If all we need to do is bombard with alpha particles then why can't we do nuclear fusion like that in the star?

In Chadwick's experiment, alpha particle from a source are bombarded on beryllium atom and fusion occurs to produce carbon atoms. My question is that if it is that easy to cause nuclear fusion, then why do physicists need such complicated machinery to do nuclear fusion, why not just bombard with alpha particles?

Since, we can control the rate of emission of alpha particles from the source, such fusion might actually be controllable, right?

I really regret if this is a stupid question, I'm just a physics enthusiast.

• Fusion isn't that hard, eg. en.wikipedia.org/wiki/Fusor The tricky part is getting out more energy than you put in. And stars only generate a tiny amount of energy per cubic metre. – PM 2Ring Apr 13 '19 at 5:35
• You can buy "neutron generators" commercial off the shelf (examples). Those are accelerator based fusion reactors. But as @PM2Ring notes, you have to put in more energy than you could, even in principle, get out of them. – dmckee --- ex-moderator kitten Apr 13 '19 at 6:26
• As PM 2Ring said, the big difficulty with imitating stars is that they only produce about 5W per cubic meter. As a comparison, sunlight at Earth's orbit is about 1300 W per square meter. The reason sunlight is so much is that the Sun is very thick. – Carl Brannen Apr 13 '19 at 8:03
• Your assumption is not true. The sun has volume $1.4\times 10^{27}$ cubic meters and about $3.8\times 10^{26}$ watts so overall it makes about 0.3 watts per cubic meter. The 5 watts per cubic meter is the figure for the fusion producing part of the sun. – Carl Brannen Apr 13 '19 at 23:44
• The usual classroom factoid for the low power density of the sun is that $5\,\mathrm{W/m^3}$ is about the power developed by a large and well tuned compost heap. – dmckee --- ex-moderator kitten Apr 14 '19 at 20:43