Looking into modafinil to help me study for/take a final exam.

Basically, I need to take several tests as part of my PhD qualifying exam. For reasons I won't get into, I didn't do so hot on the first test, and I need over 90 on the second to get the grade required so I don't have to repeat them both. I understand the material (despite not having touched it in years), I just need help buckling down and studying, "clearing my head", etc. My concentration is really not what it used to be.

I've heard that modafinil can hone one's concentration. I'm not so much interested in prolonging my wakefulness, I'd just like to be more effective while awake -- pushing out distractions, mostly.

I do have a preexisting diagnosis of sleep apnea (I have a CPAP machine but I don't find it makes a huge difference) so this might help me get a prescription for it. I'm not really opposed to buying it online, though I'm curious as to what the consequences are if it's caught. Just seizure?

Does anyone else have experience with modafinil for this sort of short-term use, or any other (hopefully easily accessible) study aid? I should mention I'm on an antidepressant -- venlafaxine -- if that makes any difference.