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Getting Beam to NuMI
(It’s a worry!)
Peter Kasper
The Fermilab Accelerator Components
• Preaccelerator
H- ions from 0 to 750 keV
• Linac
H- ions from 0.75 to 400 MeV
• Booster ( 474.2 m circumference )
Protons from 0.4 to 8 GeV
Linac beam is injected over multiple Booster turns
Magnets pulse resonantly at 15 Hz
• Main Injector ( 7 x 474.2 m circumference )
Protons from 8 to 120 GeV
Can be loaded with up to 6 Booster batches
Booster Injection System
ORBMP magnets
P+
• Booster
Booster intensity given by Linac current (50-55 mA), number of turns, and “acceleration efficiency”.
Typically 10-12 turns of 5.0-5.5e11 p/turn
Limit is determined by beam loss which is not linear w.r.t. number of turns
H-
Foil
P+
P+
P+
Ring magnet Ring magnet
Typical Injection System
• MI circumference is 7 Booster circumference
• MI intensity is determined by Booster Intensity
• Can fit only 6 Booster batches since a gap is needed to allow time for the injection kicker’s field to dissipate.
Slip stacking gets round this limitation
P+
P+
Ring magnet
Injection kicker
P+ Ring
magnet
Time Structure of Beam
• Time structure of the beam is defined by:-
Booster RF 37.8 MHz at injection 52.8 MHz at extraction
Booster beam energy = 0.994 at extraction
Booster circumference = 474.2 m
Booster batch = 474.2 52.8 / c = 84 bunches length = 84/52.8 = 1.6 sec
NuMI beam = 5 x Booster batches
NuMI Protons/Year with Run IIb
• Assume Booster can deliver 5e12 p/batch
• Assume MI accelerates 6 batches
1 batch is extracted to thep source
5 batches are extracted to NuMI
• NuMI intensity = 2.5e13 p/cycle
• Assume MI acceleration cycle is 22 Booster cycles
Total cycle time = ( 22 + 6 )/15 = 1.87 sec
• Assume 1 year = 2e7 seconds ( 63% up time )
NuMI gets 2.68e20 p/year
NuMI Protons/Year with CKM
• MI cycle time is increased by 1 sec due to time required for slow extraction to CKM
• Assume MI accelerates 6 batches
5 batches are extracted to NuMI
1 batch is extracted to CKM ( nop )
• NuMI intensity = 2.5e13 p/cycle
• Assume 1 year = 2e7 seconds ( 63% up time )
NuMI gets 1.74e20 p/year
CKM gets 0.35e20 p/year
Effect of Slip Stacking
• Assumed time line ( other scenarios possible )
Inject 6 Booster batches into MI
Use RF to vary velocities of individual batches
Allow batches to overlap then recapture Takes ~2 Booster cycles
Repeat process twice more to add 5 more batches.
• Intensity to NuMI = ( 11 - 2 ) 5e12 = 4.5e13
• Cycle time = ( 22 + 11 + 32 ) / 15 = 2.60 sec
• Gain = ( 4.5 / 2.5 ) ( 1.87 / 2.60 ) = 1.29
Demands on the Booster
• The Booster is the primary limit to NuMI’s intensity
• NuMI’s demands on the Booster
5e12 p/cycle
5.8e16 p/hr ( 7.5e16 p/hr with slip stacking )
4.3 Hz rep rate ( 5 Hz with slip stacking ) 2 conditioning cycles are needed prior to
each burst ( 15 Hz ) of beam cycles
• MiniBooNE: 5e12 p/cycle, 1.0e17 p/hr, and 7.2 Hz
• To date, only the per cycle requirement has been achieved.
Rep Rate Issues
• The main magnets cycle continuously at 15 Hz
• Other systems do not
Pulsed magnets and their power supplies etc. ORBMP ( injection magnets ) MP02 and MP02 ( extraction septa )
– Overheat at 2.5 Hz– Replacement MP02 magnet is currently under test
Kickers BEXBMP ( extraction magnets )
RF cavities and their power supplies etc.
Needed upgrades have been done or are in progress
Hourly Rate is Limited by Radiation
• It is permitted to accelerate up to 1.8e17 p/hr
BUT ...
Only if you can do so without tripping the safety system
The above ground areas around the Booster are protected by ~50 interlocked radiation detectors
• There are no well defined limits to the allowed activation within the tunnel ( 1 watt/m for SNS/ORNL )
BUT ...
Maintenance issues impose practical limits
Performance vs. Time
• Performance: Detector closest to its trip point
Normalized to trip its trip point
Scaled to an intensity of 1.2e16 p/hr (Run IIb)
Performance vs.. Cycle Intensity
• Surprisingly little if any dependency
• NuMI trip point would be at 0.2
• Green points represent best performance periods
Best Performance Periods
Best Performance Periods
• Worst detectors are protecting office space
• Offices can receive no more than 100 mr/yr
• Year averages is 5 < hourly limit
Problem Areas: West Towers
• Best performance is still 8 higher than allowed at NuMI intensities ( 1.5 trip point )
Contain office areas
Located above extraction region
• Situation has greatly improved over past several years
Steel shielding added above the extraction region
Beam notched (see later) to avoid extraction losses
• To do :-
Minor shielding upgrade factor 6
Collimator system to relocate losses
The Notch
• Create a gap (notch) in the beam ( 4-5 RF buckets ) at 400 MeV ( low energy less radiation )
• Fire extraction kickers so that the current rise time coincides with the gap being inside the magnets
• Avoids intolerable high energy losses on the extraction septum
• Good for MiniBooNE or single batch injection into MI
Booster uses notch position to set the timing
• Unsolved timing problems exist for multi-batch injection
MI sets timing based on location of 1st batch
Problem Areas: East Towers
• Best performance is 5.6 higher than allowed at NuMI intensities ( just over trip point )
Contain office areas
Losses are dominated by the creation of the notch
• To do :-
Minor shielding upgrade factor 6
Use “pinger” to create notch over multiple turns Works like resonant extraction Needs R&D to make efficient at high intensity
Collimator system to relocate losses
Activation in the Tunnel
• Have started systematically monitoring activation levels throughout the tunnel
• Several 100-200 mr/hr @ 1 ft locations have been noted
• Will become 10 worse at NuMI intensities!
No focussed plan as yet
Requires controlling actual losses E.g. collimator system
Argues against extreme measures to solve above ground problems
Conclusion
• Most Booster problems will (hopefully) be solved before NuMI runs
MiniBooNE sets the most stringent limits
• Some are unique to NuMI
Notch timing issues
• NuMI’s requests cannot be met without raising the cycle intensity
7.5e12 ( 6.3e12 with slip stacking ) gives 4e20 p/yr
This will require some R&D on space charge issues
• Assistance will be greatly appreciated!