Why did the 1985 fullerene experiments use graphite and helium gas?

The 1985 experiments conducted by Harold Kroto, Richard Smalley, and Robert Curl were designed to simulate the chemical conditions found in the atmospheres of carbon-rich red giant stars.1

Kroto had previously used microwave spectroscopy to detect long, chainlike carbon molecules in interstellar gas clouds and stellar atmospheres. To investigate how these chains formed, the researchers used Smalley’s laser-supersonic cluster beam apparatus to vaporize graphite into a plasma. By performing this vaporization in an atmosphere of helium, they were able to create and study the resulting carbon clusters, which led to the serendipitous discovery of the exceptionally stable C${}_{60}$ molecule, or buckminsterfullerene.12