Download (6.17)Super Cavities - acceleratorinstitute

Superconducting Accelerating Cavities
Accelerating cavities are at the heart of linacs and, therefore, it has been the linac people who
have been the leaders in developing ever better RF accelerating cavities. There was
considerable work on S-band, L-band, side-coupled cavities, etc. at SLAC, Los Alamos, and
many other places. At first these were room temperature cavities, but rather early on — when
superconducting materials first became available — their attention turned to the use of these
materials so as to reduce the power consumption and improve the duty cycle of their
machines.
In the 1960s work on superconducting cavities was initiated at Lausanne, CERN,
HEPL (Stanford), and Cornell. Although lead was first examined, very soon effort was
focused upon niobium. A collaboration between Karlsruhe and CERN, initiated in 1970,
produced a successful niobium cavity for a radio frequency particle separator by 1978.
Meanwhile, in the US, the main thrust of research into the use of superconducting cavities
was at HEPL at Stanford, where the first fully superconducting L-band linac was completed
(after many years of very difficult R&D) in 1977. During the same time period the Argonne
National Laboratory developed their superconducting linac. Research and development at
Cornell also greatly advanced the art.
By the 1980s there was considerable activity in quite a large number of places.
Working closely with Cornell, the Jefferson Laboratory built the re-circulating linac, CEBAF,
with superconducting accelerating cavities. Early work at Wuppertal soon led to a very long
and fruitful collaboration with DESY. Considerable advance was made, both with the type of
cavity and very fine “packaging” into cryostats. This work led to TESLA, the DESY design
of a TeV superconducting version of a linear collider that was, in fact, the design selected for
the International Linear Collider.
Superconducting RF was, at first, of little interest to synchrotron builders as their
cavities are usually of less importance in determining the size and cost of the machine than is
the case with linacs. Indeed, in a journey round a large synchrotron one might easily miss the
small fraction of the ring where the cavities are installed. The cavities do their work
incrementally over many thousands of turns and for each turn a few MeV of acceleration is
sufficient. However, their design and very type becomes much more critical for the larger
electron storage rings where they must make up the energy radiated each turn. LEP was a
prime example, originally built for 50 GeV; it was planned to double this to 100 GeV but at
the cost of providing about 3 GV of accelerating voltage per turn to balance the losses. The
normal accelerating gradient of a room temperature cavity is at best one or two MeV per m,
so that a few km of the circumference would have to be filled with cavities, and then the
power wasted in just heating up the copper vessels of the cavity would have been over 100
MW, a fair fraction of the output of a power station. Superconducting cavities were a
necessity.
The modern room temperature cavity consists of a series of cylindrical structures
joined together, with a central passage for the beam. These structures are of very special
shape so as to reduce sparking and reduce the development of unwanted oscillation modes.
The strong fields on the axis of these cavities are produced by oscillating radio waves not
unlike the acoustic modes in an organ pipe. The waves end on the copper surfaces of the
cavity — where strong currents flow to maintain the electric and magnetic fields of the
waves. These currents dissipate thermal energy in the copper just as currents in a resistor —
one cannot avoid the consequences of Ohm’s law. However, just as in a resistor, the energy
dissipated by these currents is proportional to the resistance: lower the surface resistance and
the power loss is minimized. The ultimate — actually zero resistivity — is to coat the surface
with a superconductor such as niobium. The incidental, but very important, by-product is that
the voltage gradient can also be much improved in a superconducting cavity. This
development did not come easily, but was the result of decades of R&D.
Early studies in the 1960s with lead coatings had revealed the problems of
impurities and surface imperfections and led the technology in the direction of more exotic
superconducting materials such as solid niobium or electrolytically deposited niobium. Any
slight strain due to forming the cavity shape in a press or the seams due to welding the cells
together would defeat the superconducting properties of the cavity, as would the slightest
speck of dust on the surface layer. Nowadays, in large measure due to the R&D at Cornell
and other places, superconducting cavities with large gradients and reliability of operation
have been achieved.
Early RF cavities were incorporated into machines such as those at HEPL and
ATLAS, and more sophisticated cavities were then developed for KEK (the first major
installation of superconducting cavities in a circular machine; commissioned in 1987), CESR
(Cornell), Jefferson Laboratory, LEP, HERA, and most recently TESLA. At the present time,
superconducting RF is used both in linac and synchrotron-based colliders.