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Efficient transfer of positrons from a buffer-gas-cooled accumulator into an orthogonally

oriented superconducting solenoid for antihydrogen studies

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2012 New J. Phys. 14 045006

(http://iopscience.iop.org/1367-2630/14/4/045006)

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New Jou rnal of Ph ys ics

The open access journal for physics

Ef cient transfer of positrons from a

buffer-gas-cooled accumulator into an orthogonally

oriented superconducting solenoid for antihydrogen

studies

D Comeau1, A Dror1, D W Fitzakerley1, M C George1,

E A Hessels1,5, C H Storry1, M Weel1, D Grzonka2, W Oelert2,

G Gabrielse3, R Kalra3, W S Kolthammer3, R McConnell3,

P Richerme3, A Mullers4 and J Walz4 (ATRAP Collaboration)

Department of Physics and Astronomy, York University, Toronto,

ON M3J 1P3, Canada

Forschungszentrum J lich GmbH, 52425 J lich, Germany

u u

Department of Physics, Harvard University, Cambridge, MA 02138, USA

Institute f r Physik, Johannes Gutenberg Universit t, D-55099 Mainz,

u a

Germany

E-mail: abql41@r.postjobfree.com

New Journal of Physics 14 (2012) 045006 (10pp)

Received 2 January 2012

Published 10 April 2012

Online at http://www.njp.org/

doi:10.1088/1367-2630/14/4/045006

Abstract. Positrons accumulated in a room-temperature buffer-gas-cooled

positron accumulator are ef ciently transferred into a superconducting solenoid

which houses the ATRAP cryogenic Penning trap used in antihydrogen research.

The positrons are guided along a 9 m long magnetic guide that connects the

central eld lines of the 0.15 T eld in the positron accumulator to the central

magnetic eld lines of the superconducting solenoid. Seventy independently

controllable electromagnets are required to overcome the fringing eld of the

large-bore superconducting solenoid. The guide includes both a 15 upward bend

and a 105 downward bend to account for the orthogonal orientation of the

positron accumulator with respect to the cryogenic Penning trap. Low-energy

positrons ejected from the accumulator follow the magnetic eld lines within

the guide and are transferred into the superconducting solenoid with nearly

Author to whom any correspondence should be addressed.

New Journal of Physics 14 (2012) 045006

1367-2630/12/045006+10$33.00 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft

100% ef ciency. A 7 m long 5 cm diameter stainless-steel tube and a 20 mm

long, 1.5 mm diameter cryogenic pumping restriction ensure that the 10 2 mbar

pressure in the accumulator is isolated well from the extreme vacuum required

in the Penning trap to allow for long antimatter storage times.

Contents

1. Introduction 2

2. The ATRAP apparatus 3

3. The positron magnetic guide 4

4. Performance of the e+ magnetic guide 8

5. Conclusions 10

Acknowledgments 10

References 10

1. Introduction

Large numbers of trapped antiprotons (p) and positrons (e+ ) are required for precision

antihydrogen (H) studies. The goals of such studies are to test CPT invariance [1] and to measure

the gravitational force [2] on antimatter.

Over the last 25 years, much effort has gone into capturing p into an environment suitable

for long-term storage and manipulation and for the production and study of H. During these

years, p were slowed to 5 MeV at CERN, rst by the LEAR (Low-Energy Antiproton Ring) and

then by the AD (Antiproton Decelerator), and were further slowed in a beryllium degrader. In

1986, p were rst captured in a Penning trap [3], housed in a superconducting (SC) solenoid and

then cooled to meV energies [4] by thermalizing with a plasma of electrons. Antiproton storage

times of up to 1 month [5] indicated a pressure inside the 4.2 K vacuum enclosure of less than

7 10 17 mbar. Larger Penning trap electrodes allow for a loading rate of over 105 p per 90 s

cycle of the AD [7]. Stacking of p from subsequent cycles of the AD traps a greater number

(up to 107 ) of p [8]. A pumped helium system allows for p within a 1.2 K Penning trap [9].

Antiproton plasmas have been further cooled by evaporative cooling [10] or, more recently, by

adiabatic cooling [7]. The latter allows for cooling of the p without any p losses.

The availability of radioactive isotopes that produce e+ through + decay makes

e considerably easier to obtain than p. However, trapping e+ in the same Penning trap apparatus

as p is a considerable challenge. e+ loading is restricted by the limited access and large fringing

magnetic elds of the SC solenoid, and it is essential that the cold temperature of the Penning

trap and of the p be maintained, as must the extreme vacuum needed for long-term storage of

antimatter. These challenges were rst overcome [11] by lowering a 22 Na radioactive source to

a position just above the Penning trap (inside of the high magnetic eld of the SC solenoid),

which allowed high-energy e+ to pass through a thin titanium foil into the Penning trap vacuum

system, where they were moderated by single-crystal tungsten to produce e+ of about 1 eV. At

the exit of the moderator, some of the slow e+ bind with an electron to form a highly excited

state of positronium, which is Stark ionized inside of the Penning trap, allowing the e+ to be

captured in the trap [12]. An advantage of this loading technique is that extremely low vacuum

New Journal of Physics 14 (2012) 045006 (http://www.njp.org/)

pressures can still be achieved since an isolated sealed cryogenic vacuum system is maintained.

This method allows for loading rates of up to 4 104 e+ h 1 per mCi of 22 Na [13].

An alternative method for obtaining e+ for H studies [14] uses a buffer-gas-cooled e+

accumulator [15], in which e+ from a 22 Na source are moderated in a cryogenic neon solid [16]

to produce an intense beam of slow e+ . These slow e+ are magnetically guided into a room-

temperature Penning trap where inelastic collisions with nitrogen molecules in the buffer gas

reduce the e+ kinetic energy, causing them to be axially trapped in the electric potential wells

of the Penning trap. Radial con nement is provided by an axial magnetic eld, and a rotating

transverse electric eld compresses the trapped e+ plasma to within a small radius near the

axis of the accumulator [17]. Such an e+ accumulator can collect e+ at much higher rates than

the positronium method described above, but has the disadvantage of requiring approximately

10 2 mbar of buffer gas.

The magnetic guide described in this work is used to transport e+ from the higher pressures

of the buffer-gas-cooled e+ accumulator to the extremely low pressures of the cryogenic ATRAP

Penning trap used for H studies. The e+ accumulator is located 6 m from the cryogenic Penning

trap. The magnetic eld of this accumulator is horizontally oriented, whereas the magnetic eld

of the cryogenic Penning trap is oriented vertically. This paper describes the techniques and

challenges of ef ciently transporting large numbers of e+ from the accumulator to the cryogenic

Penning trap while maintaining a vacuum suitable for antimatter con nement.

2. The ATRAP apparatus

The ATRAP cryogenic Penning trap has a vertical magnetic eld (of 1 3 T) provided by

a 152 cm high, 62 cm diameter SC solenoid. There are distinct advantages of orienting the

cryogenic Penning trap vertically. A vertical orientation simpli es the cryogenic system design

required to maintain a temperature of 4.2 K for the SC solenoid and a temperature of 1.2 K

for the Penning trap electrodes [9]. Long thermally insulating supports suspend the Penning

trap electrode stack from only a few points at the top of the trap, whereas a far larger number

of shorter supports would be required along the length of the cryogenic Penning trap if it were

oriented in the horizontal direction, resulting in much poorer thermal isolation. With the vertical

design, the ATRAP cryogenic Penning trap can maintain its operating temperature for more

than 30 h between liquid-helium re lls of the 60 liter dewar. The vertical orientation will also

facilitate higher-precision tests of gravitational forces on antimatter, since it allows far more

distance for the neutral H to accelerate under gravity before annihilating on the walls of the

apparatus.

The simplest experimental setup for introducing e+ into the p Penning trap would be to

have the e+ accumulator and the cryogenic Penning trap aligned coaxially as in [14]. In such a

con guration, the magnetic eld lines originating from the central axis of the e+ accumulator

would continue into the eld lines on the central axis of the SC solenoid, and these eld

lines would guide the e+ along this axis. The challenge that ATRAP faced in implementing

a buffer-gas-cooled e+ accumulator is that the vertical space above the cryogenic Penning trap

is restricted by clearance for a bridge crane used inside the AD hall, making such a coaxial

alignment impossible. Instead, the e+ accumulator was oriented horizontally and was placed at

the nearest location available in the AD hall (approximately 6 m away from the SC solenoid),

and a magnetic guide that has two bends (15 and 105 ) was implemented, as shown in

gure 1.

New Journal of Physics 14 (2012) 045006 (http://www.njp.org/)

6m A20

A19 T2 A21 CP2

A18

4m T19 A22 7m

V4 T18

T21

A17 S

s T17

tron V3 TP

posi 3m S6

d by T15 T

erse A10

T22

T14

Distance trav V5

T1315

T23

T11 T12 13 A4 A16

0m 1m

T10

S2 A

1 2 A1

A1A1 8m

A8 V6

A3- A7 T8

S1 x7

T1 V2 T7 5m

e+ ag

accumulator ni

T4 x2 magnification fic

T2 A1 T3 at

V1 io

1.5mm n

Figure 1. Magnetic guide for e+ . Axial magnetic elds used to guide the e+

are provided by solenoids S1 S6 and supplementary circular electromagnets

A1 A22, along with the fringing elds of the 0.15 T accumulator solenoid

and the 1 3 T SC solenoid. The fringing eld of the SC solenoid (purple

eld lines) and other external elds need to be canceled by a set of

rectangular electromagnets T1 T23 (a three-dimensional view of some of these

electromagnets is shown in the inset). A similar set of rectangular electromagnets

(not shown) cancel smaller external elds in the other transverse direction

(perpendicular to the plane of the gure). The SC solenoid eld lines shown

(both near the axis of the solenoid (in green) and far off of the axis (in purple))

do not include the effect of any of the other electromagnets.

3. The positron magnetic guide

To ensure that p and H do not annihilate quickly with residual gas in the cryogenic Penning trap

located inside of the SC solenoid, an extremely low vacuum pressure is required. With this in

mind, the e+ magnetic guide is designed to act as a pumping restriction that isolates the buffer-

gas-cooled e+ accumulator, which has pressures of 10 2 mbar, from the cryogenic Penning trap,

which was shown to have a vacuum pressure of less than 7 10 17 mbar in an earlier, closed

apparatus [5]. To maintain a low pressure in the e+ magnetic guide, a 3600 liter s 1 cryopump

(CP1 at 0.7 m in gure 1) ensures that very little of the buffer gas enters the magnetic guide.

An 80 liter s 1 turbo pump (TP at 4 m in gure 1) and a 1500 liter s 1 cryopump (CP2 at the

7 m in gure 1) provide successively lower pressures along the magnetic guide. The pressure

in the vacuum space near CP2 is 10 8 mbar. A 1.5 mm diameter, 20 mm long hole maintained

at 4.2 K, and located inside of the SC solenoid (as shown in gure 1) provides a nal pumping

restriction while allowing e+ to enter the cryogenic Penning trap. Background gas atoms and

molecules incident on this pumping restriction freeze onto its cryogenic surfaces, with only the

New Journal of Physics 14 (2012) 045006 (http://www.njp.org/)

small fraction with straight-line ballistic trajectories that do not hit these surfaces entering the

cryogenic Penning trap.

With this system in place, an exceedingly good vacuum is maintained in the cryogenic

Penning trap. An experiment where a calibrated number of p were held for 15 h in the Penning

trap and then released and counted indicated a p loss rate consistent with zero, and set a

minimum p annihilation lifetime of over 200 h. This lifetime is suf cient for precision H studies,

and con rms that we have been successful in preserving the exceptional vacuum in our system

despite the open connection to the 10 2 mbar e+ accumulator system.

The e+ must pass through the 1.5 mm cryogenic pumping restriction to enter the cryogenic

Penning trap. Figure 1 shows some fringing eld lines of the SC solenoid. One possibility for

guiding e+ from the accumulator to the SC solenoid would be to follow one of these fringing

eld lines (such as the purple line shown which intersects the e+ accumulator). However, e+

loaded along this eld line would be 11 cm from the axis of the SC solenoid, well outside of the

1.5 mm pumping restriction and outside of the cryogenic Penning trap. In order to load e+ near

the axis, they must follow the eld lines shown in green. The dark green eld line goes through

the center of the 1.5 mm pumping restriction, and the light green lines pass near the edges of the

restriction. The main function of the magnetic guide is to direct the e+ onto these central (green)

eld lines.

A series of electromagnets produce magnetic elds along the axis of the magnetic

guide. Electromagnets A1 and A2 (of gure 1, where a cross-sectional view of the circular

electromagnets is shown) continue the fringing eld of the 0.15 T accumulator solenoid, and

electromagnets A3 A7 bend these eld lines by 15 . Solenoids S1 S5 are wound directly on

the outside of the 5 cm diameter stainless-steel vacuum tubes and provide an axial eld of

0.02 T along the straight-line path to a point that is 3.5 m above the center of the SC solenoid.

Electromagnets A8 A20 produce additional axial elds in locations where vacuum pumps,

valves and anges do not allow for a continuation of the main (S1 S5) solenoids. A particularly

challenging region between S3 and S4 results from a pump and two valves (TP, V3 and V4 in

gure 1), which are necessary so that the right-hand portion of the magnetic guide (from V4 to

V5) can be removed (with breaking vacuum only between V3 and V4 and between V5 and V6)

when access is needed for repairs and upgrades of the cryogenic Penning trap apparatus.

If it were not for the fringing eld of the large-bore SC solenoid (and the earth s eld

of 5 10 5 T and elds produced by other electromagnets within the AD hall), the axial

electromagnets would be suf cient to guide the e+ along the 15 inclined straight-line path.

The fringing eld, however, is large (up to 0.015 T) at positions along the guide, and, as can be

seen from the purple eld lines in gure 1, is mostly perpendicular to the guide. To cancel these

position-dependent elds, a series of individually controllable 30 cm long rectangular windings

are placed in pairs above and below the magnetic guide (T3 T20 of gure 1). A similar set of

rectangular electromagnets (not shown in gure 1) cancel smaller horizontal magnetic elds.

Additional transverse electromagnets (T1, T2) ensure that the e+ are guided into the center of

the magnetic guide.

At the end of the 6 m long inclined section of the guide, electromagnets A21, A22, S6, T21,

T22 and T23 cause the eld line at the center of the guide to join the green eld lines of gure 1

that go through the 1.5 mm cryogenic pumping restriction.

The e+ are transferred adiabatically through the magnetic guide at low energies

(10 100 eV). At these energies, the e+ are expected to almost exactly follow the magnetic eld

lines, and this has been con rmed by numerical calculations of the trajectories. More precisely,

New Journal of Physics 14 (2012) 045006 (http://www.njp.org/)

Figure 2. Axial eld along the magnetic guide. The distance scale refers to the

scale shown in gure 1. The axial eld is 0.15 T in the accumulator (at 0 m) and

remains at values of at least 0.012 T along the entire path to the SC solenoid

(which has a 1 T eld in this example).

Figure 3. Field lines along the magnetic guide. The distance scale is that of

gure 1, and the positions of the bends in the magnetic guide are noted on the

plot. The scale of the y-axis of this plot shows the position of the eld lines

relative to an axis through the center of the guide and is expanded relative to the

x-axis by a factor of 300 to allow a clear view of the eld lines that go through

the center (dark green) and edges (light green) of the 1.5 mm cryogenic pumping

restriction. In this gure the SC solenoid has a eld of 1 T. The eld lines do not

intersect the gray areas in the gure, which show the locations of the walls of the

magnetic guide.

the e+ execute cyclotron motion (with cyclotron radii much less than 1 mm) around the eld

lines leading to helical trajectories that remain centered near the eld lines.

Figure 2 shows the magnitude of the axial magnetic eld along the length of the guide. The

eld for most of the length of the guide is 0.02 T, with smaller axial elds (as low as 0.012 T) in

the regions before S1, between S3 and S4 and after S5 of gure 1. The eld lines are shown in

gure 3. The eld lines shown are the ones that go through the 1.5 mm pumping restriction (the

same green eld lines as those of gure 1, but now with the inclusion of the magnetic elds due

to the 70 magnetic-guide electromagnets and the accumulator solenoid, and now traced back to

the e+ accumulator). The eld-line spacing (the top to bottom line in the gure) is 7.5 mm within

the e+ accumulator, expands to 20 mm through most of the magnetic guide, where the eld is

0.02 T, becomes as large as 28 mm at the lowest eld points and nally reduces to 1.5 mm inside

a 1 T eld for the SC solenoid, as shown in gure 3. The wiggles in the eld lines show that

New Journal of Physics 14 (2012) 045006 (http://www.njp.org/)

NaI scintillation signal [arb. units]

0 0.5 1.0 1.5 2.0 2.5 3.0

time [ s]

Figure 4. Signal seen on a NaI scintillating crystal for e+ that are intentionally

made to annihilate at V2 at 1 m in gure 1 (blue), at a retractable Faraday cup

below TP at 4.1 m in gure 1 (green) and at a second retractable Faraday cup

halfway between V5 and V6 at 7.8 m in gure 1 (red).

the transverse magnetic eld (mostly due to the fringing eld of the SC solenoid) cannot be

canceled perfectly using the series of nite-length rectangular electromagnets, but it can also be

seen that the eld lines stay safely away from the stainless-steel walls of the magnetic guide.

The wiggles become more pronounced when the SC solenoid is energized to its maximum eld

of 3 T.

Initially, it was a signi cant challenge to nd the current settings for the 70 electromagnets

that would connect the central eld lines of the accumulator to the central eld lines of the SC

solenoid. It is necessary to have the eld lines not intersecting the 5 cm diameter stainless-steel

vacuum tube, since otherwise the e+ would annihilate. These annihilations produce 511 keV

gamma rays, which are detected with NaI scintillating crystals and photomultiplier tubes. For a

plasma of e+ launched in a short time window from the e+ accumulator (with a kinetic energy set

by the electrostatic potential from which they are released typically 10 100 eV), the timing

of the annihilation signals depends on the distance the e+ travel before annihilating on the

surfaces. Figure 4 shows signals from a NaI detector located 11 m away from the magnetic

guide, at a position where the detector has a relatively unobstructed view of the entire magnetic

guide. From the launch energy for the e+ and the delay time to the NaI signal, it is possible

to determine the approximate position of the annihilations and therefore to determine which

electromagnet should have its current adjusted to better center the eld lines through the 6 m

long, 5 cm diameter tube and into the SC solenoid. After adjustment of the currents, there are

no longer any visible signals on the NaI detectors, indicating that a negligible fraction of the e+

hit the walls of the guide.

The 70 electromagnets of the e+ magnetic guide are energized by independent

programmable power supplies operating in constant-current mode. The voltages across shunt

resistors in series with each electromagnet are monitored and recorded every few minutes as

a means of precisely monitoring the current in the electromagnet. In addition, the voltage

New Journal of Physics 14 (2012) 045006 (http://www.njp.org/)

across each electromagnet is monitored and recorded to identify possible shorts or elevated

temperatures in the windings of the electromagnets. The active monitoring provides an

immediate warning if any of the voltages or currents have fallen out of their typical operating

range.

A series of Faraday cups provides an additional diagnostic tool for monitoring the transport

of e+ through the magnetic guide. Faraday cups that can be moved into (or out of) the e+ path

are located at 0.7, 4.1 and 7.8 m in gure 1. An additional Faraday cup that could be inserted

into the center of S5, and that could be moved along the length of S5, was critical for tuning the

magnetic eld through this region where the fringing eld of the SC solenoid and its gradient are

particularly large. Just above the cryogenic pumping restriction (shown in the inset of gure 1), a

Faraday cup segmented into four quadrants detects the charged particles that make it through the

entire magnetic guide, but are not aimed through the 1.5 mm hole. By monitoring the charges

on these four quadrants, the e+ can be guided onto the central axis of the cryogenic Penning

trap. When the electromagnets are tuned to guide the e+ onto the central axis, no e+ are found

to hit any of the four quadrants. Finally, the beryllium degrader at the bottom of the cryogenic

Penning trap of gure 1 (used to slow the p which enter from below) also acts as a Faraday cup to

monitor the charged particles that make it through the pumping restriction. A movable electron

gun can be inserted just before the accumulator solenoid to provide an intense continuous beam

of electrons that can be monitored on each Faraday cup to aid in optimizing the currents for

the 70 electromagnets of the magnetic guide. e+ could also be detected on each Faraday cup by

measuring the integrated current with a charge ampli er when an accumulated plasma of e+ are

launched from the e+ accumulator.

4. Performance of the e+ magnetic guide

The e+ magnetic guide has performed with high reliability during its ve years of operation.

Typically, 107 e+ are launched into the magnetic guide. The fact that no e+ annihilations are seen

on the NaI scintillating detectors during the 3 s period (see gure 4) that it takes for the e+ to

traverse the magnetic guide indicates that no e+ are lost as they travel between the accumulator

and the SC solenoid. Charge measurements on a Faraday cup inside the SC solenoid con rm

that all e+ successfully enter the large magnetic eld of the SC solenoid. The fact that no e+ hit

the four quadrants of the Faraday cup surrounding the 1.5 mm cryogenic pumping restriction

indicates that the e+ are tightly focused to less than this 1.5 mm diameter within the eld of the

SC solenoid.

When e+ are launched from the accumulator into the magnetic guide, the details of the

spatial and velocity distributions of the e+ before launch, and of the space charges and applied

potentials during launch, can affect the distribution of kinetic energies for the released e+ . The

launch leads to most of the e+ kinetic energy being due to the axial motion along the magnetic

guide (along the direction of the eld lines), but some of the kinetic energy is also due to

transverse motion (which leads to cyclotron orbits around the eld lines). As the e+ enter the

larger eld of the SC solenoid, the angular momentum due to this cyclotron motion is conserved

(since there is no torque), but due to the smaller cyclotron orbits in the larger eld, the transverse

velocity must be increased to maintain the angular momentum. The result is that the axial kinetic

energy is reduced. The effect slows the e+ and leads to a larger axial energy distribution for the

e+ . This axial energy distribution is measured by applying a series of increasing potentials in

an attempt to block the e+ from reaching the beryllium degrader, as shown in gure 5. The

New Journal of Physics 14 (2012) 045006 (http://www.njp.org/)

1.0

(a) (b)

6.6 eV

0.8

fraction of e+

0.6

40 50 60 70 80

energy distribution [eV]

0.4

0.2

0.0

40 50 60 70 80

blocking potential [V]

Figure 5. The energy distribution of the e+ as they enter the 1 T eld of the

superconducting solenoid. The fraction of e+ that pass by a blocking potential

barrier is shown in (a). The energy distribution of the e+ entering the cryogenic

Penning trap (b) is obtained from the derivative of the red curve.

Figure 6. Temporal spread of the e+ as they enter the SC solenoid.

gure demonstrates that the width of the e+ axial energy distribution within the SC solenoid is

7 eV, or 6% of the average value. The lowest observed energy is 50 eV, indicating that this

slowing effect is not large enough to cause so-called magnetic mirroring, in which the e+ lose

all of their axial kinetic energy, and are re ected back out of the higher eld. As expected,

no annihilations are seen with the NaI detectors at delayed times due to magnetic mirroring

of e+ .

The same factors that affect the energy distribution can also affect the arrival time of the e+

into the SC solenoid. Figure 6 shows the distribution of arrival times, as measured using a large

blocking potential that was pulsed off at a series of times. The gure shows that almost all e+

arrive in a time interval of less than 600 ns.

The combination of a small range of energies ( gure 5) and a small range of arrival times

( gure 6) makes it possible to capture most of the e+ in the cryogenic Penning trap.

New Journal of Physics 14 (2012) 045006 (http://www.njp.org/)

5. Conclusions

We have demonstrated that the large numbers of e+ available from a buffer-gas-cooled e+

accumulator can be transferred from this accumulator into a perpendicularly oriented SC

solenoid containing a cryogenic Penning trap used for H studies. The e+ are transferred along

a magnetic guide with near unity ef ciency. The time and energy distributions of the e+ as

they enter the SC solenoid are suf ciently small to allow for ef cient capture of the e+ in

the cryogenic Penning trap. Most importantly, the vacuum of this cryogenic Penning trap

is demonstrated to still be suf cient to allow for long storage times of p, despite the open

connection between this vacuum system and the 10 2 mbar of pressure in the e+ accumulator,

thus allowing for the possibility of precision H studies.

Acknowledgments

This work was supported by the National Science Foundation and Air Force Of ce of Scienti c

Research of the United States, the Bundesministerium f r Bildung und Forschung, Deutsche

Forschungsgemeinschaft and Deutscher Akademischer Austausch Dienst of Germany, and the

Natural Sciences and Engineering Research Council, Canada Research Chair program, Canada

Foundation for Innovation, and Ontario Innovation Trust of Canada.

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