Turbine behaviour

Hi
What happens with a turbine type blower at high altitude? Say atmospheric pressure's 0.5 bar instead of 1 bar, so you'd obviously need a 'big' diameter turbine to collect the air mass per second. It would be working harder than at sea level. It would be collecting the same amount, but compressing it more (say 0.5 bar to 2.0 bar).
I'm particularly interested in a turbine, not a screw/Rootes type blower.
Anyone with any experience of turbochargers fitted to aircraft?
Or any other ideas?
Cheers
Malc

I would send an e-mail to Gale Banks at Banks power.com.