What are the vacuum requirements for KP? Is it a UHV only technique?
No, as it is essentially a capacitor technique it works in vacuum, ambient (air) or controlled atmosphere (e.g. Nitrogen, Argon) in exactly the same way.
In our commercial experience 1/3 of systems are for UHV/HV use and 2/3 are for ambient/controlled atmosphere. It is common for UHV scientists to purchase two KP head units one for UHV, their intended application, and one for air. The latter for immediate hands on experience, to train on and gather information on the technique quickly. It has proven to be a most helpful allowing clear sight of the tip vibrating or the closeness of the tip to the sample. In air one can proceed quickly from very simple measurements to more complicated ones. Typically when working in a UHV environment reduced visibility means you are working entirely on the electrical signal produced by the software.
In summary the KP will work from atmospheric pressure down to the lowest vacuum pressure achievable. It is also compatible with sample temperature range of 2 - 1200 K.