Beam shaping using an ultra-high vacuum multileaf collimator and emittance exchange beamline N. Majernik1G. Andonian1 W. Lynn1 S. Kim2 C. Lorch1 R. Roussel3 S.

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Beam shaping using an ultra-high vacuum multileaf collimator and emittance

exchange beamline

N. Majernik1,∗G. Andonian1, W. Lynn1, S. Kim2, C. Lorch1, R. Roussel3, S.

Doran2, E. Wisniewski2, C. Whiteford2, P. Piot2,4, J. Power2, and J. B. Rosenzweig1

1University of California Los Angeles, Los Angeles, California 90095, USA

2Argonne National Laboratory, Lemont, Illinois 60439, USA

3SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA and

4Northern Illinois University, DeKalb, Illinois 60115, USA

(Dated: October 7, 2022)

We report the development of a multileaf collimator (MLC) for charged particle beams, based

on independently actuated tungsten strips which can selectively scatter unwanted particles. The

MLC is used in conjunction with an emittance exchange beamline to rapidly generate highly vari-

able longitudinal bunch proﬁles. The developed MLC consists of 40 independent leaves that are

2 mm wide and can move up to 10 mm, and operates in an ultra high vacuum environment, en-

abled by novel features such as magnetically coupled actuation. An experiment at the Argonne

Wakeﬁeld Accelerator, which previously used inﬂexible, laser-cut masks for beam shaping before an

emittance exchange beamline, was conducted to test functionality. The experiment demonstrated

myriad transverse mask silhouettes, as measured on a scintillator downstream of the MLC and

the corresponding longitudinal proﬁles after emittance exchange, as measured using a transverse

deﬂecting cavity. Rapidly changing between mask shapes enables expeditious execution of various

experiments without the downtime associated with traditional methods. The many degrees of free-

dom of the MLC can enable optimization of experimental ﬁgures of merit using feed-forward control

and advanced machine learning methods.

I. INTRODUCTION

One of the goals in modern accelerator physics is

the full control of particle beam distributions in multi-

dimensional space [1]. Many methods exist for trans-

verse phase space shaping, employing magnetic elements

or rigid collimators along the beamline, yet there are

fewer reliable options for longitudinal phase space tai-

loring. Designer longitudinal proﬁles of beam current

are important in many applications. For example, asym-

metric (ramped) beam proﬁles are critical for enhancing

eﬃciency in wakeﬁeld-driven acceleration concepts [2, 3]

while ramping the beam longitudinal proﬁle in the op-

posing sense is useful in mitigation of eﬀects stemming

from coherent synchrotron radiation [4]. Drive beam cur-

rent proﬁle tailoring is also consequential from the stand-

point of enhancing the ﬁnal energy output in free-electron

lasers [5].

In recent years, many methods for manipulating the

beam longitudinal proﬁle have been experimentally ex-

plored. Some of these methods introduce, then remove,

speciﬁc correlations in the beam 6D phase space. Such

beam shaping methods that have been experimentally

demonstrated include using higher-order multipole mag-

nets in a dispersive dogleg section [6], rigid masking

at high dispersion [7], dual high frequency RF mod-

ulations [8], and self-generated wakeﬁeld modulations

coupled with magnetic compression [9, 10]. In addi-

tion, direct laser shaping on the cathode has produced

∗NMajernik@g.ucla.edu

sources with controllable current proﬁles [11], while in-

verse free-electron laser interactions have demonstrated

bunch train generation at high repetition rates [12].

Finally, transverse-to-longitudinal emittance exchange

(EEX) methods have also successfully produced a vari-

ety of beam shapes by design through complex, multi-

dimensional phase space manipulations.

Speciﬁcally, transverse distribution masking combined

with EEX [13, 14] (See Figure 1) is a versatile option for

shaping the longitudinal proﬁles of high charge bunches

with a high degree of precision. In EEX, one of the trans-

verse phase-space planes of the beam is swapped with the

longitudinal phase plane. EEX is often accomplished by

placing a transverse deﬂecting cavity between two dogleg

transport sections [15, 16], although other beamline lay-

outs are possible [17]. The EEX approach allows for the

generation of high-charge bunches with current proﬁles

shaped with a precision that is diﬃcult to achieve using

other techniques [1].

The EEX beamline at the Argonne Wakeﬁeld Accelera-

tor Facility (AWA) has generated electron beams of many

diﬀerent longitudinal proﬁles [18], and recently used such

beams in the demonstration of high transformer ratios in

dielectric wakeﬁeld acceleration [19] and plasma wake-

ﬁeld acceleration [20]. The transformer ratio, the ratio

of the maximum accelerating ﬁeld to the maximum decel-

erating ﬁeld, R≡|W+/W−|, is limited to two for longi-

tudinally symmetric bunches [2]. However, using a drive

bunch that has an asymmetric current proﬁle – with a

ramp increasing in time followed by a sharp drop in cur-

rent – transformer ratios greater than two are achievable

and have been demonstrated [19–21].

arXiv:2210.02572v1 [physics.acc-ph] 5 Oct 2022

Gun, solenoid

magnets

Linac cavities

Quadrupole

magnets

MLC

YAG

B2 TDC1 B3

B4 ICT

TDC2

Spectrometer

YAG YAG

FIG. 1. AWA drive linac and EEX beamline (not to scale), adapted from [18] to include the MLC. There are six linac cavities

at the AWA, but two cavities were not used in this experimental demonstration.

(a)

(d)

(b)

(e)

(c)

(f)

FIG. 2. Simulated screen images with vertical and horizontal projections for three mask conﬁgurations. The top row (a-c)

shows the electron shadow of the mask shortly downstream of the mask while the bottom row (d-f ) shows the beam after the

EEX beamline and a transverse-deﬂecting cavity, revealing the longitudinal structure of the beam. The ﬁrst column shows a

smooth mask for a triangle current proﬁle with witness bunch, the second column is the MLC set to approximate the smooth

mask, and the ﬁnal column is a three driver bunch proﬁle with a witness.

At the AWA EEX beamline, a linear accelerator (linac)

consisting of four 1.3-GHz cavities to accelerate electron

bunches up to 43 MeV energy. A series of quadrupole

magnets are then used to control the beam transverse-

phase-space parameters at the location of the transverse

mask. After the beam is propagated through the mask,

it traverses the EEX beamline, which consists of four

dipoles and a transverse deﬂecting cavity (TDC) to en-

able a transformation which swaps the horizontal and

longitudinal phase spaces. The beam charge is monitored

in this experiment with an integrating current trans-

former, and the longitudinal phase space downstream of

the EEX beamline is characterized in a diagnostic line,

as illustrated in Figure 1.

The transverse masking has been previously accom-

plished using laser-cut tungsten masks and changing the

resultant longitudinal beam proﬁle required physically

changing the mask shape by installing newly cut masks

into the UHV beamline. This is a time-intensive pro-

cess which has prohibited quick iteration and dynami-

cal reﬁnement of the beam current proﬁle. In this pa-

per, we describe an apparatus that replaces the laser-

cut masks with a fully adjustable multi-leaf collimator

(MLC) [22, 23]. The MLC is a device with dozens of

independently actuated leaves which intercept the beam

trajectory to create a custom aperture [24–26]. MLCs

are common in radiotherapy applications to shape the

radiation beam to precisely match the shape of the tar-

get from any angle, thereby delivering an eﬀective dose

while reducing exposure to non-targeted regions. In the

context of an EEX beamline, the MLC enables rapid,

near-arbitrary control over the drive and witness current

proﬁles and spacing.

Start-to-end beam dynamics simulations are per-

formed, to compare beams produced by a practical MLC

and existing laser-cut masks, illustrating operational

equivalence between the approaches. The engineering

design, fabrication, and assembly of a forty leaf, ultra-

high vacuum compatible MLC are presented. This imple-

mentation includes a number of novel approaches which

enable unique new capabilities. Finally, an experiment

was conducted to shape and characterize various proﬁle

beams using the MLC and EEX beamline at the AWA.

The results of the MLC-based beam shaping are ana-

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Beamshapingusinganultra-highvacuummultileafcollimatorandemittanceexchangebeamlineN.Majernik1,G.Andonian1,W.Lynn1,S.Kim2,C.Lorch1,R.Roussel3,S.Doran2,E.Wisniewski2,C.Whiteford2,P.Piot2;4,J.Power2,andJ.B.Rosenzweig11UniversityofCaliforniaLosAngeles,LosAngeles,California90095,USA2ArgonneNationalLabora...

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Beam shaping using an ultra-high vacuum multileaf collimator and emittance exchange beamline N. Majernik1G. Andonian1 W. Lynn1 S. Kim2 C. Lorch1 R. Roussel3 S.

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