I see two possibilities, depending on what you want.
1) Dust — anything in the range 1mm to 1nm, depending on what properties you want it to have. Give it a small electric charge and accelerate it conventionally, perhaps in a vacuum with a plasma window at one end.
2) A relativistic beam of something that quickly decays in a way that doesn’t emit much EM radiation. (Would muons fit this description? I don’t know, I’m relying on what Wikipedia says).
1) Dust — anything in the range 1mm to 1nm, depending on what properties you want it to have. Give it a small electric charge and accelerate it conventionally, perhaps in a vacuum with a plasma window at one end.
2) A relativistic beam of something that quickly decays in a way that doesn’t emit much EM radiation. (Would muons fit this description? I don’t know, I’m relying on what Wikipedia says).