What energy alphas do you need? 10 eV or 5 MeV? Bend the alpha particles through a dipole magnet, and keep only those with the highest e/M ratio. I saw some desktop/portable setups like this with radioactive alpha emitters and electro magnets at LBL Berkeley a few years ago.
You are talking about a serious alpha particle source (30 milliamps cw at 40 KeV = 1.2 kW of alphas). You will probably need an ECR (electron cyclotron source), a LEBT (low energy beam transport) with a magnetic bend (momentum filter), probably followed by a RFQ (RF quadrupole accelerator) before a linac. 40 KeV is too low a velocity for a standing wave (DTL or drift tube) linac. What frequency is your DTL?
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