Demonstration of Cathode Emittance Dominated High Bunch Charge Beams in a DC gun-based Photoinjector

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by  Colwyn Gulliford

Demonstration of Cathode Emittance Dominated High Bunch Charge Beams in a DC
gun-based Photoinjector
Colwyn Gulliford,∗ Adam Bartnik,† Ivan Bazarov, Bruce Dunham, and Luca Cultrera
CLASSE, Cornell University
(Dated: January 19, 2015)
We present the results of transverse emittance and longitudinal current profile measurements of
highbunchcharge(≥100pC)beamsproducedintheDCgun-basedCornellEnergyRecoveryLinac
Photoinjector. Inparticular,weshowthatthecathodethermalandcorebeamemittancesdominate
the final 95% and core emittance measured at 9-9.5 MeV. Additionally, we demonstrate excellent
agreement between optimized 3D space charge simulations and measurement, and show that the
quality of the transverse laser distribution limits the optimal simulated and measured emittances.
5
These results, previously thought achievable only with RF guns, demonstrate that DC gun based
1
photoinjectorsarecapableofdeliveringbeamswithsufficientsinglebunchchargeandbeamquality
0
suitableformanycurrentandnextgenerationacceleratorprojectssuchasEnergyRecoveryLinacs
2
(ERLs) and Free Electron Lasers (FELs).

n
a
J Linear electron accelerators boast a wide range of cur- countered in this work (1017-1018 e/m3), classical rel-

6 rent and planned applications in the physical sciences. ativistic Hamiltonian mechanics, with a self-interaction
1
Examples include: x-ray sources [1–3], electron-ion cool- describedbyaspacechargepotential,sufficientlyapprox-

ers [4], Ultra-fast Electron Diffraction (UED) experi- imates the single bunch beam dynamics [12, 13]. In this
]
h ments[5–8],andfixed-targetnuclearphysicsexperiments model, the Hamiltonian for each bunch separates into
p [9]. Akeyfeatureofmanyoftheseapplicationsisthepo- a sum over N =q/e Hamiltonians of the same form, re-
-
c tential to produce beams where the initial beam quality, ducingthe6N-Densemblephasespacevolumeconserved
c setbythesource,dominatesthefinalbeamqualityatthe in Liouville’s theorem to the 6D phase space of a single
a
usage point. This has lead to the design of a next gener- bunch. In the absence of coupling between each of the
.
s ation of machines, such as high energy Energy Recovery 2D canonical phase spaces (x , P ), the conserved 6D
c i xi
i Linacs (ERLs) [2], and Free Electron Lasers (FELs) [3] phase space volume separates into three conserved 2D
s
which could provide diffraction limited hard x-rays with volumes. From a physics stand point, these three con-
y
h orders of magnitude brighter beams than modern stor- served quantities represent the most fundamental defini-
p age rings. The successful design and implementation of tion of 2D emittance. However, as a figure of merit, this
[ such machines has the potential to impact an impres- definition fails to capture the effect of distortions of the

1 sively broad range of research in physics, chemistry, bi- phase space due to non-linear fields. This typically mo-
v ology, and engineering. tivates the definition of the (normalized) rms emittance:
1 (cid:113)
For next generation high energy x-ray sources like the (cid:15) = 1 (cid:104)x2(cid:105)(cid:104)p2 (cid:105)−(cid:104)x p (cid:105)2. Note the use of the
8 n,xi mc i xi i xi
proposedLinacCoherentLightSource-II(LCLS-II)[10],
0 mechanical momenta p . Under the above assumptions,
4 thecreation(atMHzrepetitionrates)andeffectivetrans- conservation of this emxiittance follows directly from Li-
0 portofmulti-MeVbeamswithhighbunchcharges(≥100
ouville’s theorem, provided the forces on the bunch are
. pC), picosecond bunch lengths, and sub-micron normal-
1 linear.
0 ized transverse emittances represents a beam dynamics In practice, bunches in high-brightness photoinjectors
5 regime previously thought attainable only with RF gun
experience both non-linear and longitudinally correlated
1 based photoinjectors [11]. In this letter, we challenge
fields arising from space charge and time-dependent RF
:
v this assumption, and show that the DC gun-based Cor-
fields. Left unchecked, these fields lead to emittance
i nell ERL injector can produce cathode emittance dom-
X growth along the beamline. The mitigation of these ef-
inated beams which meet the bunch charge, emittance,
r fects, known as emittance compensation [12, 14], deter-
a andpeakcurrentspecificationsofanextgenerationlight mines the degree to which the cathode emittance, given
source. Indoingso, wealsodemonstrateexcellentagree-
by:
ment between measurement and simulation of the injec-
tor,andshowthatultimateoptimizationoftheemittance (cid:114)
1 MTE
in high-brightness photoinjectors may require advanced (cid:15) = σ σ =σ , (1)
n,x mc x,0 px,0 x mc2
transverse laser shaping along with the use of low intrin-
sic emittance photocathodes. dominates the beam quality downstream. Here σ is
px,0
Before discussing our experimental results, we review the momentum variance intrinsic to the cathode mate-
the definitions of the key figures of merit for beam qual- rial, which can be expressed in terms of the mean (ki-
ity in high-brightness accelerators relevant for this work: netic)energryofthephotoemittedelectrons(MTE),and
emittance and brightness. For the beam densities en- σ is the spatial variance of the laser distribution. The
x,0
2
rmsemittancemotivatesasimpledefinitionfortheaver-
TABLE I. LCLS-II Injector Specifications
agetransverse(normalized)brightness,definedgenerally
as the particle flux per unit 4D transverse phase space Bunch Charge 20 pC 100 pC 300 pC
volume [15, 16]:
95% (cid:15) 0.25 µm 0.40 µm 0.60 µm
n,x,y
I¯ Peak Current 5 A 10 A 30 A
B = , (2)
n (cid:15) (cid:15)
n,x n,y
where I¯is the average beam current. specifications shown in Table-I as our working param-
To characterize the contribution of the central core of eters [10]. For all direct phase space and longitudinal
the phase space to the emittance, as well as provide a profiledatatakenwithourEmittanceMeasurementSys-
pratical means to compare non-Gaussian beams, we de- tem (EMS), we exclusively used a 50 MHz laser to limit
fine the emittance vs. fraction curve (see [15, 17] for the beam power deposited into the interceptive EMS di-
details): foreveryareainphasespacea,wefindabound- agnostics. This laser system produces 520 nm, 1 ps rms
ing contour D(a) which maximizes the enclosed fraction pulses with comparable pulse energy to the primary 1.3
f of beam particles. The rms emittance computed for GHz laser used for full repetition rate experiments [19].
the particles inside D(a) defines the corresponding frac- Fourrotatablebirefringentcrystalstemporallyshapethe
tional emittance (cid:15)n,x(f). From this we define the core primary pulses by splitting each into 16 copies with tun-
emittance as ablerelativeintensitiessetbythecrystalrotationangles.
1 For transverse shaping, we used a beam expander and
(cid:15)cno,xre = d(cid:15)/df|f=0 = 4πρ , (3) pinholetocliptheGaussianlaserdistributionatroughly
0
thehalfmaximumintensity(truncationfractionof50%).
whereρ0 isthepeakinthephasespacedistributionfunc- All measurements were performed using a single NaKSb
tion (typically the centroid). The corresponding bright- cathode with a 140±10 meV MTE.
ness as a function of fraction and peak brightness follow Inordertodeterminetheinjectorsettingsthatproduce
directly from Eqns. (2-3): optimal emittances and peak currents, we ran Multi-
ObjectiveGeneticAlgorithm(MOGA)optimizationsus-
I¯f2 peak (cid:12)
Bn(f)= (cid:15) (f)(cid:15) (f), Bn = Bn(cid:12)core. (4) ing the 3D space charge code General Particle Tracer
n,x n,y (GPT) [20]. For each of the LCLS-II nominal charges,
In addition to defining the principle figures of merit for the optimizer simultaneously minimized both the emit-
high-brightness accelerators, Eqns. (1-4) make clear the tance and rms bunch length at the location of the EMS
importance of preserving the thermal and cathode core in the simulated injector, subject to realistic constraints
emittances: the degree of conservation sets the extent to on all relevant injector and beam parameters. For a de-
which the intrinsic cathode MTE determines the down- tailed description of our 3D injector model, refer to [17].
stream beam quality. For cathode emittance dominated Additionally, we provided the optimizer with a realistic
beams, any improvement in the MTE translates into im- simulationofthelaserdistribution,andallowedtheopti-
mediate improvement in the final beam quality. With mizer to vary the transverse pinhole size and truncation
this property in mind, we turn to the main purpose of fraction, as well as the rotation of the longitudinal shap-
this work: demonstrating that the Cornell ERL injector, ing crystal angles.
a 5-15 MeV machine featuring a DC gun followed by a The resulting Pareto fronts (shown later) provided in-
short SRF linac, can produce beams with a high degree jectorsettingsthatsimultaneouslysatisfiedboththe95%
of emittance preservation in the beam dynamics regime emittanceandpeakcurrentgoalsspecifiedbytheLCLS-
set by next generation light sources. II injector design. In all cases, the optimizer chose a 9-
Originally designed to create low emittance, moderate 9.5MeVfinalbeamenergyattheEMS.Additionally,the
bunch charge (≤77 pC) beams at high (1.3 GHz) repeti- optimizer chose 0.73 mm, 1.9 mm, and 3.5 mm pinhole
tion rate for a full hard x-ray ERL, the Cornell injector diameters, and roughly 50% for the truncation fraction
currently holds the world record for high average cur- forthethreebunchchargesrespectively. Thecorrespond-
rentfrom aphotoinjectorwithcathodelifetimessuitable ingpinholesavailableatthetimeofmeasurementwere1
for an operating user facility [18], as well as the record mm,2mm,and3.5mm. Postprocessingoftheoptimized
forlowestdemonstratedemittancefromaDCgun-based simulations showed a weak dependence of the transverse
photoinjector at bunch charges of 19 and 77 pC [17]. As emittances on the temporal shaping crystal angles. For
ofthiswork,theCornellinjectorremainslargelythesame simplicity, we tuned the crystal angles to produce a flat
as described in [17], with the most notable difference be- top temporally.
ing the current operation of the DC gun at 395 kV. Foreachbunchchargeweloadedthecorrespondingop-
Forthemeasurementsinthiswork,weusedtheLCLS- timalsettingsintotheinjectorandtunedthemachineto
II injector 95% normalized emittance and peak current producethelowestemittancespossiblewhilestillmeeting
3
thepeakcurrenttargets. Allcriticalmachineparameters chanical momenta γβ = p /mc. One striking feature
xi xi
matched those chosen by the optimizer to within 5%,
withthenotableexceptionofthepinholeusedforthe20 1
0) 10 00 ppCC 2200 ppCC 110000 ppCC 330000 ppCC
pC measurements. At these optimal machine settings, 0
0
we measured the initial transverse laser distribution at x1 0 0.5
the cathode, as well as the longitudinal electron current ((cid:96)x (a)
distribution, and both the horizontal and vertical pro- (cid:97) (cid:239)10 0
0 2 4 6 8 10
jected phases spaces at the EMS. From the phase spaces
x (mm)
we computed the emittance vs. fraction curves, and the
coreemittances. Thethermalemittanceswerecomputed 1
directly from the measured transverse laser profiles ac- 0) 10 00 ppCC 2200 ppCC 110000 ppCC 330000 ppCC
0
cording to Eqn. (1). In order to characterize the initial x10 0 0.5
temporallasershape, wemeasuredthelongitudinalelec- ((cid:96)y ((bb))
tronbeamcurrentprofileatnearzerocharge(0.02±0.01 (cid:97) (cid:239)10 0
pC)withallRFcavitiesoff. Finally,weloadedthecorre- 0 2 4 6 8 10
y (mm)
sponding machine settings and measured laser distribu-
tions for each bunch charge into our virtual accelerator
FIG. 2. Measured horizontal (a) and vertical (b) projected
GUI [17], and ran 250k macro-particle GPT simulations
phase spaces.
for comparison with measurement.
Fig.1showsthemeasuredlaserdistributionsonaCCD
seen in Fig. 2 is the overall symmetry between the hori-
camera located at the same distance from the clipping
zontal and vertical phase spaces. Fig. 3 shows the com-
pinhole as the cathode. To match the optimization re-
parisonofthemeasured(red)andsimulatedlongitudinal
currentprofiles(blue). Inadditiontotheexcellentagree-
1
2 (a) 3.5 mm mentseenbetweenmeasurementandsimulation,wenote
m) 1 mm 1 mm 2 mm that all peak current targets were met.
m 0 0.5
y ( 0 pC 20 pC 100 pC TableIIdisplaysthethermalandcoreemittanceatthe
(cid:239)2 300 pC cathode and the resulting measured 95% (Table-II(a))
0 and core emittances (Table-II(b)) at the EMS. We esti-
0 5 10 15
x (mm) mate a ±6% relative error for the thermal emittances,
and a ±10% relative error in the 95% emittances mea-
1
sured with the EMS (up to the specified resolution of ≤
A) (b)
m 0.05 µm). The random error between subsequent mea-
(
nt 0.5 surements using the EMS was typically ≤1%. We note
re thatthisdataquantitativelyreflectsthequalitativesym-
r
u
c metryseeninthephasespacemeasurements(Fig.2),and
0
(cid:239)20 (cid:239)10 0 10 20 as well as satisfies all of the LCLS-II injector emittance
arrival time (ps) targets. The table also shows the ratio of the thermal
FIG. 1. (a) The measured transverse laser distributions. (b)
Thesimulatedtemporallaserdistribution(green),theresult- 35
data
ing electron current profile at the EMS from GPT (dashed 300 pC GPT
30
blue), and the measured electron current profile (red).
25
sults as best as possible, we tuned the laser spot size on A)
the laser CCD so that the edge truncation fraction was nt (20
50% using a beam expander. Fig. 1(b) shows the mea- urre15
suredtemporalcurrentprofileoftheelectronbeamatthe c
100 pC
EMS (red), for a bunch charge of 0.02 ± 0.01 pC, and 10
with all RF cavities off. The green curve shows the sim- 20 pC
5
ulated initial temporal laser distribution (normalized to
0.02 pC) and the resulting simulated electron beam cur- 0
(cid:239)10 (cid:239)5 0 5 10
rent profile at the location of the EMS in GPT (dashed arrival time (ps)
blue).
Fig. 2 displays the measured horizontal and vertical FIG. 3. Comparison of the simulated (blue) and measured
projected phases spaces corresponding to the best mea- (red) current profiles as a function of bunch charge. Peak
sured emittances. Note the use of the normalized me- current targets are shown in black.
4
1
TABLE II. (a) Measured horizontal (vertical) thermal 95%
300 pC
emittances at the EMS location. (b) Initial and final mea-
sured horizontal (vertical) core emittances. 0.8
(a) Horizontal(vertical)projectedemittancedata.
0.6
Charge Thermal (cid:15) (µm) 95% (cid:15) (µm) Ratio (%) m)
n n µ
20 pC 0.12 (0.11) 0.18 (0.19) 67 (58) (n 100 pC
(cid:161) 0.4
100 pC 0.24 (0.23) 0.30 (0.32) 80 (72)
300 pC 0.42 (0.41) 0.62 (0.60) 67 (68) 20 pC
0.2
(b) Horizontal(vertical)projectedcoreemittancedata.
0
0 1 2 3 4
Charge Cathode (cid:15)n,core (µm) EMS (cid:15)n,core (µm) Ratio (%) rms bunch length (ps)
20 pC 0.06 (0.06) 0.09 (0.08) 67 (75)
100 pC 0.14 (0.13) 0.16 (0.16) 85 (79) FIG. 4. Optimized emittance vs. rms bunch length using
(blue) a perfect variable truncated Gaussian and variable
300 pC 0.26 (0.24) 0.30 (0.28) 87 (87)
temporaldistribution,(red)themeasuredlaserdistributions.
Measured data are shown in black.
emittance and the final 95% emittance, and the ratio likely due to the use of a pinhole size 40% larger than
of the initial and final core emittances. In all measure- the optimal value.
ments, the thermal emittances were preserved to within In this work, we have shown that optimal injector
58%-80%. Similarly, the core emittances were preserved settings found using MOGA optimizations of 3D space
within 67-87%. We point out that the roughly 80-90% charge simulations of the Cornell ERL injector produce
preservation of the core emittance for all charges except machinestatesthatpreserveboththemeasured95%and
20 pC. In this case, the finite resolution of the EMS (≤ core emittance, computed from direct phase space mea-
0.05µm)likelybecomesacontributingfactorwhenmea- surements,towithin57-87%for20,100,300pCbunches.
suring such small emittances. We conclude that the ac- Furthermore,theresultingmeasuredemittancesandlon-
tual core emittance for this bunch charge is smaller than gitudinal current profile show excellent agreement with
the quoted value, as suggested by simulation. Never- corresponding GPT simulations, and meet the stated
theless, these results demonstrate the main focus of this 95% emittance and peak current specifications of the
work: contrary to previous thought, DC gun based pho- LCLS-II injector design. Additionally, we have shown
toinjectors are capable of delivering cathode emittance that the transverse laser shape plays an important role
dominated beams at high bunch charges suitable for use in determining the optimal emittances, adding further
in next generation FELs like the LCLS-II. relevance to the recent demonstration of accurate, ar-
In order to determine the effect of the laser shape bitrary transverse laser shaping at Cornell [21, 22]. In
on the emittances, we ran a second round of optimiza- conclusion, this work shows that DC gun based photoin-
tions using the measured transverse laser distributions jectorscanproducecathodeemittancedominatedbeams
in Fig. 1(a) and the crystal angles used to create the with single bunch beam quality rivaling that produced
flattop Fig. 1(b). All other relevant injector parameters by RF gun based injectors for charges up to 300 pC, and
varied as before. Fig. 4 shows the average 100% emit- represents a significant expansion of the beam dynamics
tance, (cid:15)n = 12((cid:15)n,x+(cid:15)n,y), vs. rms bunch length at each regime for which DC gun-based injectors are applicable.
bunch charge. Shown in blue are the initial optimiza- We acknowledge Jared Maxson for his useful discus-
tionswithvariedlaserdistributionparameters,andideal sionsandinterestinthiswork. Thisworkwassupported,
transverse shape. The red curves show the results of the in part, by the LCLS-II Project and the US Department
second round of optimizations using the measured laser of Energy, Contract DE-AC02-76SF00515.
distributions (Fig. 1). The emittances corresponding to
the data in Figs. 2-3 and Table-II are shown in black.
Wenotethattheemittancegrowthduetothelaser(dis-
tancebetweenblueandredcurvesatthemeasuredbunch
lengths shown in black) increases with bunch charge, as ∗ co-first author; [email protected]
† co-first author; [email protected]
one might expect. For the 100 and 300 pC measure-
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