They need to be relatively passive, hard to detect, and locally effective. If active mechanisms within the mine make these easily detectible, they are not mines but highly visible hazards.
Most engineering problems are about making things cheaper, faster, and effective. (One can also argue that you only get choose only two of these things... But let's not get too cynical here.)
This is not ideal for a mine. Yes, it can generate a lot of energy, but it requires a lot of energy (or mass) as an input. Someone is going to notice this either way!
Fusion is effective for creating energy, but seems like the expensive option when nuclear fission is around. You can store fissile material long term and only need to get two masses close to each other (or close a neutron reflector, as per experiments and accidents with the demon core.) This can be done quickly and cheaply. Seems like a better alternative to me!
Also not ideal for a mine. You need special, active, energy consuming process just to store this (nevermind just getting ahold of enough of the stuff). This is expensive and can scream "danger here" to passing spacecraft, since you will likely need to use magnetic containment of some sort.
Once again, nuclear fission seems to be a cheaper alternative here. No active storage mechanisms required!
This Isn't a Mine
This is a defense platform. If you have effectivity for up to thousands of kilometers, require active systems to maintain, and have (for space) fixed positions, a mine is not a good analogy. It's more like a static defense, like an artillery battery or a machine gun "pill box."
The fact that this is not a mine is even in the name of the weapon: a howitzer specifically has a medium-range effectiveness (whereas artillery is long range and cannons are short range).
What Needs to Change
Semantics aside, several things need to be in place for this to be an effective mine compared to our current world and technology level.
We need a better understanding of fusion and antimatter. We need reactants and raw material to be cheaper and abundant.
For fusion, hydrogen is widely available, but we lack the knowledge to do more than a few seconds and have the reaction occur quickly. Of course, we could just use the energy from antimatter reaction to vaporize that tungsten...
For antimatter, we only have found this in theory. Where are the antimatter galaxies? Likewise, our understanding of it needs to increase as well. How can we store it? Does it need perfect normal matter pairs to annihilate? How does one feed it nicely to make a quick reaction? All this needs to be answered by practical experience.