It can be either AC or DC. Aluminum TIG welding uses AC, whereas you'd use electrode-negative DC for steel or copper. As I understand it, with aluminum you need the electrode-negative part of the waveform to transfer heat to the work piece, but you need the electrode-positive part of the waveform to clear out the crud that accumulates in the electrode-negative part. Often you set a lopsided duty cycle and use different frequencies depending on how deep you want the weld to penetrate.

If you go to 100% electrode positive you tend to heat the metal rather poorly, but can turn the end of your tungsten electrode into a molten blob -- which is usually not desirable.