update on beam measurements with the 200 ohm...

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


• 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


Measurement setup

• PIP2IT in MEBT-3.0 configuration

• Typical mode:

– 5 mA, 10 µs, 20 Hz


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


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


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


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


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


– 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


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


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


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)


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


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


• 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


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


update on beam measurements with the 200 ohm...

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