Patent ID: 7352846

Claim:
A coiled filament for an X-ray tube comprising: a central region including plural turns having a same coil pitch; and end regions which are arranged on either side of the central regions and include plural turns each of which has a coil pitch smaller than the coil pitch of the central region, wherein the coil pitches of the plural turns of the end regions are reduced one by one by a same variation from a turn close to the central region toward an outermost turn, and Δp/p is within a range of 0.015 to 0.1 and k/n is within a range of 0.3 to 0.8, where p is the coil pitch of the central region, Δp is the coil pitch variation of the end regions, n is a total number of turns of the filament, and k is a sum of numbers of turns of the end regions.