Long story short… It can’t work.
There are 3 different collisions you care about with beam matter interaction. The first is collision with electrons. The second is collision with B11. The 3rd is fusion events. The probability of these events and how much energy goes where is fairly easy to measure. So we don’t even need to make assumptions about our models. It turns out we are most likely to hit electrons and give them lots of energy which results in heat and xrays. Second is to bounce of the B11 nucleus. Even with perfect energy matching, you still have a fairly low probability of fusion. In fact the probability of a fusion event is so low you can’t get net energy out.
Its just the way the universe is wired. Over all the probability even in favorable conditions is really really low compared to all the other loss mechanisms. This is why fusion is hard. Its also why the sun is stable for billions of years……