update on beam measurements with the 200 ohm...
TRANSCRIPT
Update on beam measurements with
the 200 Ohm kicker
A. Shemyakin for PIP2IT team
PIP-II Technical meeting
31 October 2017
Outline
• Introduction
• Measurements setup
• Emittance measurements
• Long run
• Summary
10/31/2017A.Shemyakin| | Measurements with kicker2
Introduction
• PIP-II needs two wide-band,
travelling wave kickers in MEBT
– To form complicated bunch
structure for Booster injection and
multi-user operation
• In summer 2017, operation of 200
Ohm kicker with beam started
10/31/2017A.Shemyakin| | Measurements with kicker3
• Reported on July 25, 2017 meeting:
– Deflection efficiency is within
specification
– Kicker allows to form an arbitrary
bunch pattern for 10 µs pulses
Nominal beam envelope in MEBT for
passed and deflected bunches. A. Saini.
3D model of 200 Ohm kicker. A. Chen
Further questions
• Can the kicker driver fulfill the CDR requirements?
– Yes (Daniil/Greg’s report)
• How large will be the emittance growth of passed bunches?
Is it sensitive to phasing?
– These measurements give estimates
• Can the kicker survive operation with long pulses or CW?
– Partial answer came from a long run
• Remain to answer:
– Sensitivity to the bunch length
– Sensitivity to H- velocity
10/31/2017A.Shemyakin| | Measurements with kicker4
Measurement setup
• PIP2IT in MEBT-3.0 configuration
• Typical mode:
– 5 mA, 10 µs, 20 Hz
10/31/2017A.Shemyakin| | Measurements with kicker5
200 Ohm
kicker
6σ beam envelope in MEBT.
Settings #1527; 5 mA.
TraceWin. A. Saini.
Two trajectories
• The beam is deflected upstream of the 200 Ohm kicker with
dipole correctors to use the kicker aperture in optimal way
– Beam reaches the dump with low losses in both cases, with
the kicker off and on
10/31/2017A.Shemyakin| | Measurements with kicker6
Kicker off
Kicker on
Beam trajectory with the
200 Ohm kicker off and on.
OptiMX.
Beam deflection was
analyzed at the BPM with a
scope. Separation of
envelopes was analyzed
with Allison Scanner and a
“F” scraper
BPM M50B
AS F scraper
Vertical emittance growth
• Particles in different locations along the bunch may see
different transverse deflection. Possible reasons:
– Changing kicker voltage at the location of passing bunches
– Slippage of the beam with respect to the pulse
• Difference between H- speed and kicker phase velocity
10/31/2017A.Shemyakin| | Measurements with kicker7
Kicker waveform at 81.25 MHz pulsing.
Vertical axis shows voltage between plates as
measured at loads downstream of the kicker.
20-Sep-2017. D. Frolov.
Ellipses approximately indicate bunches’ 6σ
length and optimum location.
Vertical emittance growth (cont.)
• Vertical emittance grows as
– where is the deflection at the longitudinal offset z averaged
along the path through the kicker and brackets mean averaging
over all particles in the bunch
• An “effective” emittance may grow without growth in 6D
emittance (linear kick) or even in 2D emittance of an
individual bunch (constant offset along the bunch)
– The beam line is optimized for the kicker off
– The offset depends on the kicker pulse pattern
10/31/2017A.Shemyakin| | Measurements with kicker8
2 2 2 21 0 ( )y z
( )z
Difficulty with emittance analysis: too low separation
• The existing program
doesn’t calculate accurately
the emittance if separation
is not large enough
– phase areas overlap
• e.g. over ±3σ
– With one kicker, space
separation is 3.5 - 4.5 σy
• At kicker voltage of 550V
10/31/2017A.Shemyakin| | Measurements with kicker9
Phase portrait with the kicker pulsing at
81.25 MHz. 25-Sep-2017.
• For analysis, the kicker was pulsing at 81.25 MHz, deflecting
every other bunch, and phase portrait were analyzed with the
Allison scanner
“Sheet” beam
• Solution: scrape the beam vertically with 4 upstream scrapers
to decrease the vertical emittance
– To increase separation in terms of σy
– Scraping ~0.9 mA at each scraper to 1.5 mA of beam current
10/31/2017A.Shemyakin| | Measurements with kicker10
– Horizontal emittance stays roughly
the same
– Vertical emittance decreases by 5
times to ~0.04 µm
– Bunch separation is ~7.5 σy
– It allows for independent
measurement of passed and
deflected bunchesVertical F-scraper scan with the kicker pulsing at 81.25 MHz. 20-Sep-2017. The green curve
is a fit with two Gaussian profiles separated by 7.5 σy. There ~4 mm region with constant current between scraping of two trajectories.
Emittance measurement for each trajectory
• For well-separated phase portraits, emittance of both
deflected and passed bunches can be measured accurately
– By making scans over two separate areas
– A MathCad program to analyze accurately
10/31/2017A.Shemyakin| | Measurements with kicker11
Phase portrait with the kicker
pulsing at 81.25 MHz. Scraping a
5.1 mA beam to a “sheet beam”
with 1.5 mA. 20-Sep-2017.
Passed
Deflected
Emittance dilution measurements: phase dependance
• Emittance of passed bunches is equal to one with the kicker
off within measurement uncertainties (~3%)
– No measurable dependence on the kicker phase within ±0.5 ns
• Emittance of deflected bunches is measured to be slightly
higher
10/31/2017A.Shemyakin| | Measurements with kicker12
Comparison of emittances of passed and
deflected bunches with the case of the
kicker off for 3 different kicker phases
with respect to the beam.
The kicker pulsing at 81.25 MHz. Sheet
beam. 10-Oct-2017.
5%
Phase of maximum deflection
81.25 MHz vs a step pulsing
• Deflecting every other bunch is the most challenging mode
for the kicker to stabilize the voltage after switching
• If the kicker pulse is a step in the middle of the beam pulse,
the time-dependent effects should be negligible
– About half of beam is passed and another half is deflected
• Observed differences are likely related to Allison Scanner
peculiarities
10/31/2017A.Shemyakin| | Measurements with kicker13
Comparison of cases with the kicker off,
pulsing at 81.25 MHz, and with a step
pulse. Sheet beam. 10-Oct-2017.
Perturbation of passing bunches
• The deflection of passing bunches results in an increase of
effective emittance since it depends on the kicking pattern
– Can be accurately measured with the sheet beam by removing
the deflected bunches and tracing the passed bunches with
BPMs
• 0.3 mrad for 81.25 MHz pattern (negligible for step pattern)
10/31/2017A.Shemyakin| | Measurements with kicker14
Measurements
Comparison of trajectory of passing
bunches in BPMs with OptiMX. 81.25
MHz; sheet beam. 13-Oct-2017. A. Saini.
– Vs 7.7 mrad for
deflected bunches
– For the un-scraped
beam, would give 2%
emittance increase
Future work for kicker characterization
• How sensitive the phase portraits to the bunch length?
– In part, generation of transverse tails
• How sensitive the phase portraits to the H- energy?
– Testing the alignment between the kicker phase velocity and H-
ions speed
10/31/2017A.Shemyakin| | Measurements with kicker15
Notes on kicker protection from the beam
• The main tool was planned to be kicker protection electrodes
– Copper plates on both sides of each kicker with slits of 13 mm
in Y (vs 16 mm gap between kicker electrodes); +100V bias
– Provide reasonable readings with kicker off, but so far the RF
noise with the kicker pulsing is large (~0.5 mA equivalent)
– Used comparison of beam currents at the entrance and exit of
MEBT for long-pulse operation
10/31/2017A.Shemyakin| | Measurements with kicker16
• Presently good to protect from
~5% accidental loss
• See ~20 µA current loss to the
electrodes (at 10 mA operation)
not removable with steering
– Not tolerable by the present design
in CW if created by 2 MeV ions Protection electrodes
Long run with kicker pulsing at CDR parameters
– Was run by MCR
– 5 mA x 0.55 ms x
20 Hz
– 24 hours total
– 2 interruptions, 1
hour total (96%
availability)
– No problems with
the kicker
10/31/2017A.Shemyakin| | Measurements with kicker17
B50BY
dump
DCCT
28 hrs
B3
vacuum Current comparison
AS vacuum
RFQ
0.5 mA/div for currents;
2%/div for current
comparison; 2.E-8 Torr/div
for vacuum; 1 mm/div for
BPM.
Summary
• The 200 Ohm kicker satisfies the CDR requirements
– The deflection amplitude should be enough for 6-sigma
separation in the two-kicker scheme
– Emittance growth for passed bunches is low
• Including the case with the phase shifted by ±0.5 ns
– Kicker survived a long run with a 100W beam and pulsing
• Time to start updating the kicker mechanical design
– Correct issues found at assembly
– Improve the design of protection electrodes
10/31/2017A.Shemyakin| | Measurements with kicker18