Superconducting Radio Frequency - Q Vs E

Q Vs E

When using superconducting RF cavities in particle accelerators, the field level in the cavity should generally be as high as possible to most efficiently accelerate the beam passing through it. The Q_o values described by the above calculations tend to degrade as the fields increase, which is plotted for a given cavity as a "Q vs E" curve, where "E" refers to the accelerating electric field of the TM₀₁ mode. Ideally, the cavity Q_o would remain constant as the accelerating field is increased all the way up to the point of a magnetic quench field, as indicated by the "ideal" dashed line in the plot below. In reality, though, even a well prepared niobium cavity will have a Q vs E curve that lies beneath the ideal, as shown by the "good cavity" curve in the plot.

There are many phenomena that can occur in an SRF cavity to degrade its Q vs E performance, such as impurities in the niobium, hydrogen contamination due to excessive heat during chemistry, and a rough surface finish. After a couple decades of development, a necessary prescription for successful SRF cavity production is emerging. This includes:

• Eddy-current scanning of the raw niobium sheet for impurities,
• Good quality control of electron beam welding parameters,
• Maintain a low cavity temperature during acid chemistry to avoid hydrogen contamination,
• Electropolish of the cavity interior to achieve a very smooth surface,
• High pressure rinse (HPR) of the cavity interior in a clean room with filtered water to remove particulate contamination,
• Careful assembly of the cavity to other vacuum apparatus in a clean room with clean practices,
• A vacuum bake of the cavity at 120 °C for 48 hours, which typically improves Q_o by a factor of 2.

There remains some uncertainty as to the root cause of why some of these steps lead to success, such as the electropolish and vacuum bake. However, if this prescription is not followed, the Q vs E curve often shows an excessive degradation of Q_o with increasing field, as shown by the "Q slope" curve in the plot below. Finding the root causes of Q slope phenomena is the subject of ongoing fundamental SRF research. The insight gained could lead to simpler cavity fabrication processes as well as benefit future material development efforts to find higher T_c alternatives to niobium.