You could use a 1:10 transformer to turn the variable (AC) voltage from 1V to 10V and then use a rectifier (diode bridge + smoothing capacitor) to transform that into [10*sqrt(2) - 2*diode turn-on voltage drop] V, which would be somewhere around 12V (DC) depending on how you design it.
Is this alternator home made? The output seems to be a very low voltage and I wonder why. Could there be a fault?
(Or are you measuring the DC component when you should be measuring the AC component?)