Electrical principles, magnet components and schematics, risks to

and from magnets, protection MOPS Training Session 1

21.8.2008KHM

The nice ideas and pictures are stolen from M. Wilson , A. Siemko., R. Denz and P. Schmueser. The mistakes and the rest of it are mine.

Apologies for the quality of pictures and talk. It had to be prepared in a hurry, parallel to HC.

Electrical principles, magnet components and schematics, risks to

and from magnets, protection

MOPS Training Session 121.8.2008

KHM

Outline

Components in a typical circuitEnergiesRisksEnergy Management (Protection)Quench DetectionReminder

The basic components:Consider a superconductor, already immersed in LHe:

The basic components:Consider a superconductor, already immersed in LHe:

As such pretty useless, but the picture is incomplete, anyhow:

The basic components:Consider a superconductor, already immersed in LHe:

We need: Current leads and all the warm partsWe will have in addition: Inductance, resistance and capacitance

A single wire in details

LCC

CRR

R

01020

30405060

708090

1st Qtr 2nd Qtr 3rd Qtr 4th Qtr

EastWestNorth

LCC

CRR

R

Stored magneticenergy

Stored electrical energy

Frequency dependence

A single wire in detail

Stored Magnetic Energy

LHC dipole magnet (twin apertures) E = ½ LI2 L = 0.12H I = 11.5kA E = 7.8 x 106 Joules

the magnet weighs 26 tonnes

so the magnetic stored energy is equivalent to the kinetic energy of: 26 tonnes travelling at 88km/hr

Stored Magnetic Energy

LHC dipole magnet (twin apertures) E = ½ LI2 L = 0.12H I = 11.5kA E = 7.8 x 106 Joules

the magnet weighs 26 tonnes

so the magnetic stored energy is equivalent to the kinetic energy of: 26 tonnes travelling at 88km/hr

Stored Magnetic Energy

In a sector we have 154 magnets…in LHC we have 154*8 magnets

with a total stored energy of

E=9.6 GJ

Stored Magnetic Energy

In a sector we have 154 magnets…in LHC we have 154*8 magnets

with a total stored energy of

E=9.6 GJ

This corresponds a 100 000 to ship running at 27 knots.

Stored Magnetic Energy

In a sector we have 154 magnets…in LHC we have 154*8 magnets

with a total stored energy of

E=9.6 GJ

This corresponds a 100 000 to ship running at 27 knots.

Stored Magnetic Energy

Magnetic energy can be converted to electrical energy by a fast change of

the current(break of busbar, opening of a switch….).

U=L dI/dt

3.6.03 K H Mess, LHC days 2003 15

3.6.03 K H Mess, LHC days 2003 16

In 2003:

About 15…20% of all cold tested magnets have isolation problems.

They can (with some exceptions) not be used in the tunnel.

Why are these faults not detected earlier in the manufacturing?

Reason 1: The faults are produced during cool down. (heater, omega)

Reason 2: It is difficult, because we use Helium or measure lousy transmission lines.

In 2008:

Not all were found during the tests!!!

Back to the basics

Consider a superconductor, already immersed in LHe:

18

Kamerlingh Onnes liquifies for the first time (1908) Helium and studies the temperature dependence of the

electrical resistance of metals. (1911)

Below a critical temperature the

resistance (voltage drop) seems to disappear. He calls the phenomenon “Superconductivity”.Nobel Price in 1913

19

Critical Temperature, Meissner Ochsenfeld

Critical Temperature c

)0(25.3 cBk

Critical Field Bc: Type 1 superconductors show the Meissner effect. Field is

expelled when sample is cooled down to become

superconducting.

Low temperature superconductivity is due to a phase transition. Phase

transitions happen to keep the relevant thermodynamic energy

(Gibbs energy) low.Here pairs of electrons of opposite

momenta and spin form a macroscopic (nm) boson, the Cooper

Pair.The binding energy determines the

critical temperature.

where kB = 1.38 10-23 J/K is the Boltzmann's constant

and (0) is the energy gap (binding energy of Cooper pairs)

of at = 0

Type 1 superconductors are useless for magnets!

The thermodynamic energy due to superconductivity Gsup increases with the magnetic energy, which

is expelled i.e. with B2

Gsup reaches Gnormal at the maximal field Bc, which is small. (~0.2 T)

20

London Penetration depth, Coherence Length

•Very thin (<) slabs do not expel the field completely. Hence less energy needed.

•Thick slabs should subdivide to lower the energy.•But we pay in Cooper pair condensation energy to build sc

boundaries of thickness energy .•We gain due to the not expelled magnetic energy in the

penetration depth .•There is a net gain if > .

Material In Pb Sn Nb

24 nm 32 nm 30 nm 32 nm

360 nm 510 nm 170 nm 39 nm

Ginzburg Landau refine the argument::

If the ratio between the distance the magnetic field penetrates ( )

London penetration depth

and the characteristic distance Coherence length

over which the electronic state can change from superconducting to

normal is larger than 1/2, the magnetic field can penetrate in the form of discrete fluxoids - Type 2

Ginzburg Landau refine the argument::

If the ratio between the distance the magnetic field penetrates ( )

London penetration depth

and the characteristic distance Coherence length

over which the electronic state can change from superconducting to

normal is larger than 1/2, the magnetic field can penetrate in the form of discrete fluxoids - Type 2

The coherence length is proportional to the mean free path of the conduction electrons.

2 is the area of a fluxoid. The flux in a fluxoid is quantised.

The upper critical field is reached, when all fluxoid touch. Bc2=0/(22).

Hence, good superconductors are always bad conductors (short free path).

Type 2 Superconductors are mostly alloys. Transport current creates a gradient in the fluxoid

pattern. Fluxoids must be movable to do that. However not too much, otherwise the field decays …..

Here starts the black magic.

23

Current Density

108

64

2 24

68

1012

1416

Field T

1

2

3

4

5

6

7

Current density kAmm-2

temperature K

The current (density) depends on the field and on the

temperature and is a property of the sample. (here shown for

NbTi)

24

Working Point and Temperature Margin

10

8

6

4

2 2

4

6

8

10

Field T

1

2

Blue plane: constant temperature, green plane: constant fieldRed arrow: “load line”= constant ratio field/current

If the “working point” leaves the tent (is outside the phase transition) => “Quench”

•Too far on the load line: •Magnet Limit

•Energy deposition increases temperature

•Temperature margin

Deposited Energy: 2 mJ ~106 p/m

~1 A4 sheet falling 4 cm

•Movement•Eddy current warming

•Radiation (all sorts)

Material Constants, Copper

Copper Resistivity Copper Thermal Conductivity

Low ρ High λ

Material Constants, specific heat

Scales differ, Specific heat of He is by far bigger than of Cu

Compares with Water 4.2 J/g K

0.1

10Cu He

4

27

Quench Development

),(),,(

,,2

2

2

2

2

2

zTQzTJgz

T

y

T

x

T

t

zyxTC zr

•Heat Capacity <= small•Heat Conductivity, radial<= small

•Heat Conductivity, longitudinal<= good•Cooling<= depends

•The Quench expands (if the current is above the recovery limit)

•The Temperature at the origin (Thot-spot) continues to rise

spothotT

T

dtJdTT

TCtJ

TC

T

dt

dT

0 0

22

)(

)()(

)(

)(

Only material constants,

can be calculated.

Measurement of the max

temperature (MIITS)

Material Constants, specific heat

Highest at the point and around the boiling point

Water

Slide 29Introduction to testing the LHC magnets - Info Sessions 2002

Magnet Quench – Quench Signal

Threshold

10ms validation window P

R O T E C T I O N

Introduction to testing the LHC magnets - Info

Sessions 2002, A. Siemko

30

How to keep the temperature down?

•Keep the MIITS down by Heatcapacity and Resistivity (too late now)

•Keep the MIITS down by shortening the current flow

•Increase the bulk resistivity (Heating, spread the energy)

•Fast, complicated, energy into He

•Bypass the energy of the rest of the sector (if applicable)

using Diodes or Resistors•Using Resistors <= Attention, introduces a time

delay L/R and Quench back

•Extract the energy (External Resistors and Switches)•Slow, energy into air/water, needed to protect the

diodes

High temperature results in:Movement, frictionInsulation damageMagnet destruction

31

Voltage

High resistance means high I*R and high L*dI/dt

High voltage is dangerous for the insulation

Local damage => ground short or winding shortGlobal damage => Diodes reverse voltage

Voltage taps

Overvoltage can be/ can develop to be a global phenomenon.

Can cause considerable damage.

3.6.03 K H Mess, LHC days 2003 32

Voltage breakdown

- U +

Current I

3.6.03 K H Mess, LHC days 2003 33

Voltage breakdown

3.6.03 K H Mess, LHC days 2003 34

U.V. light

Electron avalanche

Ne(x)=Ne(0)* ex

Ion Bombardment

Per electron (ed-1) ions hit the Cathode

In total

ed/(1-(ed-1))

Breakdown for (1-(ed-1)) = 0 , ed>> 1 => e d ~ 1

3.6.03 K H Mess, LHC days 2003 35

U.V. light

Electron avalanche

Ne(x)=Ne(0)* ex

Ion Bombardment

Per electron (ed-1) ions hit the Cathode

In total

ed/(1-(ed-1))

Breakdown for (1-(ed-1)) = 0 , ed>> 1 => e d ~ 1

is proportional to the density n. It varies with the

field E (geometry!)

and depends on the gas

3.6.03 K H Mess, LHC days 2003 36

E

nB

Aen

1

1ln11 de d

E

nBAdn

ln1

1lnln

Combine it to obtain:

In uniform gaps E=V/d

11lnlnln ndA

dnBVBreakDown Paschens

law

3.6.03 K H Mess, LHC days 2003 37

E

nB

Aen

E

nBAdn

ln1

1lnln

Combine it

11lnlnln ndA

dnBVBreakDown

Paschens law

98.0)*(*2.1 dV Approx. in LHC-PM-ES-1, in kg/l and m

3.6.03 K H Mess, LHC days 2003 38

In air at this density

Vb=6.6kV !!!

3.6.03 K H Mess, LHC days 2003 39

Values differ, because of different

Cathodes and geometries

A Data Compilation

100

1000

10000

100000

1E+20 1E+21 1E+22 1E+23 1E+24

Bre

akd

ow

n V

olt

ag

e [V

]

Number Density * Distance [m -̂2]

Paschen Curve Helium and N2

Bortnik et al Gerhold et al ES N2

Minimal detectable distance for various scenarios in He

1 bar2

bar

6 bar

4.2 K gas

Liquid He

Breakdown Distance for various conditions

0.01

0.1

1

10

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Voltage [V]

Dis

tan

ce [

mm

]

Distance @ 1 bar Distance @ 2 bar Distance @ 6bar Distance @ cold Distance in Lhe

3.6.03 K H Mess, LHC days 2003 42

• The break down voltage of air is 6 * bigger than that of He.

• Tests at elevated voltages run into problems at other spots.

• Magnets that have seen Helium, may not be tested again at “air voltages”.

• Voltages during operation (quench) may be locally higher than can be applied globally. Interturn shorts are particularly difficult.

• We have observed problems with the heater strips.

3.6.03K H Mess, LHC days

200343

Evidence of the insulation deficiency

3.6.03 K H Mess, LHC days 2003 44

• The break down voltage of air is 6 * bigger than that of He.

• Tests at elevated voltages run into problems at other spots.

• Magnets that have seen Helium, may not be tested again at “air voltages”.

• Voltages during operation (quench) may be locally higher than can be applied globally. Interturn shorts are particularly difficult.

• We have observed problems with the heater strips.

Energy Management

• Divide et impera!• Treat sectors separately!• Detect resistive the transistion asap• Divide the energy in a magnet over many windings, using heaters (if necessary).• Guide the energy of all other 153 (or so) magnets around using a diode or resistor.• Protect the diode by a fast extraction of the energy.

Slide 46Introduction to testing the LHC magnets - Info Sessions 2002

Voltage over one aperture

Spike

Irreversible quench

Introduction to testing the LHC magnets - Info

Sessions 2002, A. Siemko

Slide 47Introduction to testing the LHC magnets -

Info Sessions 2002

Example of the mechanical activity in dipoles

Circa 1 spike per 1ms

Quench - What Went Wrong?

• Abnormal voltage signals recorded during the provoked quench

Courtesy: A. Siemko

How does it look at LHC?

Symbolic Circuit

Inventory• Current Leads

– 13 kA– 6 kA– 600 A– 120 A in DFB– 120 A in

magnet– 60 A in magnet

• Busbars– Big busbars– Small busbars

Difficult, because CL need a working cooling environment to run current. To establish this the load parameters have to varied, which in turn requires various currents through a working magnet circuit.

To be discussed.

Form part of the circuit, but tested only globally.

Inventory• Magnets

– 13 kA circuits– 6 kA circuits– 600 A circuits– 120 A circuits– 60 A circuits

Inventory• Magnets

– 13 kA circuits

– 6 kA circuits– 600 A

circuits– 120 A

circuits– 60 A circuits

“Easy”, Freddy takes care.

The 60 A circuits and most 120 A circuits ( including the current leads and bus bars) are protected by the overvoltage detection of the powerconverter.

Its AB-PO.

Inventory• Magnets

– 13 kA circuits– 6 kA circuits– 600 A circuits– 120 A circuits– 60 A circuits

The 120 A MO and the 600 A circuits have a “global quench protection”, that means the current is measured and the first and second derivative are calculated to predict the inductive voltage. Note that the inductance depends on the current.

Difficult

Global Quench Protection

DSP 24 bit ADC

Δ VΔ V

L dI/dt

InterlockFieldbus

Inventory• Magnets

– 13 kA circuits– 6 kA circuits– 600 A circuits– 120 A circuits– 60 A circuits

6 kA quadrupoles

ΔU

ΔU

Long voltage tap, Problems to be expected

Inventory• Magnets

– 13 kA circuits– 6 kA circuits– 600 A circuits– 120 A circuits– 60 A circuits

13 kA busbar protection

Courtesy R. Denz

Note that the reference magnets have to represent an average magnet!

Problem after a quench!

Local quench detector for main magnets

Courtesy R. Denz

Note that only one of the two channels isVisible in the CCC.

The “hidden” card may have “seen” things, invisible for you

Summary

What is special with superconducting circuits?Large inductance, large stored energy, low resistance, long time constants, extremely high current density

What are the specifically dangerous issues?Shorts, opening connections, high voltage, high energy density, hydraulic problems

Keep on telling the operation crew:

62

We are pulling a tigers tail!.

H. Brechna, Superconducting Magnet Systems, Springer, Berlin 1973P. Schmueser, Superconducting magnets for particle accelerators,

Rep. Prog. Phys. 54 (191) 683M. N. Wilson, Superconducting Magnets, Clarendon Press, Oxford,

1983See also his lectures here and at CAS

A.Siemko, Introduction to testing the LHC magnets - Info Sessions 2002

http://nobelprize.org/nobel_prizes/physics/laureates/1913/onnes-lecture.pdf

http://www.bnl.gov/magnets/Staff/Gupta/cryogenic-data-handbook

KHM et al, Superconducting Accelerator Magnets, World Scientific, Singapore, 1996

References