Can someone explain to me how to use the attached equation?

How is the value of alpha, theta and q found?

 

Symbolism differs between authors and years. Can you post the context of this equation or a reference to it?

 

I am trying to calculate the load current of a Single-Phase RL Half-wave rectifier with
parameters of:

R = 10
L = 31.8mH
alpha = 45
theta = 2*pi*50
q = 0.999

and I am plugging in these values into the equation and I am getting a negative value of 24. I am not sure if I am putting it into the calculator correctly.