10 The Largest Accelerators and Colliders of Their Time
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fields above 4 T owing to saturation effects in the nearby iron yoke. 2 With the
aim in mind of combining the virtues of both design lines, while avoiding the
relative drawbacks, the idea was conceived to take the well-proven aluminumcollared coil of the first design line and put an iron yoke immediately around the
collars. Our main motivation was magnet safety in case on a quench (breakdown of
superconductivity).
Quench protection is an important task in any large superconducting (sc) magnet
system. Accelerator magnets have slim coils with a small copper-to-superconductor
ratio. They are definitely not cryostable (cryostability means that the copper in the
cable is able to carry the full current). If a HERA dipole quenches it is mandatory
to ramp down the current of 5000–6000 A with a time constant of less than a
second to prevent overheating and possible destruction of the coil. The protection of
a single magnet is straightforward: if a quench is detected the coil is connected
to a dump resistor via a thyristor switch and the current decays exponentially
I(t) = I 0 exp (−t/τ ). The stored magnetic energy is dissipated in the dump resistor.
In an accelerator this simple solution is not possible since the magnets in a long
string are connected in series. In HERA this string is a 45 ◦ arc comprising 52
dipoles and 26 quadrupoles. Current leads from the sc coils to the room-temperature
environment are only installed at the ends of the string but not at individual magnets.
Owing to the large inductivity of the 54-magnet string a long decay time constant
τ ≈ 20 s must be chosen in order to limit the induced voltage against ground
potential to less than 1000 V. The quenched coil would burn up during such a
slow current decay, hence an electric bypass must be provided. Here the great
advantage of a cold iron yoke comes in: it is easy to provide such a bypass by
mounting a superconducting cable, reinforced with a copper bus bar, in a groove at
the outer rim of the yoke. The bypass conductor is connected to the main current
conductor via a “cold” silicon diode inside the liquid helium cryostat. 3 The diode
has a threshold voltage of about 1 V at liquid helium temperature, hence the current
flow through the bypass is zero as long as the magnet coil is in the superconducting
state (during particle accceleration the ramp speed must be chosen so low that
the inductive voltage stays well below the 1 V threshold). However, a quenched
coil develops rapidly a resistive voltage exceeding 1 V, and then the main current
switches automatically over to the bypass. The Tevatron possesses an active quench
protection system which is more complicated and shall not be discussed here.
Potentially dangerous are quenches at localized spots in the coil. If the normal
zone does not propagate fast enough along the coil it may happen that the stored
magnetic energy is dissipated in the vicinity of the quench origin leading to
local overheating. To avoid this, so-called “quench-heaters” are installed which are
electrically heated when a quench has been detected. Their task is to spread the
normal zone over the entire coil. Figure 10.14 shows that a warm-iron magnet may
2 It has found out later in the LHC and RHIC projects that the detrimental effects of iron saturation
can be alleviated by punching a suitable hole pattern into the iron yoke laminations.
3 The cold diode concept was invented and thoroughly investigated at BNL.
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fields above 4 T owing to saturation effects in the nearby iron yoke. 2 With the
aim in mind of combining the virtues of both design lines, while avoiding the
relative drawbacks, the idea was conceived to take the well-proven aluminumcollared coil of the first design line and put an iron yoke immediately around the
collars. Our main motivation was magnet safety in case on a quench (breakdown of
superconductivity).
Quench protection is an important task in any large superconducting (sc) magnet
system. Accelerator magnets have slim coils with a small copper-to-superconductor
ratio. They are definitely not cryostable (cryostability means that the copper in the
cable is able to carry the full current). If a HERA dipole quenches it is mandatory
to ramp down the current of 5000–6000 A with a time constant of less than a
second to prevent overheating and possible destruction of the coil. The protection of
a single magnet is straightforward: if a quench is detected the coil is connected
to a dump resistor via a thyristor switch and the current decays exponentially
I(t) = I 0 exp (−t/τ ). The stored magnetic energy is dissipated in the dump resistor.
In an accelerator this simple solution is not possible since the magnets in a long
string are connected in series. In HERA this string is a 45 ◦ arc comprising 52
dipoles and 26 quadrupoles. Current leads from the sc coils to the room-temperature
environment are only installed at the ends of the string but not at individual magnets.
Owing to the large inductivity of the 54-magnet string a long decay time constant
τ ≈ 20 s must be chosen in order to limit the induced voltage against ground
potential to less than 1000 V. The quenched coil would burn up during such a
slow current decay, hence an electric bypass must be provided. Here the great
advantage of a cold iron yoke comes in: it is easy to provide such a bypass by
mounting a superconducting cable, reinforced with a copper bus bar, in a groove at
the outer rim of the yoke. The bypass conductor is connected to the main current
conductor via a “cold” silicon diode inside the liquid helium cryostat. 3 The diode
has a threshold voltage of about 1 V at liquid helium temperature, hence the current
flow through the bypass is zero as long as the magnet coil is in the superconducting
state (during particle accceleration the ramp speed must be chosen so low that
the inductive voltage stays well below the 1 V threshold). However, a quenched
coil develops rapidly a resistive voltage exceeding 1 V, and then the main current
switches automatically over to the bypass. The Tevatron possesses an active quench
protection system which is more complicated and shall not be discussed here.
Potentially dangerous are quenches at localized spots in the coil. If the normal
zone does not propagate fast enough along the coil it may happen that the stored
magnetic energy is dissipated in the vicinity of the quench origin leading to
local overheating. To avoid this, so-called “quench-heaters” are installed which are
electrically heated when a quench has been detected. Their task is to spread the
normal zone over the entire coil. Figure 10.14 shows that a warm-iron magnet may
2 It has found out later in the LHC and RHIC projects that the detrimental effects of iron saturation
can be alleviated by punching a suitable hole pattern into the iron yoke laminations.
3 The cold diode concept was invented and thoroughly investigated at BNL.