Journal ArticleDOI

# A compact linear Paul trap cooler buncher for CRIS

15 Jan 2020--Vol. 463, pp 375-377

AbstractA gas-filled linear Paul trap for the Collinear Resonance Ionisation Spectroscopy (CRIS) experiment at ISOLDE, CERN is currently under development. The trap is designed to accept beam from both ISOLDE target stations and the CRIS stable ion source. The motivation for the project along with the current design, simulations and future plans, will be outlined.

Topics: Ion trap (51%)

### 1. Introduction

• Bunched beams are essential to Collinear Resonance Ionisation Spectroscopy (CRIS) [4, 5, 6] because the required high-power lasers are only available with repetition rates of the order of 100 Hz, meaning the ion beam must also be pulsed to remove duty-cycle losses.
• ISCOOL has been essential to the success of the CRIS experiment, providing bunched beams with <1 eV energy spread and <5 µs width.
• This would allow for continual optimisation of trapping parameters and efficiency with stable isotopes, improving beam quality and transport efficiency for radioactive beam experiments.

### 2. Prototype RFQcb overview

• Anticipating the available lab space, the current prototype design uses a short trapping region, 20 cm in length, with four cylindrical rods at 5.3 mm internal radius.
• The ions will be trapped longitudinally using DC potentials applied to four printed circuit boards inserted between the rods.
• Preprint submitted to Elsevier April 4, 2019.
• As the figure shows, when a 20 mm side length cube of 3D printed PLA was placed in the vacuum chamber, the pressure decreased to 2 × 10−7 mbar after 5.5 hours of pumping, equal to the pressure reached by the empty chamber within the same pumping time.
• The transmission efficiency at A = 70 was 54% and the final simulated emittance of the cooled beam at this mass was 1.5 πmm mrad.

### 3. Outlook

• The benefits of installing an independent gas-filled linear Paul trap at the CRIS beamline have been explained and the design outlined is under development.
• Simulations have indicated the feasibility of a short trap design and have informed possible design improvements.
• The design offers a cost effective and high efficiency method to trap ions for radioactive beam studies.
• This work also illustrates the possible uses of 3D printed parts in vacuum, enabling cheaper and faster methods for design prototyping.

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The University of Manchester Research
A compact linear Paul trap cooler buncher for CRIS
DOI:
10.1016/j.nimb.2019.04.054
Document Version
Accepted author manuscript
Link to publication record in Manchester Research Explorer
Citation for published version (APA):
Ricketts, C., Cooper, B., Edwards, G., Perrett, H., Billowes, J., Binnersley, C., Cocolios, T., Flanagan, K., Garcia
Ruiz, R. F., de Groote, R. P., Gustafsson, F. P., Koszorús, Á., Neyens, G., Vernon, A., & Yang, X. F. (2019). A
compact linear Paul trap cooler buncher for CRIS. Nuclear Instruments & Methods in Physics Research. Section B:
Beam Interactions with Materials and Atoms. https://doi.org/10.1016/j.nimb.2019.04.054
Published in:
Nuclear Instruments & Methods in Physics Research. Section B: Beam Interactions with Materials and Atoms
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A compact linear Paul trap cooler buncher for CRIS
C.M. Ricketts
a,
, B.S. Cooper
a,b
, G. Edwards
a,b
, H.A. Perrett
a,b
, J. Billowes
a
, C.L. Binnersley
a
, T.E. Cocolios
c
, K.T. Flanagan
a,b
,
R.F. Garcia Ruiz
d
, R.P. de Groote
f
, F.P. Gustafsson
c
,
´
A. Koszor
´
us
c
, G. Neyens
c,d
, A.R. Vernon
a
, X.F. Yang
e
a
School of Physics and Astronomy, The University of Manchester, Manchester, M13 9PL, United Kingdom
b
Photon Science Institute, Alan Turing Building, The University of Manchester, Manchester, M13 9PY, United Kingdom
c
KU Leuven, Instituut voor Kern- en Stralingsfysica, B-3001 Leuven, Belgium
d
EP Department, CERN, CH-1211 Geneva 23, Switzerland
e
School of Physics and State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China
f
Department of Physics, University of Jyv¨askyl¨a, PB 35 (YFL) FIN-40351 Jyv¨askyl¨a, Finland
Abstract
A gas-ﬁlled linear Paul trap for the Collinear Resonance Ionisation Spectroscopy (CRIS) experiment at ISOLDE, CERN is currently
under development. The trap is designed to accept beam from both ISOLDE target stations and the CRIS stable ion source. The
motivation for the project along with the current design, simulations and future plans, will be outlined.
Keywords: Ion trapping, Laser spectroscopy, 3D printing
1. Introduction
Gas-ﬁlled linear Paul traps are used at a variety of radioac-
tive beam facilities to provide bunched beams with low emit-
tance to a range of experimental setups [1, 2, 3]. Bunched
beams are essential to Collinear Resonance Ionisation Spec-
troscopy (CRIS) [4, 5, 6] because the required high-power lasers
are only available with repetition rates of the order of 100 Hz,
meaning the ion beam must also be pulsed to remove duty-cycle
losses. The pulsed-time structure also reduces optical pumping
to atomic dark states, further increasing ionisation eciency
[7]. Minimising the ion beam energy spread via in-trap buer-
gas cooling reduces the Doppler broadening of the resonant ex-
citation steps, leading to higher precision in extracted hyperﬁne
technique, see the contributions from
´
A. Koszor
´
us et al. and
A.R. Vernon et al. in these proceedings [8, 9].
CRIS at ISOLDE [10] currently utilises the ISCOOL gas-
ﬁlled linear Paul trap (RFQcb). ISCOOL has been essential to
the success of the CRIS experiment, providing bunched beams
with <1 eV energy spread and <5 µs width. When fully opti-
mised the transmission eciency of ISCOOL can reach 70%,
however for lower masses the usable eciency decreases sig-
independent RFQcb for the CRIS beamline, to be placed af-
ter the CRIS ion source (CRISIS) [12]. Installing a dedicated
RFQcb would decouple the CRIS beamline from the rest of
ISOLDE; ions injected into the trap from ISOLDE and from
CRISIS would have the same properties when they reach the
CRIS interaction region. This would allow for continual opti-
misation of trapping parameters and eciency with stable iso-
topes, improving beam quality and transport eciency for ra-
dioactive beam experiments. Frequent reference measurements,
christopher.ricketts@manchester.ac.uk
performed by extracting a stable reference isotope from ISOLDE,
are required during experiments on radioactive atoms to keep
track of slow drifts in wavenumber readout. Switching masses
on the ISOLDE separator for this purpose is time consuming
(approximately 6 minutes per mass change); during recent CRIS
campaigns, between 5% and 10% of the total beamtime was
spent cycling the magnets. In addition, more than 33% of the
remaining time was spent adjusting bending and focussing elec-
tric potentials to maximise ion beam transmission through CRIS.
A dedicated trap would allow CRISIS to be used for the refer-
ence measurements, and would almost entirely eliminate the
beam transport optimisation required at the start of radioactive
beam experiments. This period is when the yield of radioac-
tive cases is at its maximum and additional data collection at
this point would have a disproportionate eect on the overall
signal signiﬁcance. As the majority of background counts at
CRIS scale with the beam current, for example non-resonant
laser ionisation of isobaric contamination, the total background
observed is proportional to the interaction region atom through-
put, hence the signal signiﬁcance scales with the product of the
beam intensity, measurement time and total eciency.
2. Prototype RFQcb overview
Anticipating the available lab space, the current prototype
design uses a short trapping region, 20 cm in length, with four
cylindrical rods at 5.3 mm internal radius. Sine or square wave
RF driven at frequencies of the order of MHz will be applied
to the rods to trap the ions radially, while helium buer gas (at
pressure in the range 10
3
to 10
1
mbar) will be used to reduce
the amplitude of the radial oscillation. The ions will be trapped
longitudinally using DC potentials applied to four printed cir-
cuit boards inserted between the rods. The rods and PCBs are
mounted on two 3D printed endcap pieces.
Preprint submitted to Elsevier April 4, 2019

Figure 1: The current linear RFQ design. Upper - schematic of the trap design including the buer gas containment cylinder (dark grey) around the trapping region,
the 3D printed components (light yellow) and the PCB electrodes (dark yellow). 70% of the losses occur in the region indicated by the red box. Lower - SIMION
simulation showing electric potential contours (red) and A = 70 ion trajectories starting at the beam waist, at 0.034 mbar buer gas pressure (blue). The ions travel
from left to right. Losses are minimal during extraction.
We have demonstrated that polylactic acid biopolymer (PLA)
used in 3D printers is vacuum compatible. Figure 3 shows
the pressure over time in a 3-way T-piece vacuum chamber
with 440 L s
1
pumping capacity. As the ﬁgure shows, when
a 20 mm side length cube of 3D printed PLA was placed in
the vacuum chamber, the pressure decreased to 2 × 10
7
mbar
after 5.5 hours of pumping, equal to the pressure reached by
the empty chamber within the same pumping time. This pres-
sure is compatible with the vacuum requirements of the RFQcb.
Residual gas analysis was performed at 3.9 × 10
8
mbar with
the PLA sample present and at 6.7 × 10
8
mbar when the cham-
ber was empty. No signiﬁcant change was seen in the compo-
sition of the gas after adding the sample, indicating that out-
gassing of the PLA is not signiﬁcant at these pressures. The
RGA spectra are shown in Figure 4. Flow rate calculations in-
dicate the 4 mm apertures on either side of the trapping region
will provide the required dierential pumping to reach pres-
sures below 5 × 10
6
mbar outside the trap with two stages of
dierential pumping and up to 0.1 mbar internal buer gas pres-
sure.
The entire trap is biased to a potential slightly below 30 kV
to decelerate the incoming beam from 30 keV to approximately
100 eV. At 30 keV, the Doppler broadening of the resonant exci-
tation lineshape is comparable to the natural linewidth of many
resonant transitions and a larger platform operating voltage is
not required. SIMION simulations, shown in the lower part of
Figure 1, have indicated that more than 70% of the ion beam
losses occur during the injection-deceleration phase due to the
narrow input aperture required for dierential pumping and col-
lisions with residual buer gas. To mitigate these losses, a se-
ries of PCB ring electrodes were introduced into the model to
produce a longitudinal potential structure of the form V(z) =
V
0
(1 e
αz
). This produces a transverse focussing eect with
a focal point at a position proportional to 1 [13]. The re-
sults of the simulations are shown in Table 1 and an output
emittance plot for A = 70 is shown in Figure 2. These used
an input 30 keV focussed beam with 20 π mm mrad transverse
emittance and a Gaussian radial proﬁle with 0.5 mm standard
deviation. The ions were released from the simulated trap after
2 ms. The transmission eciency at A = 70 was 54% and the
ﬁnal simulated emittance of the cooled beam at this mass was
3. Outlook
The beneﬁts of installing an independent gas-ﬁlled linear
Paul trap at the CRIS beamline have been explained and the de-
sign outlined is under development. Simulations have indicated
the feasibility of a short trap design and have informed possi-
ble design improvements. The design oers a cost eective and
high eciency method to trap ions for radioactive beam stud-
ies. This work also illustrates the possible uses of 3D printed
parts in vacuum, enabling cheaper and faster methods for de-
sign prototyping.
Acknowledgements
The authors would like to thank Colin Reed and Andrew
McFarlane for preparing the PLA printed component and vac-
uum testing samples. This work was supported by the ERC
2

Mass, A Pressure / mbar Transmission probability / % Acceptance / π mm mrad Output emittance / π mm mrad
12 0.012 11 10 16
70 0.034 54 20 1.5
Table 1: Simulated RFQ trapping parameters with total ion transmission probabilities, the input beam acceptance and output beam emittance after reacceleration at
optimised buer gas pressures. A 30 keV focussed input beam with 20 π mm mrad transverse emittance and a Gaussian radial proﬁle with 0.5 mm standard deviation
was used. The ions were released after 2 ms.
Figure 2: A plot showing the displacement along the transverse x-axis vs the
velocity angle relative to the beam axis in the xz-plane, where z is aligned along
the beam axis, of the transmitted A = 70 ions in a SIMION simulation of the
RFQcb (green points). The ellipse showing the RMS transverse emittance in x
is also included (red line).
Figure 3: Pump down curves for an empty chamber (yellow) and for the same
chamber with a 20 mm side length cube of PLA (orange). The chamber con-
taining the PLA sample reached a pressure equal to that of the empty chamber,
2 × 10
7
mbar, after 5.5 hours of pumping, indicated by the pink dashed line.
Features in both curves below the order of 2 × 10
2
s are related to dierent
pumping stages turning on.
Figure 4: A plot showing the partial pressure at each mass detected by a residual
gas analyser of the empty vacuum chamber at 6.7 × 10
8
mbar (red) and the
vacuum chamber containing the PLA test cube at 3.9 × 10
8
mbar (black). No
signiﬁcant change in the RGA spectrum is seen after adding the PLA sample,
indicating that outgassing from the sample is not signiﬁcant at these pressures.
Consolidator Grant No. 648381 and the Science and Technol-
ogy Facilities Council Grants ST/P004423/1 and ST/S002316/1.
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3
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TL;DR: In collinear laser measurements the signal-to-noise ratio has been improved by a factor of 2 x 10(4), allowing spectroscopic measurements to be made with ion-beam fluxes of approximately 50 ions s(-1).
Abstract: A new method has been developed for increasing the sensitivity of collinear laser spectroscopy. The method utilizes an ion-trapping technique in which a continuous low-energy ion beam is cooled and accumulated in a linear Paul trap and subsequently released as a short ( $10--20\ensuremath{\mu}\mathrm{s}$) bunch. In collinear laser measurements the signal-to-noise ratio has been improved by a factor of $2\ifmmode\times\else\texttimes\fi{}{10}^{4}$, allowing spectroscopic measurements to be made with ion-beam fluxes of $\ensuremath{\sim}50\mathrm{ions}{\mathrm{s}}^{\ensuremath{-}1}$. The bunching method has been demonstrated in an on-line isotope shift and hyperfine structure measurement on radioactive ${}^{175}\mathrm{Hf}$.

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Abstract: A gas-filled segmented linear Paul trap has been installed at the focal plane of the high-resolution separator (HRS) at CERN-ISOLDE. As well as providing beams with a reduced transverse emittance, this device is also able to accumulate the ions and release the sample in bunches with a well-defined time structure. This has recently permitted collinear laser spectroscopy with stable and radioactive bunched beams to be demonstrated at ISOLDE. Surface-ionized 39, 44, 46K and 85Rb beams were accelerated to 30keV, mass separated and injected into the trap for subsequent extraction and delivery to the laser setup. The ions were neutralized in a charge exchange cell and excited with a co-propagating laser. The small ion beam emittance allowed focussing in the ion-laser overlap region, which is essential to achieve the best experimental sensitivity. Fluorescent photons were detected by a photomultiplier tube as a frequency scan was taken. A gate (typically 7-12μs wide) was set on the photomultiplier signal to accept the fluorescent photons within the time window defined by the bunch. Thus, using accumulation times of 100ms, the dominant contribution to background due to continuous laser scattering could be reduced by a factor of up to 4×104 .

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