Two Bunch Injection Issues Franz-Josef Decker 6-Dec-2004 Goals: 1.Two bunch injection will reduce the time to inject from scratch. 2.Two bunch injection might create the possibility to run the Linac at a lower rate a) 15 or 20 Hz will save some energy [~$ per day ?]. b) The lowest rate should not need to have heaters on most of the time, tests were planned for Oct-Dec Issues for Linac / Injector: 1.For e – most hardware is capable for two bunches like with SLC beams: DR kickers are flat, energy curve (PSK_HER) is rising, … 2.Issues with feedbacks, BPMs, stability, … has to be studied. 3.Positrons need more attention: PSK_Scav, Li19 kicker, SDR injection and extraction kicker. Issues for PEP-II: 1.BIC 2.Fast filling and slow feedbacks for faster IR change due to heating

First two-bunch pair at the end of the injection line 7-Nov-2003

General Issues 1.Shortest bunch separation: 16* (sub-harmonic buncher) and 6* (PEP-II) gives 3*5.6 ns = 16.8 ns (8 PEP-II buckets apart). 2. Fill PEP-II with one pattern [by2_2b] and then go to the normal trickle pattern [by2]; e.g.: by2_2b: 0:3400:16=1; 2:3400:16=1; 4:3400:16=1; 6:3400:16=1; 8:3400:16=0.01; 10:3400:16=0.01; 12:3400:16=0.01; 14:3400:16=0.01; 0.01 fools the BIC so it cannot complain about charge in the wrong bucket 3.Kicker set-up: The fast kickers (~60 ns) for the scavenger in Li19 and the positrons in the SDR have to be timed 8.4 ns later and raised by about 5%. The beams will then be on the raising or falling edge and more susceptible to timing jitter. (A longer term solution is to upgrade the hardware to NDR style kickers.) 3.PSK timing: Currently the HER and LER beams have a PSK timing of about 220 ns, which means they are injected into the Linac early, the RF will reach its peak 220 ns afterwards. This gives them about 6% less energy compared with the peak or the same energy as a beam with a BNS phase offset (20  ). The rising amplitude will compensate the beam-loading similar to E-158 running: 4.5E11 for 250 ns == 4.5E10 for 25 ns == 3.0E10 for 16 ns. So the charge per bunch should stay constant: 10% variations gives 0.05% in  E/E.

More General Issues 3.(More PSK): The scavenger (and FFTB) beam have currently no PSK timing offset, which means that the second bunch would have about 0.5% less energy. To compensate this effect there are two possible adjustments: a) lower the energy of the scavenger line 25  23 GeV, (reducing the positron production), and introducing some PSK offset (introducing some mismatch in early linac). b) Reduce BNS phase and raise PSK timing (increased wakefield-amplified jitter). 4.Positron production: This is probably the most not-understood issue. In the front-end at the gun and injector area we have special pulsed timing and pulsed phases to adjust for amplitude and phase loading. The positron capture section doesn’t have this and the question is how bad will it be? 0.5% energy loading corresponds to about 0.3  phase loading, which seems small enough. 5. Diagnostic and BPMs: The Nov-2003 test showed the beam already on the PEP-II injection timing scope before any sign of a second beam was seen in the front end. BPM response was not yet studied. This will impact how we setup and operate our steering feedbacks.