Electrical discharge in low-pressure argon

Shown here is an experimental low-pressure argon discharge tube (T4) used for plasma studies. The lamp was made using components taken from standard compact fluorescent lamp production. Here the lamp runs at around 0.17 A on an HF 13 W fluorescent tube driver and its discharge shows a relatively diffuse profile due to a fill pressure of 4.0 mbar, similar as in standard fluorescent tubes (minus mercury, then). The light emission has a characteristic lilac hue and the plasma radiates far less UV radiation than the usual argon-mercury mix, which is the reason why mercury-starved fluorescent tubes turn reddish-purple and lose most of their brightness.