ess mebt layout & chopper rise time - from the point of...
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ESS MEBT Layout & Chopper Rise Time- From the Point of View of Beam Physics -
Ryoichi Miyamoto (ESS/AD/BPG)On behalf of the ESS/ESS-Bilbao MEBT team
Jan 26th, 2015MEBT Chopper Workshop
Outline
● Layout(s)● Chopper rise/fall time● Conclusions/Questions
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2 layouts: chopper & quad integrated vs separated
● ESS MEBT lattice structure:
(Triplet) + (Chopper+Dump) + (Triplet) + (Diagnostics Box) + (Quadruplet)● The chopper quad was originally considered to surround the chopper
(like LINAC4) but this has been questioned.● Two configurations are tried and the difference is minor from the point of
view of beam physics, as seen in the following slides.
Integrated (2014.v2) Separated (2015.v0)
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Summary of two layouts
● Integrated– Chopper specs:
● Voltage: ≥ 4.5 kV● Plate length: 500 mm● Plate gap: 20 mm● Rise/fall time: ~10 ns
– Tuning of the chopper section slightly harder.
– Beam quality● Transverse: roughly the same● Longitudinal: slightly worse
– Slightly higher voltage and shorter plate better and the above situation may change. (Good Tunability and beam quality if 7-8 kV is achieved.)
● Separated– Chopper specs:
● Voltage: ≥ 4.0 kV● Plate length: 500 mm● Plate gap: 20 mm● Rise/fall time: ~10 ns
– Tuning of the chopper section slightly easier.
– Beam quality:● Transverse: roughly the same● Longitudinal: slightly better
– Slightly higher voltage and shorter plate better and the above situation may change.
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Comparison of two layouts: distributions (MEBT out)
● The transverse better for the “Integral” and the longitudinal better for the “Separated”.
Integrated Separated
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Comparison of two layouts: distributions (DTL out)
● Through the DTL, the transverse become roughly the same. The longitudinal is still better for the “Separated”.
Integrated Separated
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Comparison of two layouts: emittance and haloIntegrated Separated
(Matching not fine-tuned for these layouts yet.)
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Rise/fall time and partially-chopped bunches
● Partially-chopped bunches are of concern for beam losses and a major factor to determine the rise/fall time.
● No clear threshold in terms of the beam losses and it seems rather a political decision of how much we are allowed to use the 1 W/m.
● SNS and JPARC are having no issue.
4.2 kV
1.5 kV
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Losses for various partial voltages
● Losses mainly in DTL and almost none in the SC sections (even with errors).● The bad voltage range is ~1-2.5 kV.● Rise/fall time definition with 10-90% seems good enough. (Certainly 5-95%.) ● Lattice errors enhances the (confidence level) losses by roughly an order or magnitude.● Practically no loss with the 2nd and 3rd scrapers. (Should think the pessimistic case?)
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Total losses during 10, 20, and 30 ns rises
● Linear 10, 20, and 30 ns rises are considered.● 10 ns → ~1.1 kV per RF cycle → 1 bunch in the bad range of 1-2.5 kV. ● Some differences between 10 and 20 ns but 20 and 30 ns are quite similar.● Should be verified by including the errors.● Hard to draw the threshold. (Do we care the peak or total ??)
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Conclusions/Questions● Chopper parameters wanted by a beam physicist
– Voltage: ≥ 4 kV– Plate length: ≤ 500 mm– Plate gap: 20 mm– Rise/fall time: ~10 ns (20-30 ns OK ??) for 5-95% (10-90% seems OK)
● Two layouts are compared and for an around 4 kV voltage range the separated chopper and quad configuration seems better (although the difference isn't major).
● Which is the recommended technology choice for our MEBT chopper (and quad) to satisfy above parameters in terms of– Feasibility– Reliability– Design effort– Cost– …
● Is there anything else to do on the beam physics side?– Compromise between the voltage and rise/fall time?