8 Accelerator Engineering and Technology: Accelerator Technology
381
multi-gap vacuum cavities are used where very large accelerating gradients are
required, like in a future linear collider projects. Section 7.5 above is dedicated to
the design of high gradient accelerating cavities for linear colliders.
8.2.7 Superconducting Cavities
Superconductivity denotes the effect of vanishing electrical resistivity in some
materials at cryogenic temperatures. Superconductivity is applied in many present
day high energy accelerators in coils to create very large (DC) magnetic fields (Sect.
8.1.3). If also the RF resistance were zero, one would be able to establish RF fields
as cavity mode in a superconducting cavity without feeding any RF power, i.e. the
shunt impedance would become infinite. The RF surface resistance is not exactly
zero though, but it still can be made small enough to reach Q values in the order of
10 10 , which makes RF superconductivity still extremely attractive.
It should be noted that even if the power lost in a superconducting cavity is much
smaller than in normal-conducting cavities, this power has to be cooled at cryogenic
temperatures, which is much less efficient. As a rule of thumb, an AC power for the
refrigerator plant of about 1 kW is necessary to cool a lost RF power of 1 W lost at
2 K (Sect. 8.3.5).
The more serious limitation for superconducting RF cavities is however the
maximum possible accelerating field: the RF magnetic field at the cavity surface
of the accelerating mode is equal to a surface current density, which must stay
below a critical value. For niobium, the best RF superconductor known today, this
effect limits the accelerating gradient to below 50 MV/m. But already accelerating
gradients above of 5 MV/m require extraordinary care, since also other effects like
field emissions from impurities on the surface or multipactor induced quenches
have to be dealt with. In order to obtain very large accelerating fields, a very
complex technology has been developed over the last decades, primarily driven by
the TESLA collaboration (and later the ILC Global Design Effort) with the aim to
develop cavities for a linear collider. This R&D allowed pushing the practical values
for reliably obtained accelerating gradients from a few MV/m to levels close to the
above mentioned limit; the accelerating gradient considered practical for the ILC is
31.5 MV/m. Also the European XFEL, now under construction near Hamburg, uses
this technology. The developed technology includes the forming and welding of
niobium sheets, different abrasive and non-abrasive, chemical and electrochemical
cleaning processes, a special technique with ultra-pure water applied under high
pressure, and the strict application of clean-room methods.
Since Sect. 7.5.2 above is dedicated to superconducting accelerating structures
for linacs, we will here give an example of a superconducting cavity using a different
technology, which has been initially developed for LEP at CERN and is now also
used for LHC. The cavities are fabricated from sheet metal copper—a process well
understood—and eventually sputtered from the inside with a thin layer of Nb. The
advantage of this technique is that the good thermal conductivity of copper will
381
multi-gap vacuum cavities are used where very large accelerating gradients are
required, like in a future linear collider projects. Section 7.5 above is dedicated to
the design of high gradient accelerating cavities for linear colliders.
8.2.7 Superconducting Cavities
Superconductivity denotes the effect of vanishing electrical resistivity in some
materials at cryogenic temperatures. Superconductivity is applied in many present
day high energy accelerators in coils to create very large (DC) magnetic fields (Sect.
8.1.3). If also the RF resistance were zero, one would be able to establish RF fields
as cavity mode in a superconducting cavity without feeding any RF power, i.e. the
shunt impedance would become infinite. The RF surface resistance is not exactly
zero though, but it still can be made small enough to reach Q values in the order of
10 10 , which makes RF superconductivity still extremely attractive.
It should be noted that even if the power lost in a superconducting cavity is much
smaller than in normal-conducting cavities, this power has to be cooled at cryogenic
temperatures, which is much less efficient. As a rule of thumb, an AC power for the
refrigerator plant of about 1 kW is necessary to cool a lost RF power of 1 W lost at
2 K (Sect. 8.3.5).
The more serious limitation for superconducting RF cavities is however the
maximum possible accelerating field: the RF magnetic field at the cavity surface
of the accelerating mode is equal to a surface current density, which must stay
below a critical value. For niobium, the best RF superconductor known today, this
effect limits the accelerating gradient to below 50 MV/m. But already accelerating
gradients above of 5 MV/m require extraordinary care, since also other effects like
field emissions from impurities on the surface or multipactor induced quenches
have to be dealt with. In order to obtain very large accelerating fields, a very
complex technology has been developed over the last decades, primarily driven by
the TESLA collaboration (and later the ILC Global Design Effort) with the aim to
develop cavities for a linear collider. This R&D allowed pushing the practical values
for reliably obtained accelerating gradients from a few MV/m to levels close to the
above mentioned limit; the accelerating gradient considered practical for the ILC is
31.5 MV/m. Also the European XFEL, now under construction near Hamburg, uses
this technology. The developed technology includes the forming and welding of
niobium sheets, different abrasive and non-abrasive, chemical and electrochemical
cleaning processes, a special technique with ultra-pure water applied under high
pressure, and the strict application of clean-room methods.
Since Sect. 7.5.2 above is dedicated to superconducting accelerating structures
for linacs, we will here give an example of a superconducting cavity using a different
technology, which has been initially developed for LEP at CERN and is now also
used for LHC. The cavities are fabricated from sheet metal copper—a process well
understood—and eventually sputtered from the inside with a thin layer of Nb. The
advantage of this technique is that the good thermal conductivity of copper will