Today, I sat for a 30-minute meditation session, and towards the end of the 30- minutes (since I heard the timer bell ring a few minutes after the start of the experience), I started to feel an intense acceleration in the mind, and just after that, everything else clearing out of my mind. At the same time, I felt my heart beating faster and I am making heavy breaths. I could really pay attention to the heavy breaths and I was keeping my mind there. And then, I saw some soft, satisfying light, lighting up from beneath my eyes (like lighting a torch from below when your eyes are closed). I got really scared of the entire thing, I tried to continue, but I couldn't because I felt I was going to blow up and ended the session. The whole thing was not a calm experience. It was like a free fall. Is this something normal or something that I should be worried about?
When stored accumulate stress dissolves, pure energy can rise up into the brain, causing this experience of "acceleration", "cleansing out of the mind" and "softening calmness".
The "heart beating faster" and "heavy breaths" is both exhilaration and the beginnings of fear (since the mind has not experienced this before).
Finally, the "fear" became strong and broken the meditation; because the mind could not handle the loss the control and the loss of self.
Unless you have a history of mental illness, the experience should not be worried about. It was a perfectly normal & ordinary meditation experience. If fact, the experience was a sign of progress (in terms of the calming & purification process).
When the body & mind cleanse themselves in meditation of accumulated stresses, these "special effects" will happen and then subside.
When the Buddha taught meditation, he provided the following basic instruction to be followed always; 99% of the time:
There is the case where a monk remains... ardent, clearly comprehending & mindful — putting aside greed & distress with reference to the world.
You correctly identified your experience and your reactions so that is a win/win. It could be worse: like go for years having only some mild euphoria that masks your true feelings and clouds your mind as to the fact. I know, I was there. If you always have negative reactions that you can't get around, don't force it. Since you are following a good teacher (Ajahn Braham) it's not a worry if something rare happens. Sometimes experience is like eating cool ice cream, other times it's like touching an electrical socket.