# Presentation: 041313

## Booster Issues for NuMI

*Eric Prebys*
- FNAL Beams Division

## The Basics

- 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)

- *MR fixed target, back when life was cheap

- Factor 8!

## Ramped element limits

- 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 Issues

- 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 “Notch”

- 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)

- Notch Off!

- Run II Needs

- MiniBoone+RunII

- Almost there!

- NuMI+RunII

## Integrated Dose Trips (factor 5 lower)

- Extraction

- Notch!!

- Shielding currently being installed in critical areas (first floor of East and West Booster Towers)

- BooNE

- NuMI

## 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.

## Collimators

- 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.

## Better Beam Characterization

- Injection Line diagnostics upgraded substantially during shutdown.
- Space charge studies underway.
- Transition revisited?

## Loading The Main Injector

- 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...)

## Proton Timelines

- 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

## Summary of Proton Ecomomics

- Booster Hardware Issues

- Radiation Issues

- NUMI “baseline” = 13.4E12 pps x 2E7 s/year  2.7E20 p/year

- *assuming 5E12 protons per batch

## 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”.

## Active cogging

- Detect slippage of notch relative to nominal and adjust radius of beam to compensate.

- Allow to slip by integer turns, maintaining the same total time.

- Does not currently work at high intensities.
- Still do not really understand the problem.  Suspect TCLK drift relative to magnet phase, but no correlation seen.

## > 5E12 ppp ????

- 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!!*

## Summary

- 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.