| Eric Prebys | |
| FNAL Beams Division |
| One of two electrostatic pre-accs accelerates H- ions to 750 keV. | ||
| The linac accelerates these ions to 400 MeV | ||
| The ions are injected over several (up to 12) turns into the booster, and passed through a foil to strip off the electrons. | ||
| The booster accelerates the protons from 400 MeV to 8 GeV. | ||
| The booster lattice is in an offset 15 Hz (line/4) resonant circuit. This sets an overall quantum of time for the whole accelerator (“tick”, “click”, “clink”). | ||
| Instantaneous 15 Hz rep rate routinely achieved. | ||
| Average rep rate limited by | ||
| Power dissipation of ramped elements | ||
| Above ground radiation (safety issues) | ||
| Below ground radiation (activation issues) | ||
| Some numbers: | ||
| Historical high*: 3E12 ppp @2.5 Hz (3E16 pph) | ||
| Run II needs: 5E12 ppp @.7 Hz (1.3E16 pph) | ||
| BooNE+Run II: 5E12 ppp @5.7 Hz (1E17 pph) | ||
| NuMI + Run II: 5E12 ppp @3.2 Hz (6E16 pph) | ||
| Linac chopper: 15 Hz | |||
| ORBUMP Magnets: 7.5 Hz (lots of work to go to 15Hz) | |||
| Booster RF: 7.5 Hz (Maybe go to 15 if we use existing cooling lines). | |||
| BEXBMP: 15 Hz | |||
| Extraction kickers: 15 Hz | |||
| MP02 extraction septum: 2.5 Hz (New PS -> 5 Hz, New magnet + PS -> 7.5Hz, + more cables -> 15 Hz. ~6/2002) | |||
| -> We currently take 7.5 Hz as a practical limit for BooNE and beyond. | |||
| Radiation Limitations | |||
| Above ground (want to avoid turning towers into controlled access area). | |||
| Shielding | |||
| Reduce beam losses | |||
| Below ground (must avoid making booster elements too hot to handle). | |||
| Reduce beam losses | |||
| The Booster uses multi-turn injection, resulting in a continuous beam around the ring. | |
| If beam is passing through the extraction septum while it is ramping, some of it will be steered into beam elements. | |
| This is the single largest source of radiation resulting from the booster. | |
| Solution: Early in the cycle, the old extraction kicker is pulsed, blowing a “notch” in the beam. Extraction is timed to coincide with the notch. | |
| Problem: although it’s a factor of 20 better to lose the beam early in the cycle, it’s still not negligible (more in a minute) |
Above Ground Rad. Trip Monitors (courtesy P. Kasper)
Integrated Dose Trips (factor 5 lower)
Bottom Line for Above Ground Radiation
| It looks like with a combination of shielding and careful beam handling, we should be able to keep above ground radiation to acceptable levels, even at BooNE+Run II intensities. |
| In order to reduce the uncontrolled beam loss when creating the notch, the idea is to excite a resonant instability and then scrape the beam away at a well-positioned collimator. | |
| Collimators have been commissioned, but unfortunately didn’t make it in time for this shutdown. | |
| Most of the prep work has been done, so installing the collimators should be able to be done in about a day, and will probably get done before BooNE. | |
Ramped Closed Orbit Corrections
| The main beam elements ramp with the momentum, but up until now, the corrector elements have been operated DC | ||
| Beam can “wander” by up to a few cm’s during ramp. | ||
| Ramping control cards were installed during the shutdown. | ||
| Closure control program almost ready. Still needs to be tested. | ||
| Correctors not powerful enough to steer the beam all the way through the cycle. Still, should help. | ||
| Injection Line diagnostics upgraded substantially during shutdown. | |
| Space charge studies underway. | |
| Transition revisited? |
| The main injector circumference is exactly 7 times the booster circumference, so there’s room for 7 booster batches. | |
| BUT, one slot must remain empty to allow the injection kicker to ramp down. -> max 6 booster batches/M.I. Cycle (unless we do slipstacking…) |
| Everything measured in 15 Hz “clicks” | |||
| Minimum M.I. Ramp = 22 clicks = 1.4 s | |||
| MiniBoone batches “don’t count”. | |||
| Cycle times of interest | |||
| Stack cycle: 1 inj + 22 MI ramp = 23 clicks = 1.5 s | |||
| NuMI cycle: 6 inj + 22 MI ramp = 28 clicks = 1.9 s | |||
| Full Slipstack cycle (total 11 batches): | |||
|
6 inject + 2 capture (6 -> 3) + 2 inject + 2 capture (2 -> 1) + 2 inject + 2 capture (2 -> 1) + 1 inject + 22 M.I. Ramp ---------------------- 39 clicks = 2.6 s |
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Timing: The One NuMI Specific Booster Problem
| In order to Reduce radiation, a “notch” is made in the beam early in the booster cycle. | |
| Currently, the extraction time is based on the counted number of revolutions (RF buckets) of the Booster. This ensures that the notch is in the right place. | |
| The actual time can vary by > 5 usec! | |
| This is not a problem if booster sets the timing, but it’s incompatible with multi-bunch running. | |
| We must be able to fix this total time so we can synchronize to the M.I. orbit. | |
| This is called “beam cogging”. |
| Detect slippage of notch relative to nominal and adjust radius of beam to compensate. |
| Early losses are extremely non-linear with number of turns of injection. | |
| Space charge effects are typically blamed, but the details are not well understood. | |
| A study group is working on this. | |
| Too late for BooNE, by maybe NuMI? | |
| Increasing the ppp would allow you to almost double your total protons before hitting the BooNE rad limits!! |
| Reasonable booster assumptions: | ||
| 5E12 ppp | ||
| Ave 7.5 Hz rep rate | ||
| Up to 1E17 pph w/o tripping rad alarm (BooNE needs) | ||
| Assuming BooNE runs as planned, the one NuMI-specific booster issue is that of beam cogging! | ||
| With those limits, NuMI can expect 2.7E20 p/year | ||
| Slipstacking (if it worked perfectly!) would increase this by about 30%. | ||
| Increasing the ppp could increase NuMI’s delivered protons. | ||