal force on each coil 403 MN, vertical force on half coil 205 MN, discharge time 11 s.

6 coils to generate poloidal and field and the field for plasma stability: maximum field 5.8 T.

1 central solenoid

Total weight of the system: 10130 t

The cost of the SC coil system is about 30% of the total cost of the machine

26

ITER

27

ITER

28

“Normal” conductors (copper, aluminum, ..) can not be utilized to generate the magnetic field necessary for the plasma confinement due to the excessive joule power loss

Superconducting magnets need to be utilized.

Superconductivity

30

Superconductivity history

1911 Kamerlingh-Onnes finds transition from normal state to superconducting state of a mercury sample at 4.19 K

1957 Bardeen, Cooper e Schrieffer state a microscopic theory of susperconductivity (BCS theory)

1973 Superconductivity of Nb3Ge at 23.2 K

1986 Bednorz and Mueller find superconductive state in La2-xBaxCuO4 at 30 K

1987 Superconductivity of Y-Ba-Cu-O (YBCO) at 93 K

1988 Superconductivity of Bi-Sr-Ca-Cu-O (BSCCO) at 125 K

2001 Superconductivity of MgB2 at 40 K

31

Properties of superconducting materials

Type I superconductors

Low transition temperature Type II superconductors

High transition temperature Type II superconductors

Losses in transient regime

32


At temperatures lower than the critical one the electrical resistivity is nil (< 10-21 m)

33


The superconducting state is a new phase of the material

Thermal conductivity vs. temperature

Heat capacity vs. temperature

34


Hext

R

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 0.2 0.4 0.6 0.8 1 1.2

r/R

H/H

ext

Rr

H

H

ext

exp

Perfect diamagnetism (Meissner effect): the magnetic flux density inside a type I superconducting material is nil.

Superconducting screen currents (supercurrents) are presents which flow in a shell, with thickness of about the penetration length, near the surface of the sample.

4

1

)0()(

cT

TT

= penetration length

35


Magnetization characteristics

H

B

Hc

B = 0(superconducting state)

B = 0 H(normal state)

H

M

Hc

M = -H(superconducting state)

M = 0(normal state)

From a macroscopic point of view the phenomenon can be modeled with a volume magnetization of the superconducting material.

36


A type I superconductor is not only a perfect conductor

Zero field cooling

Perfect conductor Superconductor

Field cooling

Perfect conductor Superconductor

37


The superconducting state is destroyed when magnetic flux density becomes larger than a critical value Bc (critical field)

012

0

Jwhen

T

TBB

ccc

The superconducting state is destroyed when current density becomes larger than a critical value Jc (critical current density)

0

0

extc

c BwhenT

TBJ

38


The critical surface defines all the possible operating condition for the superconducting state to be present

T

J

B

Bc0

Tc0

B T

Jc

Jc0

39


Type I superconductors are not useful for applications:

Due to the fact that current density is confined in a small shell near the surface, transport current is too low for applications.

Critical magnetic field is too low.

Elem. Tc0

(K)Bc0

(mT)Elem. Tc0

(K)Bc0

(mT)Elem. Tc0

(K)Bc0

(mT)

Al 1.18 10.5 Zr 0.61 4.7 Cd 0.517

2.8

Ti 0.40 5.6 Nb 9.25 206.0 Hg() 4.15 41.1

V 5.40 141.0 Mo 0.92 9.6 Hg() 3.9 33.9

Zn 0.85 5.4 Tc 7.8 141.0 Pb 7.20 80.3

40

BCS theory

The BCS theory (proposed in 1957 by Bardeen, Cooper e Schriffer) state a quantistic and microscopic model of the superconducting state in the metallic material.

Couples of “super-electrons” can move in the material without loss due to collisions with the crystal lattice by means of a binding force connected with vibration of the crystal lattice (phonon).

The energy of the couples of “super-electrons” is lower than the energy of the fundamental state of a single electron. The energy reduction is proportional to the critical temperature of the material.

The binding force between two “super-electrons” vanishes at distances larger than the “coherence length”

41

Type II superconductors

When coherence length () is lower than the penetration length () magnetic field can penetrate in the superconducting material

x

normal material type I superconductor material

B ns

0

x

normal material type II superconducting material

B ns

0

42


Material Tc (K) (nm) (nm)

Cd 0.56 760 110

Al 1.18 550 40

Pb 7.20 82 39

Nb 9.25 32 50

Nb-Ti 9.5 4 300

Nb3Sn 18 3 65

YBa2Cu3O7 89 1.8 170

43


Hext

R

When Hext < Hc1 (lower critical field) Type II superconductor undergoes Meissner effects as type I superconductor

When Hc1 < Hext < Hc2 (upper critical field) magnetic field penetrates into the superconducting material (mixed state)

When H > Hc2 superconducting state is destroyed

44

Magnetic phase diagram


T

Hc0

Hc2(T)

Mixed state

T

Type II

B = 0Meissner effect

Hc1(T)

H

Hc0

Hc(T)

B = 0Meissner effect

Type IH

45


In type II superconductors, in the mixed state, magnetic field is concentrated in normal region (fluxoids) with the size of the coherence length, surrounded by currents (vortexes) flowing in the superconducting region of the material.

The magnetic flux connected to each fluxoid is equal to:

0 = h/2e = 2.0678 10-15 Wb

When the upper critical field is reached the fluxoids occupy all the volume of the material

46

Abrikosov lattice in MgB2, 2003

Bitter DecorationMgB2 crystal, 200G

First image of Vortex lattice, 1967

Bitter DecorationPb-4at%In rod, 1.1K, 195G

U. Essmann and H. TraubleMax-Planck Institute, Stuttgart Physics Letters 24A, 526 (1967)

L. Ya. Vinnikov et al.Institute of Solid State Physics, ChernogolovkaPhys. Rev. B 67, 092512 (2003)

http://www.fys.uio.no/super/vortex/


http://www.fys.uio.no/super/vortex/essmann.html

http://link.aps.org/abstract/PRB/v67/e092512

http://link.aps.org/abstract/PRB/v67/e092512.jpg

http://www.fys.uio.no/super/vortex/essmann.html

47

Magnetization characteristics


Vortex structure can be modeled from a macroscopic point of view by means of a volume magnetization.

48

Macroscopic model

From a macroscopic point of view, when average values of electromagnetic quantities over volume with size larger than the coherence length and the penetration length, the following usual Maxwell equations can be considered

MB

HB

EJH

0

;;t

Vortex can not be modeled by means of the the current density J in this approach.

Each superconducting material is characterized by electrical E = E(J) and magnetic M = M(H) properties

Most of the models considers M = 0

dVV

dVV

dVV

dVV

VV

VV

xjJxeE

xbBxhH

1;

1

1;

1

49


From a macroscopic point of view, in a type II superconductor, in the mixed state, when a transport current density is flowing, an electric field is present and a Joule dissipation of electric energy into heat occurs.

I

x E

NbTi - T = 4.2 K, B = 5 T

0.0E+00

2.0E-05

4.0E-05

6.0E-05

8.0E-05

1.0E-04

1.2E-04

1.4E-04

1.6E-04

1.8E-04

2.0E-04

0.0E+00 5.0E+08 1.0E+09 1.5E+09 2.0E+09 2.5E+09

J (A/m^2)

E (

V/m

)

50


Joule dissipation (electric field) is due to movement of vortexes.

Two forces are applied to the vortexes:

I

FL

Fp

Lorentz force FL is directed normally to the directions either of the magnetic field and of the transport current density

“pinning” force Fp opposes to any movement of the vortexes and is connected to the lattice imperfections

0 vE n

51


When temperature is much lower than the critical one, fluxoid motion is very slow (“Flux creep” region) and the electric field is negligible

When temperature overcomes the critical one fluxoid motion is fast and electric field is large (“Flux flow” region)

NbTi - T = 4.2 K, B = 5 T

0.0E+00

2.0E-05

4.0E-05

6.0E-05

8.0E-05

1.0E-04

1.2E-04

1.4E-04

1.6E-04

1.8E-04

2.0E-04

0.0E+00 5.0E+08 1.0E+09 1.5E+09 2.0E+09 2.5E+09

J (A/m^2)

E (

V/m

)flux creep

flux flow

52


The critical current density (Jc) is defined as the current density corresponding to the critical value of the electric field (Ec)

The value of the critical current density depends on the choice for the value of the critical electric field.

Two different values for the critical electric field are utilized

Ec = 10 –4 V/m

Ec = 10 –5 V/m

NbTi - T = 4.2 K, B = 5 T

0.0E+00

5.0E-06

1.0E-05

1.5E-05

2.0E-05

0.0E+00 5.0E+08 1.0E+09 1.5E+09 2.0E+09 2.5E+09

J (A/m^2)

E (

V/m

)Ec

Jc

53

High temperature superconductors

Bednorz and MuellerIBM Zuerich, 1986

1900 1920 1940 1960 1980 2000 0

50

100

150

200

Tem

per

atu

re,

TC

(K)

Year

Low-TC

Hig

h-T

C

164 K

La-214

Hg-1223

Hg V3Si

54

High temperature superconductors (HTSC)

The critical temperature is feasible for operation with liquid nitrogen

Large upper critical field

Brittle, low ductility and malleability Strong anisotropy Long and costly manufacturing process Low value of the critical current density (2 104 A/cm2 at 77K, in

direct current regime, without external field, against 105 A/cm2 at 4.2K for metallic superconductors)

Jc is strongly dependent on strain

55

Typical structure of ceramic superconductors

YBCO YBa2Cu3O6 YBCO YBa2Cu3O7

Perovskite ABX3

56

BSCCO

BSCCO Bi2Sr2Can-1CunOy

Conducting layersCu O

Non-conducting layers

57

Anisotropy

BSCCO-2223 Jc vs. applied magnetic field

The field is parallel to CU-O planes

The field is normalto CU-O planes

58

Magnesium boride

Tc40 K

MgB2

J. Akimitsu, Symp. on Transition Metal Oxides, Sendai, Jan 2001

59

Magnesium boride

Main characteristics of MgB2:

High machinability (wires can be easily manufactured)

Well known manufacturing technology

Low cost

Critical temperature feasible for operation with liquid hydrogen

Low electrical properties at high value of the magnetic field

60


Presently, in the devices for controlled thermonuclear fusion, the more utilized materials are NbTi and Nb3Sn

HTS materials are utilized in the current leads of the coils

61

Cryogenics

Heat rejection to ambient

( Qh )

Heat absorption( Qc )

Fluid expansionto reduce

temperature

Work done onprocess fluid

( W ) Power input

QHX

SC load at Tc

ch

cCarnot TT

T

Carnotideal

1COP

)3.01.0(

COPCOP ideal

real

efficiency : COP = Coefficient of Performance : W

Qc

1W

COP cQ

62

Cryogenics

OPERATING TEMPERATURE

CARNOT COP (Watt Input per

Watt Lifted)

"TYPICAL" COP FOR >100 WATT HEAT LOADS

(Watt Input at 300 K per

Watt Lifted at Top) 273 K 0.11 ~ 0.4

200 K 0.52 ~ 2 150 K 1.01 ~ 4 100 K 2.03 ~ 8-10 77 K 2.94 ~ 12-20 50 K 5.06 ~ 25-35 40 K 6.58 ~ 35-50 30 K 9.10 ~ 50-75

Treject = 303 K

63


When a supercondutor is immersed in a time dependent magnetic field (due to external coils or to a transport current flowing in the superconductor itself), due to the fluxoids motion, electric power is dissipated into heat in the superconducting material.

64


Infinite slab in an alternate magnetic field parallel to the main surfaces of the slab

2a

x

z

y Ba

tBtB Ma sin

Magnetic field penetrates into the superconducting slab starting from the outer surface. A current density equal to the critical current density of the material flows in the region occupied by the magnetic field (critical state model).

Q = Energy loss per cycle per unit volume

65


x

BM

p

2t

x

BM

p

t

x

- BM

p

3

2t

x

p

2

t

0

2

0

22

2

3

2,,

10

0

MM

t

t

a

pa

yy

BBdxtxJtxE

aQ

aJBJ

Bp cp

c

M0

0

Bp = minimum magnetic flux density change which fully penetrates into the slab

1p

M

B

BIf magnetic field does not fully penetrates into the slab

66


x

Bp

2t

2

3t

2

t

x

BM

x

BM- 2Bp

x

- BM

x

- BM+2Bp

x

BM

If magnetic field fully penetrates into the slab

0

2

20

22

0

2

3

212,,

10

0

MM

t

t

a

yy

BBdxtxJtxE

aQ

1p

M

B

B

The lower is the slab thickness the larger is and the lower are the losses

67

“flux jump” instability

Q = Energy loss per unit volume corresponding to a change T of the temperature

T

TT

aTJQ

c

c

0

2200

3

Effective heat capacity is lower than the real one

TJx

Bc

z0

x

BM T = T0 +

T

T = T0 Qs

2

142

110

cccc T

T

T

TJTJ

In a first approximation : 0

0 TT

TTTJTJ

c

ccc

TCT

TT

aTJQ

c

cs

0

2200

3Energy balance (adiabatic case)

0

2200

3 TT

aTJCC

c

ceff

68

“flux jump” instability

When Ceff = 0, at a small heat input corresponds a large increase of the temperature

0

2200

3 TT

aTJCC

c

ceff

Typical values for NbTi:

Jc = 1.5 109 A m-2

= 6.2 103 kg m-3

C = 0.89 J kg-1 K-1

Tc = 6.5 K (B = 6 T)

The smaller is the depth a of the slab the more stable is the superconductor

a < 115 m

NBTi e Nb3Sn Cables

70

Superconducting cables

CICCRutherford cable

71

Cable in Conduit Conductor (CICC)

The most utilized cable in the winding of the devices for the controlled thermonuclear fusion is of the multi-filamentary, multi-stage type, cooled by liquid helium which is forced to flow in the channel where the SC strands are jacketed (cable-in-conduit conductor - CICC).

Typical multi-filamentary, multi-stage structure

N. of cabling stages: 5

N. of Strands: 1350

Cabling pattern: 33556

Twist pitches (mm):

80, 140, 190, 300, 440

72

Strand

Each strand is made of a lot of superconducting wires (more than one thousand, with a diameter lower than 10 m), twisted and immersed in a matrix of normal material (typically copper)

The strand structure is necessary :

To prevent flux-jump instability

To reduce hysteresis losses

To reduce power dissipation during quench (transition to normal state of the superconductor in the strand)

73

Strand modelling

I

E J

kE s

n

c

sc Jsign

J

JE

In superconductor

kE mm JIn copper

IAJAJ mmss

kJ

ms

mm

ms

ss AA

AJ

AA

AJ

Experimental strand characterization is made by measuring its critical current ( Ic) and its current sharing temperature (Tcs)

From previous equation the elctrical characteristics E-J of the strand is obtained

JEE

74

Critical current measurement

I

V

L

A

+

The critical value of the electric field is not fixed; typical values are: Ec = 10-5 V/m, Ec = 10-4 V/m

ttI L

tVtE

At the critical current the value of the electric field equals the critical value (Ec).

s

cc A

IJ At the critical conditions is Jm << Js thus:

cc IEE

IEE

75

Current sharing temperature measurement

I

V

L

A

+ T(t)

ttT L

tVtE

The temperature correspondig to the critical value of the electric field is the measured current sharing temperature (Tcs)

csc TEE

TEE

76

Current distribution

The cable critical current / current sharing temperature measurements are similar to the strand measurements.

Non-uniform distribution of the current among the strands of the cable reduce the value of the critical current / current sharing temperature

A non-uniform distribution of the current among the strands of the cable is due to:

Non-uniform contacts of the strands at terminations of the cable and at joints between two cable-segments.

Electro-motive forces due to transient magnetic field.

77

Terminations / joints

In terminations/joints not all the strands touch the current exchange surface; thus current distribution can not be uniform

78

Current distribution

Current can redistribute among the strands along the cable, because the strands are not insulated and touch each other into the cable. The lower is the transversal contact resistance per unit length between the strands, the higher is the current redistribution.

The lower is the transversal resistance per unit length between the strands, the more uniform is the current distribution

but ..

The lower is the transversal resistance per unit length between the strands, the larger are the losses due to coupling currents circulating among the strands

79

NbTi strand

NbTi is a metallic alloy with good mechanical properties; it is easy to process by conventional extrusion and drawing techniques.Given its superconducting properties, it is well suited for the production of fields in the 2 to10 T range and requires liquid-helium cooling.

80

NbTi strand

1 mm

Cold extrusion

Thermal treatement

A Cu-stabilized, NbTi multifilament composite wire is fabricated in three main steps: production of NbTi alloy ingot (typically 80 cm hight and 20 cm diameter) production, extrusion and drawing of mono-filament billet. production, extrusion and drawing of multi-filament billet.

81

NbTi strand

TB

Bb

T

Tt

cc 20

;

bbt

B

CTBJc 11, 7.10

7.1202 1 tBTB cc

Bc20 (T) 15.07

Tc0 (K) 8.99

C0 (A T m-2) 4.78011011

1.96

2.1

2.12

I0 (A) 0.846

q 0.5925

q

cc I

IIn

0

1

The electrical characteristics of aNbTi strand can be modeled by means of the Bottura scaling

82

Nb3Sn Strand

Nb3Sn is an intermetallic compound; it is formed by thermal diffusion of Sn in Nb (Sn consentration should be in the range 18 % - 25 %). The process requires high temperatures (about 700 °C). It is well suited for the production of fields in the 10- 21T range

Some of the main process which are utilized to manufacture Nb3Sn are the followings:

Bronze process,

Internal Sn process,

Power-in-Tube process.

Nb3Sn is brittle and difficult to machinery. To overcome these problems the “wind and react” technique can be used. The coil is realized with the strand before Nb3Sn formation, then the thermal process takes place for the entire coil.

83

Nb3Sn Strand

84

Nb3Sn strand

During cool down process from the reaction temperature (about 700 °C) to operating temperature (about 4.2 K), due to the different value of the thermal expansion coefficients of the materials in the strand (Nb3Sn, Cu), a strain (thermal strain) is generated in the materials: Nb3Sn is compressed (SC - 0.27 %).

L0

Cu Cu

Nb3Sn

LCu

LSC

L0

Cu Cu

Nb3Sn

L

Cu

CuCu L

LL

SC

SCSC L

LL

T = 700 °C T = 4.2 K

85

Nb3Sn strand

The Nb3Sn electrical characteristic is strain sensitive ( is the uni-axial strain):

kE s

BTn

c

sc Jsign

BTJ

JE

,,

,,

Durham scaling

wcc cccTT

14

43

32

2** 10 *

cT

Tt

tcccBTB cc 110,0, 44

33

22

*2

*2 ,*

2 TB

Bb

c

w

u

cccAA 44

33

2210

qpm

ccc bbTBtTABTJ 1,1,, 13*

2

22*

sc BTJTrBTn ,,,1,,

86

Nb3Sn strand

87

Experimental tests towards ITER

To test the design of the ITER machine experimental activities have been performed / are performed on small size test systems Tests of short cable segments and joints/terminations (TFMC-

FSJS, CSMC-FSJS, PF-FSJS, PFIS) at CRPP Losanna – Switzerland

Tests on model coils:

TFMC (Toroidal Field Model Coil) at FZK – Karlsruhe – Germany - 2001

CSMC (Central Solenoid Model Coil) at JAERI - Naka – Japan - 2000

PFCI (Poloidal Field Conductor Insert) presso JAERI - Naka – Japan – just concluded

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SULTAN Test Facility (Switzerland)

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Sudden quench in NbTi cable

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WIC-130909

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Sudden quench shows that the current redistribution among the strands of the cable is too low.

At a large value of the current, the quench of the cable occurs and it is not possible to measure the critical current.

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When current was lower than 45 kA (PFISnw) and 38 kA (PFISw), it is not possible to measure a critical current and/or a current sharing temperature, but only a quench current

The value of the quench current is significantly lower than the estimation of the critical current supposing uniform current distribution.

Sudden quench in NbTi cable

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Degradation of the characteristics of Nb3Sn cable

The critical current of the Nb3Sn cables tested in the SULTAN facility is significantly lower of the critical current measured in the characterization of the strand at the same operating condition (temperature, field).

The current-sharing temperature of the Nb3Sn cables tested in the SULTAN facility is significantly lower of the current-sharing temperature measured in the characterization of the strand at the same operating condition (field, current).

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A possible mechanism for the degradation of the characteristics of Nb3Sn cable is the strain pattern which is present in the strand at operation in the cable due to the bending action of the Lorentz force.

Each strand is maintained in its position by the forces from the other strands at points whose distance is about 5-10 mm, depending on the twist pitch.

Cross sectio of TFI, in the most stressed region

Cross sectio of TFI, in the lesst stressed region

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The experiments performed in Japan and The Netherland on a single strand confirm a strong reduction of electrical properties due to bending effects.


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Future developments

Nb3Al use: properties are not strain sensitive

HTS use: critical field extremely high

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