Compared to traditional thermal-cutting sources, high-power industrial laser beams can concentrate a high-power beam incident on a work piece within a very small focus spot, typically less than 0.020 in. (0.51 mm) in diameter. There are four main types of assist-gases used for cutting: pure reactive gas, such as oxygen and hydrogen; pure inert gas, such as argon and helium; neutral gases, such as nitrogen, which are not chemically reactive with a majority of metals and are low-cost alternatives to inert gases, and any combination of the above, such as air (21% O2 + 78% N2 + 1% Ar). Nozzles are consumables and care must be taken to limit their cost. Most nozzles are made of copper alloy that is easily machined. Laser output is characterized by three main parameters: peak power, pulse duty, and pulsing frequency.