超导物理学2.ppt

Lecture 2: Controlling training and fine filaments
Controlling training
causes of training - release of energy within the
minimum propagating zones MPZ and minimum quench energy MQE
Fine filaments
screening currents and the critical state model
flux jumping
ization and field errors
quench initiation in LHC dipole
time
resistance
Martin Wilson Lecture 2 slide1
Superconducting s for Accelerators JUAS Feb 2003
recap training
poor quality winding
good quality winding
Martin Wilson Lecture 2 slide2
Superconducting s for Accelerators JUAS Feb 2003
Training of an LHC dipole
Martin Wilson Lecture 2 slide3
Superconducting s for Accelerators JUAS Feb 2003
Causes of training: (1) low specific heat
the specific heat of all substances falls with temperature
at , it is ~2,000 times less than at room temperature
a given release of energy within the winding thus produce a temperature rise 2,000 times greater than at room temperature
the smallest energy release can therefore produce catastrophic effects

300K
Martin Wilson Lecture 2 slide4
Superconducting s for Accelerators JUAS Feb 2003
Causes of training: (2) Jc decreases with temperature
but, by choosing to operate the at a current less than critical, we can allow a temperature margin
at any given field, the critical current of NbTi falls almost linearly with temperature
- so any temperature rise drives the conductor into the resistive state
Martin Wilson Lecture 2 slide5
Superconducting s for Accelerators JUAS Feb 2003
Causes of training: (3) conductor motion
Conductors in a are pushed by the ic forces. Sometimes they move suddenly under this force - the 'creaks' as the es on. A large fraction of the work done by the ic field in pushing the conductor is released as frictional heating
typical numbers for NbTi:
B = 5T J = 5 x 108 -2
so if d = 10 mm
then Q = x 104 -3
Starting from qfinal =
work done per unit length of conductor if it is pushed a distance dz
W