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Space Charge Neutralization

Space Charge Neutralization. Implications re pulsing or chopping the beam. TRIUMF 500 MeV cyclotron. 300 keV, < 1 mA injected beam (V sc < 1.2 volts) Space charge effect is small except near inflector We use all electrostatic optics: neutralization is very small.

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Space Charge Neutralization

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  1. Space Charge Neutralization Implications re pulsing or chopping the beam

  2. TRIUMF 500 MeV cyclotron • 300 keV, < 1 mA injected beam (Vsc < 1.2 volts) • Space charge effect is small except near inflector • We use all electrostatic optics: neutralization is very small. • This allows us to use a 1 kHz pulser. It has 3 functions: • For tuning at peak intensity without using dangerous average intensities. • To run at varying intensity without changing the tune. • To always have a 1 kHz component in the beam so that its intensity can be monitored (with a magnetic core transformer) online, for interlock protection against unexpected losses.

  3. CIAE 100 MeV cyclotron • Injected beam is 40 keV, few mA. (Vsc ~ 25 volts) • Neutralization is unavoidable and extremely beneficial: • Without it, it is very hard to keep beam focused. At 8 mA, a 4 mm diameter beam doubles in size with a drift of 12 cm. • With it, beam optics is insensitive to beam intensity. This helps for tuning up • But: need to make sure all components visible to beam are within about 0.25 volt of ground (to give 99% neut), so no electrostatic optics allowed at all. • And: Neutralization time is on order of 100 us at 1.e-6 Torr, so if pulser is used, shortest pulses must be long compared with this time. E.g. 1% duty needs < 10 Hz. For pulses >> 1 kHz, it is the same as turning neutralization off. • Chopping in MHz region cannot be done in injection line, except with a loss of about a factor of 100 avg. intensity.

  4. Effect of pulsing beam at 12 keV, 1 mA. Upper trace is current measured on aperture, lower is voltage applied to deflection plates. Horizontal is time, 50 us/div. From EPAC88, p.949

  5. An alternative is to design with no neutralization. This requires clearing electrodes all along injection line, so it is more convenient to simply use electrostatic optics. • Need quadrupole separation of about 10 cm to retain option of ~1 mA 100 MeV operation. • With ~2 mA from source, and large bunching effect, this is essentially like the Cyclone 30 design: 10 cm quad separation needed only near inflector. • With ~10 mA from source, don’t need bunching, but need very many quadrupoles. • With no neutralization, there is no cancellation of higher order space charge forces, so beam will not inject as cleanly. Not guaranteed to work at ~1 mA cw or 100 uA pulsed: may have to drop down by some factor.

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