RESONANT STRIP LINE BPM FOR ULTRA LOW CURRENT MEASUREMENTS

Proton beams used in proton therapy facilities like PROSCAN have extremely small currents of an order of nanoamperes, which create a challenge for a precise beam position measurements due to their extremely low signal level and subsequent bad signal per noise ratios. For suitable power levels with these currents, pickups need to have a high shunt impedance, something, which is difficult to design for wide band devices. So for a new strip line BPM design, the coupling of the signal outputs to the electrode was deliberately mismatched to create a resonance at the second harmonic of the RF frequency at 145 MHz. The optimum Q-factor to use is given by the coupling between the BPM electrodes leading to t oaQo f 50, an overall shunt impedance of 2.9 kOhms and power output levels of an order of -120 dBm at the design current of 1 nA. A prototype of the device has been manufactured, first measurement results will be presented.