Transcript SJ_FETS_RFQInputDist_22-08-12

RFQ GPT Input Beam
Distributions
Simon Jolly
22nd August 2012
RFQ Input Beam Matching
• At the last FETS meeting, I showed the results
of taking the matched Twiss parameters at the
end of cell 3 and tracking them upstream to the
RFQ input datum (upstream face of entrance
flange).
• Lots of variation in input acceptance due to RF
phase variation.
• Alan went back and generated matched
parameters at the end of cell 3 for 32 different
phases, covering 2 pi of phase in regular steps.
• I then tracked all of these back upstream at
60 mA, using 2D waterbag distribution and
2Dline spacecharge, to find matched Twiss
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RFQ Input Datum
• A number of small changes to
RFQ entrance region:
Design
Matcher
End of cell 3
(matched
parameters)
– Rounding off of matching section
nose makes radial matcher
shorter.
– Keeping distance from entrance
flange to start of radial matcher
the same means distance to end
of radial now shorter.
• The important numbers:
Real Matcher
RFQ Input
Datum
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– RFQ input datum is outer face of
main end flange (not insert).
– RFQ input datum is 21 mm from
design RFQ input at start of
design radial matcher (14 mm
thick, 7 mm gap).
– RFQ input datum is 42.7698 mm
from end of radial matcher.
– Nothing downstream of this point
has changed…
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Horizontal Acceptance (Old)
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Horizontal Acceptance (New)
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Vertical Acceptance (Old)
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Vertical Acceptance (New)
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Emittance Variation (Old)
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Emittance Variation (New)
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Twiss Parameters: Beta-X (Old)
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Twiss Parameters: Beta-X (New)
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Twiss Parameters: Beta-Y (Old)
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Twiss Parameters: Beta-Y (New)
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Twiss Parameters: Alpha-X (Old)
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Twiss Parameters: Alpha-X (New)
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Twiss Parameters: Alpha-Y (Old)
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Twiss Parameters: Alpha-Y (New)
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Matched Parameters: Results
• We now have matched
Twiss parameters at the
RFQ input datum!
 ax = 3.8263; ay = 3.4091.
 bx = 0.15996;
by = 0.14152.
• All phases very similar:
much less variation than
previously.
• Alpha’s and Beta’s now
converge around a
single value.
• Having sorted the
transverse, it’s time to fix
the longitudinal…
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RFQ Input Beam Distribution
• Since my very first RFQ simulations in GPT, I have been using the
same method of starting the beam in GPT:
– 2D distribution generated using either:
• Particle distribution from input file (Alan).
• setWBemittance function with matched emittances/Twiss parameters.
– 2D “stretched” into 3D by using a GPT setZdist statement:
• Beam has same transverse dimensions along its length.
• Beam looks cylindrical in 3D.
• This distribution is good for the spacecharge simulation – ghost
bunches appear the same in front/behind – but BAD for getting the
transverse parameters right!
• Cylindrical beam has correct Twiss parameters only at the start, but
back changes dimensions depending on emittance.
• However, no alternative proposed for previous simulations.
• Need to make sure we’re getting the right input beam since RFQ
transmission seems to be quite sensitive to input conditions.
• Needed to explore 3 different input methods…
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1: setZdist
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•
•
•
Beam started using GPT “setZdist”
statement.
All beam has same initial
dimensions.
No discontinuities with space
charge.
Beam
Wrong transverse dimensions:
generated
beam arrives in “cone” from last
backward
solenoid.
s from
Beam too dense by the time rear of
input
bunch enters RFQ.
RFQ start
Beam shrinks
from emittance
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RFQ start
Beam
Direction
Space charge
is always right
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2: setTdist
• Beam started using GPT
“setTdist” statement.
• Beam always has correct
initial dimensions at input
datum.
• Space charge sees
discontinuities.
• Beam created in steps:
strange space charge
effects.RFQ start
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RFQ start
Beam
generated
forwards
from input
Beam
Direction
Space charge has
discontinuities for first few
periods
Space charge very
strange during beam
creation
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3: setFile
RFQ start
• Beam started using GPT
“setFile” statement.
– Create 2D beam distribution
using matched parameters.
– Track backwards using
spacecharge2Dline to get
space charge right, creating
many 2D slices with screens.
– Output beam distribution to
Matlab.
– Interpolate (x,y,Bx,By,Bz) data
from multiple screen data at
random z-positions.
– Write GDF file to re-input
data.
Beam
loaded in
single
step
Beam
Direction
Space charge has
discontinuities for first few
periods
• Beam always has correct
initial dimensions at input
datum.
• Space charge sees
discontinuities.
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Input Simulations: Initial Results
• Ran usual simulations to check differences between
input method:
– Transverse waterbag distribution using matched Twiss
parameters.
– Beam tracked to RFQ output datum to measure
transmission.
• Very odd results: >97% 3 MeV transmission for setZdist
and setTdist but only ~88% for setFile:
– Results should be very similar!
– Most realistic is “setFile”, so why 10% transmission
loss…?
• Had to go on a 2 week detour to find the errors:
– Extract cylindrical beam from “setZdist” and reintroduce
using “setFile”: results should be identical.
– Create my own conical beam distribution (original was
from Juergen) to make sure the problem wasn’t with the
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beam distribution
itself.
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Results: Transmission
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Results: 60 mA Transmission
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Results: 60 mA Exit Emittance
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Results: 60 mA Peak Power Loss
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SetZdist: Input Beam
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SetZdist+setFile: Input Beam
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SetTdist: Input Beam
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SetFileSJ1: Input Beam
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SetFile: Input Beam
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SetFileSJ1: Input Beam
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SetTdist: Input Beam
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Conclusions
• Input distributions results finally make sense:
– setZdist gives slightly worse transmission than
setFile, which is slightly worse than setTdist.
– Methods are virtually identical at 60 mA.
– Differences in transmission a result of incorrect
beam from Juergen: beta right, alpha wrong
(something he has confirmed).
• After an exhaustive effort, RFQ simulation
parameters are now fixed:
– Beam started at RFQ input datum.
– Transverse waterbag from matched Twiss
parameters.
– Longitudinal distribution from setTdist.
• Now for the acceptance…
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Paper 1: RFQ Integrated Design
• Paper will cover modelling background for our integrated RFQ
design method.
• This is mainly RFQSIM -> Inventor -> Comsol -> GPT -> Matlab,
but also includes sections on bulk CAD design and
electromagnetic/thermal simulations.
• Half written: just waiting for other people to fill in some sections:
– Introduction
– *Vane Modulation Parameter Generation (APL – RFQSIM)
– *RFQ Mechanical Design (PJS)
– Vane Tip Modulation CAD Design (SJ)
– *Electromagnetic Cavity Simulations (SL)
– *Thermal Modelling (SL)
– Beam Dynamics Simulations (SJ)
• Field Mapping (SJ - Comsol)
• Particle Tracking in GPT (SJ)
– Conclusions (SJ)
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Paper 2: FETS RFQ Design
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Paper will cover all steps we went through to design FETS RFQ.
Will refer to previous integrated design paper, so no need to describe methods
again, but needs to include all information showing how much work we’ve done
on the various aspects of the design.
I will take as much as I can from the conference papers, but will need help
filling in gaps as there are several things that have been presented at FETS
meetings I couldn’t find in PAC/EPAC papers.
Outline will be similar:
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Initial parameter generation and design limitations (APL + RF/klystron)
Basic CAD design (PJS)
Cold model construction and bead pull (SJ/PJS)
Electromagnetic cavity simulations (SL)
Thermal simulations and squirt nozzle/cooling design (SL/PJS)
Vane tip CAD modelling (SJ)
Beam dynamics simulations, inc RFQSIM/CAD modelling comparison (SJ)
Final CAD design, including tuner design, RF feedthroughs etc and final RFQ
parameter comparison (SJ/PJS/APL)
– Anything else…
•
As Juergen suggested, this paper should include everything but also refer to
conference papers…
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Paper 3: Fringe Fields/Tolerances
• Paper will cover all the “edge effects” that have
come largely from the CAD modelling.
• Try to show how really starts to interfere on
some of the “optimised” areas of the RFQ
design.
• Juergen’s work on the effect on the beam
energy spread from the matching section fringe
field: I will run some simulations (suggestions
please…).
• All the simulations I’ve done recently checking
the alignment and machining tolerances.
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