Arc of Infinity

You may be looking for the television serial or Rondel, a human guard from the planet Varos.

Rondel, known anciently as the Arc of Infinity, was an inter-galactic region of space devoid of all stellar activity. It was the former location of a collapsed Q star, possessed of great quantities of quasi-magnetism when the star burnt out. It served as the gateway to the dimensions. Omega utilised the Arc as his base, his anti-matter existence shielded by the quasi-magnetism.

Amsterdam was located on the curve of the Arc when Omega shifted it, Amsterdam being below sea level to maintain pressure for fusion conversion. Using a fusion converter from Gallifrey, Omega planned to become matter again. The Arc later shifted again. (TV: Arc of Infinity)