1. Imperial College London 1 6. Injection into and ejection from circular machines PREACCELERATOR ACCUMULATORRING PARTICLESOURCE INJECTION 1 EJECTION INJECTION 2 SYNCHROTRON

2. Imperial College London 2 The main problem for beam injection : Liouville Injected beam Beam after first circulation Vaccum chamber d Deflection magnet (dipole) Injected beam Acceptance d At the entrance of the beam into the Vaccum chamber the distance a of the injected beam is larger than the aperture d of the Vaccuum chamber. Therefore the lower limit of the acceptance has to be: It is impossible to inject particles into an already occupied phase space volumewithout loosing the old or new ones:

3. Imperial College London 3 Longitudinal phase space stacking Free phase space volume Circulating beam zz Injected beam zz zz z zz The number of available phase space cells is then given by : The stacking of several beam bunches into the longitudinal phase space of the circular machine is typically used for protons and heavy ions. The kicker then has to produce rectangular pulses of  z length and a rise time much faster than the gap between the bunches in the ring.

4. Imperial College London 4 Transversal phase space stacking Injected beam Septum Kicker 1Kicker 2 Accumulated beam 2. Injection 3. Injection 4. Injection5. Injection6. Injection 1. Injection For short rings a longitudinal stacking is problematic and therefore a transversal stacking in phase space is used. This is possible because usually the beam emittance delivered by the accelerator is much smaller than the acceptance of the ring. A system of 3 dipole magnets (septum and kicker) is used together with the movement of the centre of gravity of the beam due to betatron oscillations to “paint” the phase space.

5. Imperial College London 5 Non Liouvillian stacking – charge exchange injection Circulating beam Stripping foil Deflecting dipole Injected beam Deflecting dipole To avoid the problems at injection point and to increase the availble current in the ring a charge exchange injection is used. Therefore Liouville is not valid and no longer a limiting factor. This scheme is usually used with proton drivers accelerating H - and stripping it to H + using foils or lasers.

6. Imperial College London 6 Electron stacking using radiation damping Injected beam Septum Accumulated beam Kicker 1 Kicker 2 Accumulated beam Acceptance 77 Injected beam xx  For light particles like electrons radiation damping can be used to avoid problems with Liouville. The setup is similar to normal transverse phase space stacking. With a kicker the accumulated beam is shifted in phase space in a way that the injected beam is within the acceptance. The acceptance is shifted back then and the injected beam is captured. Due to radiation damping the amplitude of the betatron oszillation is reduced and the new injected particles move towards the center of the acceptance.

7. Imperial College London 7 Fast dipole magnets (kicker) for beam injection Fast switch (Thyratron, MOS-FET) R Kicker dipole C I(t) U0U0 Beam To create the required pulsed (typ. 1  s) magnetic dipole fields a low inductivity dipole (no iron) is aroused by the discharge current of a capacity trough a fast switch. Typical values are B~0.05-0.1 T, I ~1-5 kA, L ~ 1-10  H, C ~ 10-100 nF, U ~10-50 kV

8. Imperial College London 8 Shielding of fringe field to reduce influence on circulating beam Injected beam Current bar Coil (DC&pulsed Coil (pulsed) Eddy current shield pulsed current Circulating beam To prevent or reduce the influence of the magnetic dipole field on the circulating beam two main techniques are used. One is the use of current bars at the edge of the dipole. The current intensity is chosen to reduce the fringe fields at orbit to zero. This is suitable for DC and pulsed operation. Often only a eddy current shield is used to reduce the influence of the fringe fields (pulsed mode operation only). Those shielded dipole magnets are then called septum.