Principle Reducing energy spread –Thermalization ions loose energy through interaction with a gas –Confinement by electric field Optimization through the Mathieu equation : mass Inner radius Angular frequency of RF voltage Amplitude on RF voltage Particle charge Condition for stability: q < 0.908 Optimum ISOLDE: q = 0.6
Ion motions Ions are interacting with a gas. Through the collisions they loose energy and change direction. Buffer gas
Ion motions Adding a oscillating RF field confines the ions in the gas, along the axis of the quadrupole. V RF ~
Continuous mode 1.Ions are interacting with a gas. 2.Adding a oscillating RF field confines the ions in the gas. 3.The electrodes have all a potential to drag the ions through the RFQ. V RF ~ 80eV 5-2V/cm ~1E-2 mbar 4 He 10 -2 mbar l/s EXTRACTION INJECTION
Bunching mode 1.Ions are interacting with a gas. 2.Adding a oscillating RF field confines the ions in the gas. 3.Pulsing the last electrodes (HRS.AX(23,24,25)(A/B)) V RF ~ 80eV 5-2V/cm ~1E-2 mbar 4 He 10 -2 mbar l/s EXTRACTION INJECTION Trapping 50V
Summary Three elements: RF quadrupolar field Radial confinement DC potentials Extracting ion in bunches or in continuos mode Buffer gas Ion motion cooling
Results Transmission: –50 % A>23 –80% for A>40. Space charge limit of 1E8 ions/bunch. –The maximum limit of ions in the RFQCB before the ions are lost. Cooling time < 1msec –At ISOLDE short lived isotopes with half-lives of a few ms are explored. Bunch width ~ 30 µsec –For the time window of the experiments.
Why do ISOLDE need an ion cooler and buncher? Large beam emittance –Low transmission to experiments –Low efficiency in mass spectrometers ~35 ·mm·mrad 95% emittance
Why do ISOLDE need an ion cooler? Small beam emittance –High transmission to experiments. During tests we had 100 % transmission between the RFQCB and LA2 beam line! –High efficiency in mass spectrometers Mistral ISOLTRAP –Small beam spot Decay/collection experiments. ~2 ·mm·mrad 95% emittance
Why do ISOLDE need an ion buncher? Finite time definition of beam pulses –Gating on the beam pulse background suppression. –Decay experiment T 1/2 Charge particle decay spectran Background suppression factor~1E4 –COLLAPS (less laser power) Singles (continuous counting) Bunched:12µs gated spectra
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