I actually didn't understand what you mean.
We are technical college in Saudi Arabia and the equipment, training kits and documentation is for this company I posted their link which is located in Germany.
This is their catalog which I think they are a professional company, they have good kits and they actually design training labs.
But I'm not re-writing their documentation. I quoting from the manual. And I did some experiments of uncontrolled/controlled rectifiers and it worked ok until now.
I just wanted to know why those currents are negative. But I didn't do that experiment yet. I'm not in the lab right now, I'm in my weekend vacation and revising the manual.
I just wondered the engineering explanation about those currents. But it's not very necessary for me to justify now, I can wait until I reach it, or do a quick test later.
Also, the answer in the red say it's because I'm measuring it from the negative side with respect to ground, so that it's my understanding is the reason that the currents in the oscilloscope graph are negative.