The cooling of beams of exotic nuclei (both in energy spread and in transverse oscillations) is critical to downstream mass spectrometry devices and can be provided by collisions with light gases as in the Radio Frequency Quadrupole Cooler (RFQC). As in other traps, several electromagnetic systems can be used for beam deceleration confinement and deceleration, as a radiofrequency (rf) quadrupole, a magnetic solenoid and electrostatic acceleration. Since rf contributes both to beam cooling and heating, operational parameters should be carefully optimized. The LNL RFQC prototype is going to be placed inside the existing Eltrap solenoid, capable of providing a magnetic flux density component Bz up to 0.2 T, where z is the solenoid axis. Setup progress and related rf component development are reported; in particular simple matching boxes are discussed; the differential gas pumping system is also described

G. Maero

L. Bellan

L. Pranovi

M. Cavenago

M. Comunian

M. Maggiore

M. Rome&apos

N. Panzeri

AIR Universita degli studi di Milano

A RFQ COOLER DEVELOPMENT∗
M. Cavenago†, L.Bellan, M.Comunian, M. Maggiore, L. Pranovi, INFN-LNL, 35020 Legnaro, Italy
G. Maero, N. Panzeri, M. Romé, Dip. Fisica, Univ. MI and INFN-MI, 20133 Milano, Italy
Abstract
The cooling of beams of exotic nuclei (both in energy
spread and in transverse oscillations) is critical to down-
stream mass spectrometry devices and can be provided
by collisions with light gases as in the Radio Frequency
Quadrupole Cooler (RFQC). As in other traps, several
electromagnetic systems can be used for beam decelera-
tion confinement and deceleration, as a radiofrequency (rf)
quadrupole, a magnetic solenoid and electrostatic accelera-
tion. Since rf contributes both to beam cooling and heating,
operational parameters should be carefully optimized. The
LNL RFQC prototype is going to be placed inside the ex-
isting Eltrap solenoid, capable of providing a magnetic flux
density component Bz up to 0.2 T, where z is the solenoid
axis. Setup progress and related rf component development
are reported; in particular simple matching boxes are dis-
cussed; the differential gas pumping system is also described.
INTRODUCTION
In the context of manipulation of beam of exotic (or ra-
dioactive) nuclei [1], the rms energy spread ∆E must be
limited to the order of 1 eV for two reasons: the need of
achieving a large mass resolving power m/∆m  104 in
order to separate isobars and the efficiency of injection into
charge breeders [2], which requires a small ∆E < 5 eV; see
for example equation (8) in [3]. Since the typical energy
spread from plasma ion sources suitable for radioactive nu-
clei production, under installation in the INFN-LNL project
SPES (Selective Production of Exotic Species [1, 4]), is
∆E ∈ [5, 20] eV, a cooling device should be inserted before
high resolution mass separation (HRMS), as for example a
Radiofrequency Quadrupole Cooler (RFQC).
In a RFQC, ion beam with energy Eb and charge state
1+ is stopped by collisions with gas (typically He pressure
p = 1 to 10 Pa); the beam is maintained focused in x, y by
a radiofrequency quadrupole and drifts toward extraction
thanks to an applied electrostatic field Ez < 100 V/m, to be
carefully optimized. Since gas collisions are more effective
at low energy, the RFQC is placed on a high voltage platform
to have E < 200 eV (preferably 100 eV) at injection, and
optics of injection and extraction needs also very challenging
optimizations [5]. Therefore a RFQC prototype was built
at LNL [6] and is going to be tested in the Eltrap facility
of Milan University and INFN, where an axial Bz is also
available, further improving the transverse ion confinement.
Note that HRMS requires an xx ′ rms geometrical emittance
below 0.7 pimm-mrad at 260 keV, equivalent to 2 pimm-mrad
at 25 keV. Ion source transverse rms emittance [4] is in the
∗ Work supported by INFN-Group 5
† cavenago@lnl.infn.it
range [6, 20] pi-mm-mrad for 25 keV Ar1+; therefore it needs
to be cooled too.
Let Vr f the peak rf voltage applied at electrodes with r0
their inner radius; as well known averaging over rf cycles
gives a confinement effect [7, 8], equivalent to an effective
potential Vp (r) = 12mω
2
Mr
2/e with the macromotion angu-
lar frequency
ωM ≡ kq ω√
8
, kq =
4e|Vr f |
mω2r20
(1)
where kq is the Mathieu parameter; a sufficient condition
for motion stability is kq < k1 = 0.91.
Even if the RFQC concept is well established since sev-
eral decades, rules for quantitative design and prediction of
ion transmission are still missing; the large complexity of
the equipment makes development and experimental tests
both necessary and challenging. Our set-up emphasizes ver-
satility of components and diagnostic, as described in the
following.
THE ELTRAP RFQC SETUP
Except for the beam emittance meter and the second fara-
day cup where the RFQC output ion beam is analyzed, all
other beamline elements are mounted on a rigid plugin,
which can be assembled, wired, and tested outside the vac-
uum chamber, and then inserted into the CF250 flanged
Eltrap solenoid bore. The plugin major parts (following
ion travel direction, see Fig. 1 in [8]) are: the alkali metal
ion source, a CF63 flanged 10 kV insulator ceramic break,
a CF250/63 transition where several auxiliary flanges are
provided for feedthroughs (mainly electrical connections),
a machined beam where einzel lens and drift tubes are sup-
ported with insulated PEEK posts and a gas tight housing
box for RFQC is placed; the plugin is about 2 m long, and
slides on spheres to facilitate insertion into Eltrap.
Let φ be the electric potential with respect to the vacuum
chamber, and φe = φ(0, 0, ze) the ion emitter potential, with
ze the ion emitter position. To simplify RFQC connections,
the ion source chassis is floated to (a negative voltage) V0
respect to vacuum chamber, which is the same potential of
the drift tubes electrode V2 and V4. Then the acceleration
voltage V (z) defined as
V (z) = φ(0, 0, ze) − φ(0, 0, z) = φe − φ(0, 0, z) (2)
is mostly due to V0; for the example of a Eb = 5 keV beam,
we plan φe = 0.2 kV and V0 = −4.8 kV. This configuration
complicates source operation; but we also have the advantage
that einzel lens voltages V1 = −0.2 kV and V2 = −1.3 kV
can be conveniently produced by USB powered modules,
and accuracy of the measured φe −V1 focussing voltage can
9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW Publishing
ISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-THPAL008
07 Accelerator Technology
T06 Room Temperature RF
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Figure 1: (a) The plugin parts: note ion source, injection beam-line and RFQC; (b) zoom on the RFQC input, with box
moved aside for better visibility.
Figure 2: The extraction beam-line from RFQC, beam diagnostic and gas pumping systems
reach values below 100 mVrms (that is, 2×10−5 parts of Vb).
The same applies to the injection lensesV5 andV6. Electrode
V7 and V8 are the end plates (within 100 V from ground) of
the gas enclosure (RFQC box) of the RFQC; the rest of the
RFQC box is grounded. A first faraday cup can be inserted
in drift tube V4; ion source can provide over Ii = 1000 nA
of Cs+ ions, even if Ii ≤ 200 nA is a suggested limit when
the emittance meter is used.
For Eltrap logistic, themore convenient insertion direction
for the emittance meter results the horizontal (note the figure
breaks in Fig. 2). Extraction beam line is formed by plate
V8, lens V9, drift tube V10, emittance meter (at potential V11)
and the second faraday cup (under construction), see fig 2.
The emittance meter is mounted on a 40 kV ceramic break
on a CF200 cross, so that its voltage is adjustable; anyway
operation with emittance meter and drift V10 connected to
V0 seems the better choice.
The alkali ion source has a negligible ion energy spread
(0.4 eV) so that any ion energy Eb distribution can be re-
constructed by scanning φe. It can be also argued that, by
scanning the second faraday cup voltage Vd and recording
the current Io (Vd), a sort of discrimination of the ion energy
Ed after the RFQC can be obtained; in other words, the
second faraday cup can approximate (or be replaced with)
a retarding field energy analyzer. This ambitious plan will
clearly request a long data collection phase to get the full
kernel K characterizing the RFQC
I ′o (Vd)
max Io
≡ f (Vd) =
∫
dVb K (Vd,Vb; p,Vr f ) g(Vb) (3)
where Vb = Eb/e is the ion acceleration voltage, g(Vb) [or
f (Vd) ] is the distribution of input ions in Vb [or output ions
in Vd], and ′ indicates the derivative. Ion source estimated
xx ′ rms geometrical emittance is about 5 pi mm-mrad at
5 keV, which is equivalent to the previously stated 0.7 pi
mm-mrad at 260 keV (and to an ion transverse temperature
of 0.2 eV). Among heating effects in RFQC, we have gas
collisions [9]: even if Cs+ transfer energy (to He), collisions
may contribute an increase of vx and vy , that is of transverse
temperature (in the sub-eV scale), so that a Monte Carlo
module must be added to ray-tracing simulation [8]. The
9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW Publishing
ISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-THPAL008
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07 Accelerator Technology
T06 Room Temperature RF
Figure 3: The multiplexer, connecting the 40 electrodes to
12 input pins; capacitors may have equal values, C = 1 nF
in the example tested, as well as all resistors have equal
resistance R = 0.47MΩ.
Figure 4: Some matching box schemes: A) low power, with
output balance by ferrite core transformers; B) air core;
C) air core, with output balance improved by two variable
capacitors.
CF200 cross also connects to the 550 l/s pump for Helium
differential pumping: it is envisioned that gas escaping from
the V7 and V8 beam holes accumulates in antechambers (see
fig 2) and is conveyed to pump region by 20 mm diameter
tubes. The gas finally escaping towards beamlines is, of
course, a drawback of the RFQC cooler concept, and is to be
pumped by standard 330 l/s pump of the Eltrap machine. It
is planned that the RQFC box be also connected to the plugin
flange by two smaller tubes (gas tight): one for He injection,
one for pressure p measure; after suitable pre-regulation
near atmospheric pressure and rupture disks, He input will
be dosed by a mass flow controller, interlocked to pumps.
With the completion of the first faraday cup this March,
the RFQC is almost ready for wiring test and insertion. Un-
fortunately the test of ion source in a separated test stand
evidenced a deterioration of Cs emitter, so that installation
is delayed.
Electric and RF Fields
It is usually argued that rf voltage is bounded by Vr f <
k1mω2r20/e by stability, so that the use of larger frequencies
allows the use of larger voltages; actually 4 MHz is more
than enough, since we get Vr f < 4 kV for our geometry
(r0 = 4.5 mm), while breakdown consideration suggests far
lower voltages. We note that, at constant rf voltage, in order
to increase the ponderomotive potential Vp, use of lower
frequencies is advantageous, since macromotion frequency
ωM scales as 1/ω thanks to eq. (1); from [8] simulations,
a value Vp (r0)  2.5 V is sufficient in our geometry; this
translates to Vr f  0.2 kV at 4 MHz.
Since the RFQC has 10 sections of four electrodes each,
in principle we have to supply 40 rf voltage and 40 dc bias,
with 40 high voltage feedthroughs. In practice, we use mul-
tiplexing, see Fig. 3, so that only two rf voltages and 10
DC bias need to be supplied; moreover the RFQC is held
near ground potential, to simplify insulation and cabling
tasks; the multiplexer circuitry in vacuum consists of capaci-
tors and resistors, with a thermal sink through mica washers
and the RFQC mounting frame. This multiplexer has a flat
response in the MHz range.
Note that RFQC itself is typically a largely reactive load,
say an equivalent circuitCp = 200 pF in parallel with Rp = 2
kΩ from very preliminary RFQC test cabling for each of the
two multiplexer rf inputs; we can add a contribution of the
order of 100 pF for the cables between RFQC and matching
box MB. Note that an older design philosophy called for
an oversized rf amplifier with class A output, so that the
RFQC (an unmatched load) could be driven without any
matching network (or box); a power rating Pr f  0.5kW
was envisioned. We observed that we have to supply two
rf counter-phase voltages, which naturally lead to the use
of transformers, and therefore of a matching box (MB). In
our new design philosophy, the matching box allows to use
a single output 50Ω matched amplifier to drive the two rf
channels, with obvious economy in the amplifier. A MB
prototype capable of providing Vr f  50 V with Pr f =
1 W was developed, based on broad-band coaxial cable
transformers (with small MnZn ferrites), see circuit A in
figure 4. Precisely, in a test with RFQC and a multiplexer
based on common resistors, with the rf generator set for
1 Vrms output on a matched load, we have input voltage
V 0
r f
= 0.94 Vrms at resonance f = 3.95MHz, with outputs
V 1r f = 4.58 Vrms (phase delay 23
0 to generator trigger) and
V 1r f = 4.69 Vrms (phase delay -157
0 as requested). After
disconnecting the RQFC, the resonance of MB becomes
f = 5.1MHz.
A more robust air-core matching box is under construc-
tion. A > 50W rf generator seems suitable with reasonable
margins. On the other hand, tuning of matching box, sta-
bility of rf voltage, and RFQC cabling becomes strongly
correlated, so that the RQFC cabling should be completed
and tested at low voltages, before the finalization of the full-
power matching box.
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9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW Publishing
ISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-THPAL008
07 Accelerator Technology
T06 Room Temperature RF
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ISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-THPAL008
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T06 Room Temperature RF


A RFQ Cooler Development

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A RFQ Cooler Development

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