Managed by UT-Battelle for the Department of Energy SNS MEBT : Beam Dynamics, Diagnostics, Performance. Alexander Aleksandrov Oak Ridge National Laboratory

Managed by UT-Battellefor the Department of Energy

SNS MEBT : Beam Dynamics, Diagnostics, Performance.

Alexander Aleksandrov

Oak Ridge National Laboratory

2 Managed by UT-Battellefor the Department of Energy

Introduction: the SNS Project

Parameters:

P beam on target 1.44MW

I beam aver. 1.44mA

Beam energy 1 GeV

Duty factor 6%

Rep. rate 60Hz

Pulse width 1ms


Introduction: the SNS Front End Systems

IS/LEBT

RFQ

MEBT

65kV; 48mA; .2 π mm mrad

2.5MeV; 38mA; .21 π mm mrad

2.5MeV; 38mA; .27 π mm mrad

3.8m

3.6m

12 cm

No beam diagnostics

No beam diagnostics

2 BCMs6 BPM5 WS

Slit/collect

Designed and build by Berkeley N L

1ms

60Hz


MEBT functions

Matching beam from RFQ to DTL– 3 quads and 1 buncher would be sufficient

Place for chopper – Requires relatively long drifts

– Increases compexity: match RFQ to chopper – chopper – match to DTL

Place for diagnostics– Try utilizing free space as much as possible.


SNS chopper functions

Ion source transient

Chopper window

single mini-pulse

Providing gaps for ring extraction kicker rise time – 700ns ON, 300ns OFF pattern at ~1MHz

Macro-pulse shaping– cut off beam during the ion source transient

– create single mini-pulse for turn-by-turn measurements in the ring

– decimate mini-pulses (N-on-M-off ) to reduce beam current

– ramp up current at the beginning of the macro-pulse to help LLRF system in compensating beam loading effects


SNS Front End chopping system

time

MEBT chopper (10ns, 1e-4)

curr

ent

LEBT chopper

(50ns, 1e-2)

Chopper LEBT MEBT

Ion energy ~25 keV 2.5 MeV

= v/c ~ 0.0073 .073

Max Voltage 3 kV 2.5 kV

gap ~ 14 mm 18 mm

Effective length

~ 27 mm ~350 mm

Max deflection 14 o 1.07 o

Time of flight ~ 12 ns ~ 17 ns

Two stage: LEBT + MEBT


2.5MeV MEBT

Schematic MEBT Layout

RMS beam envelope in MEBT. Simulation (solid) and wire scanner measurements (dots).


SNS MEBT chopper/anti-chopper arrangement

Theoretically can eliminate partially chopped beam

Requires identical choppers, power supplies and anti-symmetric transverse optic


SNS MEBT Layout

3.7 m of 14 quadrupole magnets; 4 buncher cavities; chopper system; diagnostics

Design beam envelope in the MEBT


Front End Systems in the FE building


MEBT BPM and Beam Box


Wire Scanner and MEBT Beam Box


New diagnostics

Anti-chopper proved to be unnecessary

Additional diagnostics is added into MEBT “d-box” (former “anti-chopper” box):– In-line beam stop/Faraday cup

– Horizontal and vertical in-line emittance scanners

– Pencil beam apertures (2mm ,1.5mm, 1mm)

– Laser based longitudinal profile measurement (temporary decommissioned)

– View screen (never was used)

– In line Fast Faraday (tested prototype only)

– Several ports for future developments


Beam diagnostics box replaced the anti-chopper


MEBT D-box

Slits

Harps

LaserPorts


MEBT diagnostics scrapers (low power)

collimator position

collimator design

Manually actuated

Observe collimation effect on:

1. MEBT emiitance device

2. Linac wire scanners

3. Loss monitors


MEBT Performance

MEBT was commissioned with 6% duty factor beam

50mA peak current transport with no measurable losses demonstrated

Anti-chopper has been removed as unnecessary (based on simulations)

X-ray radiation from the rebunchers exceeds the expected levels – Established Radiation Area around the MEBT. No

access allowed during operation

RF amplifiers do not provide design power for Rebunchers #1 and #4– Considering new design based on IOT or tapping RF

power from the RFQ klystron (more in T. Hardek’s talk)

MEBT chopper doesn’t perform to specifications yet


How we tune MEBT

RFQ tuning

MEBT rebunchers tuning

MEBT quads tuning


RFQ tuning

No diagnostics to measure absolute transmission No diagnostics to know what is going on inside Can characterize resulting beam in the MEBT

422 RF cells, strongest space charge Defines longitudinal and ,in large part, transverse emittances

Only one parameter to set : RF amplitude– Fit output current vs. RF power curve to PARMTEQ model to find the set point

RF field amplitude [%]

Tra

nsm

issi

on

[%

]


Characterizing RFQ output beam using MEBT wire scanners

Measure transverse profiles using 5 wire scanners

Search for input Twiss parameters to best fit model to measured data

Repeat several times with different quad settings

Twiss parameters have been stable since 2003


MEBT tuning Set strengths of 14 quadrupole magnets

– Establishing proper beam profile for chopper operation– Matching beam to DTL– Use design quad values usually (~5% PS calibration

accuracy)– Verify with wire profile measurements

Set strengths of 6 hor. and 6 ver. dipole steerers – for minimum beam centroid offset at 6 BPM locations

Set amplitude and phase of 4 rebuncher cavities– Set amplitude to design or max available (~80% of nominal)– Set phase to non-accelerating bunching phase using phase

scan– Verified once in 2004 with laser longitudinal profile scanner


MEBT transmission is always good

Beam current after RFQ (red), MEBT (blue)


MEBT rebuncher phase scan


Ambiguities during the set up

MEBT rebunchers phase– Resulting set point depends on BPMs selected for

scan – (5º - 10º uncertainty)

BPM05

BPM10

Example of MEBT rebuncher scan producing different results with different BPMs


Beam envelope measurements

First wire scanner in the MEBT

Last wire scanner in the MEBT

Gaussian core + ‘tail’ dependent on MEBT tuning


MEBT to DTL transverse matching

Vertical Beam profile measured at DTL3 exit for different matching quadrupole settings

Approaches Gaussian shape when best match is achieved

FPAE057, TPAT030


Beam profile measurements after DTL tank3 for various Q13,Q14 settings


Measurement of MEBT chopper kick strength

Kick strength is measured by measuring beam deflection at chopper target location

Kick strength of the prototype is ~15% below design value

New design will meet design specs with ~15% margin


MEBT chopper efficiency

Simple strip line of solid copper (~16 ns TOF)


Vertical beam position along the pulse. Chopper is OFF

Vertical beam position along the pulse. Chopper is ON

Chopper OFF

Chopper ON

LEBT chopper performance (II)

Measured effective beam size increase due to large fraction of partially chopped beam. With 750 Ohm resistor.

Much better now with 50Ohm resistor


Beam emittance after MEBT. Vertical (top) and horizontal (bottom)

Measured emittance after MEBT

Dependance of RMS emittance upon peak beam current

Design value = .3 mm mrad(RMS normalized)


Emittance along the pulse

Emittance at the MEBT exit is not sensitive to beam current


mode-locked laser diagnostics for longitudinal profile measurement

Longitudinal bunch profile in MEBT

Longitudinal bunch profile (blue dots) and Gaussian fit (red line).

Bottom right: rms bunch length vs. the rebuncher phase

(measurements – squares, simulation – stars, solid line – quadratic fit).


MEBT Collimation Experiments

Scrapers

downstream BCM

upstream BCM30

25

Emittance scan without scraper Emittance scan with scraper


Transverse tails in DTL

horizontal a = 8.6 sigma

Asymmetric tail in horizontal plane

Clear effect of MEBT scraper

Do not see effect in vertical plane from horizontal scraping – weak coupling

DTL wire scanners usable to 10-3 level (~ 4 sigma)

Significant portion of transverse tail originate in Front End

vertical a = 16 sigma


Symmetric tails in both plane

No measurable effect of MEBT scraper

CCL wire scanners usable to 10-2 level (~ 3 sigma)

Significant portion of transverse tail in CCL does not originate in Front End

or wire scanner measurements not reliable yet

Transverse tails in CCL

horizontal a = 13 sigma

vertical a = 12 sigma


PARMILA vs measurements


Do we understand beam dynamics in our linac well enough?

good model requires– Accurate computation algorithm– Accurate representation of beam line elements (strength, position, edge

fields, etc. ) More complicated in longitudinal plane as element parameters (RF settings)

are not given by found from the model itself

– Reliable verification Good beam diagnostics tools

Good model should be able – To provide good agreement with measured beam parameters – To predict change of beam parameters in response to changes in beam

line elements– To provide above capabilities from end to end, not only piecewise – Accuracy of agreement should be well within RMS size

We use different codes for centroid motion and RMS simulations– Online Model, PARMILA, IMPACT


Accuracy of beam modeling in different parts of the linac

Transv.

centroid

Transv.

RMS

Long.

centroid

Long.

RMSRFQ ? not so good ? ?

MEBT good good not so good not so good

DTL good not so good ? ?

CCL very good not so good very good not so good

SCL not so good ? good not so good


Do we understand halo?

Expected in transverse plane– Much studied during design process

– Observe non-Gaussian transverse profiles Do not have diagnostics to measure halo extent below ~ 1%

– Does not seem to be a source of losses so far DTL serves as a collimator ?

– Do not have reliable information on losses inside DTL Probably responsible for some ring injection losses

Did not expect in longitudinal plane– Very low level

– Have only limited direct diagnostics tools ( CCL BSMs )

Problem of initial distribution for simulations– “reference” distribution used at design stage seems not

adequate

Documents

Managed by UT-Battelle for the Department of Energy SNS MEBT : Beam Dynamics, Diagnostics, Performance. Alexander Aleksandrov Oak Ridge National Laboratory