CERNISOLDE / HIE ISOLDE

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Isolde Target ModulatorPrototype development

prepared by T. GharsaTE-ABT-EC

CERN

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Why modulate the target at ISOLDE ?

• Precision high voltage (60kV +/-1V) has to be held at the target and ion source with respect to the grounded extraction electrode to provide the needed acceleration to the secondary ionized isotope beam.

• Ionization of air around the target during and immediately after beam impact discharges the effective target capacitance and reduces the target voltage level.

• To ensure extraction of short life-time isotopes this voltage is required to recover to its stable value within few milliseconds.

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Existing HV modulator

U (V)

upstream side

1ms

60kV

Modulation

t (s) ~40µs ~30µs

Target current recovery Plateau

~60µs 6ms

Tperiod=1.2s

Beam impact

• Presently a custom-built positive 60kV d.cpower supply (ASTEC) is connected via a pulse transformer and thyratron switch to a resonant circuit such that prior to beam impact the target is fully discharged by a resonant circuit in 35 ms, which then restore the voltage close to its nominal value within a further 200 ms [1].

Finally the load voltage returns to its nominal value +/- 1V within 5 to 6 ms still allowing the detection of very short life-time isotopes.

[1] D.C. Fiander, A. Fowler, in 20th Power Modulator Symposium, 1992, pp.173-176.0

During critical period of beam impact the target voltage is modulated to zero.

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Reasons for modulator upgrade

• Heavier dynamic loading generated from secondary particles are observed with new target materials and neutron converters.

• Because of the limited power output of the power supply the recovery time cannot always be respected.

Heavier dynamic loading is observed with new target material and neutron converter.

Recovery time for the new modulator is challenging :

- recovery 5ms (or less) even with neutron-converter targets

- recovery time as fast as technically achievable, with 2ms being of "substantial" benefit

THE CHARGE PUMP (CP) MODULATOR

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ADVANTAGES• Best recovery time even with dynamic load• Requires a single low current high precision

power supply (High power supply not necessary)

• A single HV switch is required (compared to C modulator which uses 3)

• HV power amplifiers can be easily stacked in parallel to increase current output.

DISADVANTAGES• Target is not discharged at impact

(risk of voltage breakdown)• takes time to recharge buffer

capacitor (~1s) between proton pulse

CP Modulator Prototype development

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• Custom built tank enclosure

• Tank is filled with dielectric oil

• Has a second ‘skin’ for oil leak protection

• Contains the buffer capacitor bank, HV precision divider, static and dynamic load

Tank housingCapacitor bank400nF 75kVdc (hivoltcapacitors)

75kVdc high precision dividerVD75-20Y-BDSC-KC-BEA/PB/F (Ross Engineering)

Viewing window

Essex HV connector

Oil temp gauge

Pressure sensor Oil filter

Filler cap

EAPPC2014: A charge-pump 60kV modulator for the ISOLDE target extraction voltage , R. A. Barlow1, B. Bleus1, A. Fowler*1, H. Gaudillet1, T. Gharsa1, J. Schipper1

1800mm

3D model Tank housing

High Precision 65kV -40mA power supply

Power amp. 1.4kV 100mA

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CP Modulator prototype development

1800 mm

Fig 2 : HV tank integration

The CP modulator HV tank is assembled in B867 HV cage JURA 5

High voltagepower supply

(HVPS)

Fig 4 : Storage capacitor & HV ROSS divider

Fig 3 : Behlke HV switches & Dynamic load

- Prototype HW completed as expected- Control HW and software under development (J. Schipper, R. Barlow)

BUILDING 867, Prevessin, CERN

CONCLUSION

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• Target ionisation is becoming increasingly severe hence the need for a new type of modulator.

• A charge pump type modulator appears to be the best solution for fast recovery

• Serviceability and repair should be simpler with the new modulator technology.

• Prototype functionality still to be validated with test loads.

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