The Quality Factor of a Cavity

nine-cell superconducting cavity

The measured niobium surface resistance in a nine-cell superconducting cavity plotted as a function of TC / T. Here T is the temperature of the helium bath and TC = 9.2 K the critical temperature of niobium. The residual resistance of 3 nΩ corresponds to a quality factor Q0 = 1011.

quality factor of the best cavity

The quality factor of the best cavity as a function of the accelerating field Eacc. The data were taken at helium temperatures between 1.6 and 2.0 K. In the linac of FLASH the cavities are operated at about 15 - 20 MV/m, far below their limit.

nine-cell superconducting cavity

Top: a cylindrical cavity with longitudinal electric field for particle acceleration. The magnetic field lines are concentric circles around the axis.
Bottom: longitudinal cut and photo of the nine-cell superconducting cavity, which is made from pure niobium and cooled by superfluid helium of 2 K. The resonance frequency is f0 = 1.3 GHz. The electric field lines are shown at the instant when an electron bunch has just entered the first cell. The length l of a cell is chosen such that the field direction has inverted when the relativistic bunch has moved to the next cell. This is fulfilled for a cell length equal to half the radio-frequency wavelength, l = c/(2f0). Thereby it is ensured that the particles receive the same energy gain in each cell.

The quality factor of a cavity is defined as the ratio of the resonance frequency to the width of the resonance curve:
Q0 = f0 / Δf. It is inversely proportional to the surface resistance and amounts to Q0 > 1010 for niobium cavities at 2K. In principle the quality factor should stay constant when the field in the cavity is raised from zero to an upper limit which is reached when the radio-frequency magnetic field approaches the critical magnetic field of the superconductor. For niobium at 2K the critical field is Bc ≈ 200 mT, corresponding to a maximum accelerating field Eacc ≈ 45 MV/m. In practice, however, the excitation curve Q0 = Q0(Eacc) usually ends at a lower field due to “dirt effects” such as contamination of the inner cavity surface or field emission of electrons. By applying the cleanroom techniques of the semiconductor industry during the assembly and preparation of the cavities one can almost achieve the physical limit of the superconducting material.