A charge-pump 60kV modulator for the ISOLDE target extraction voltage

The ISOLDE facility at CERN provides radioactive ion beams to a number of experimental stations. These ions are produced by a metal target, floating at 60 kV, which is impacted by a 1.4 GeV high intensity proton beam. The ions are then accelerated by a grounded extraction electrode to 60 keV, before transport to the experimental area. During proton beam impact extremely high ionisation of the volume around the target gives rise to significant leakage current which results in loss of charge on the effective target capacitance of approximately 6 nF. If short life-time isotopes are to be studied, the 60 kV must be re-established within a maximum of 10 ms. Recharging the target capacitance to 60 kV and to the required stability of better than 10-4 precludes a direct charging system and an alternative method of re-establishing the 60 kV is used. The present system [1], in operation since 1991, employs a resonant circuit which is triggered 35 µs prior to beam impact. This circuit transfers the charge on the effective target capacitance to a buffer capacitor and reduces the target voltage to zero; the resonance then restores the target voltage to within a few percent of its nominal value within a further 200 µs. Finally the high precision 60 kV d.c. power supply brings the target voltage back to the required ±0.6 V tolerance within a total of 6 ms. In recent years, new types of ion sources (neutron converter targets) have been developed which present ever increasing ionisation loads with the result that it is becoming impossible to respect the maximum 10ms voltage recovery time. Future increases in beam energy to 2 GeV and higher intensity beams will further aggravate the situation. To mitigate the problem new circuit topologies have been conceived and developed. One promising development is a ''Charge pump modulator''. In this circuit a 400 nF buffer capacitor and the target capacitance are charged to 60 kV by a low power, high precision d.c. power supply (HVPS). Immediately prior to beam impact the HVPS is disconnected from the target and buffer capacitors using a 90 kV rated semiconductor switch. During beam impact the target capacitance is rapidly discharged to ~54 kV after which the buffer capacitor, which is partially isolated from the beam impact ionisation by virtue of a series-connected 3.3 kΩ resistor, begins to re-establish voltage on the target. After 1ms a feedback loop controlling an auxiliary high voltage amplifier, which applies a voltage in series with the buffer capacitor, is switched in. This additional voltage brings the target voltage back to the required ± 0.6 V tolerance within 5 ms. Finally, when the target has recovered sufficient high impedance, the feedback loop is opened and the HVPS is re-connected to the target to maintain the stable 60 kV voltage.