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First acceleration of a proton beam in a side coupled drift tube linac
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2015 EPL 111 14002
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July 2015
EPL, 111 (2015) 14002
doi: 10.1209/0295-5075/111/14002
www.epljournal.org
First acceleration of a proton beam in a side coupled
drift tube linac
C. Ronsivalle1(a) , L. Picardi1 , A. Ampollini1 , G. Bazzano1 , F. Marracino1 , P. Nenzi1 , C. Snels1 ,
V. Surrenti1 , M. Vadrucci1 and F. Ambrosini2
1
2
ENEA - Via Enrico Fermi, 00044 Frascati, Rome, Italy
University La Sapienza-Roma I - Piazza Aldo Moro 5, 00185 Roma, Italy
received 3 March 2015; accepted in final form 3 July 2015
published online 29 July 2015
PACS
PACS
PACS
41.75.Lx – Other advanced accelerator concepts
41.75.Ak – Positive-ion beams
87.56.-v – Radiation therapy equipment
Abstract – We report the first experiment aimed at the demonstration of low-energy protons acceleration by a high-efficiency S-band RF linear accelerator. The proton beam has been accelerated
from 7 to 11.6 MeV by a 1 meter long SCDTL (Side Coupled Drift Tube Linac) module powered
with 1.3 MW. The experiment has been done in the framework of the Italian TOP-IMPLART
(Oncological Therapy with Protons-Intensity Modulated Proton Therapy Linear Accelerator for
Radio-Therapy) project devoted to the realization of a proton therapy centre based on a proton linear accelerator for intensity modulated cancer treatments to be installed at IRE-IFO, the
largest oncological hospital in Rome. It is the first proton therapy facility employing a full linear
accelerator scheme based on high-frequency technology.
c EPLA, 2015
Copyright The application of S-band (3 GHz) linac technology in
accelerators designed for proton therapy has been proposed since the first years of the 1990s [1,2]. The use
of high frequency increases simultaneously Radio Frequency (RF) efficiency (thanks to the relation with shunt
impedance Z ∼ f 1/2 ) and maximum allowed accelerating gradient, providing the design of compact therapy
units. The beam currents required for cancer treatments
are orders of magnitude lower than the ones required by
nuclear physics applications (a few nA vs. a few mA).
Such low current magnitudes allow the design of accelerating RF structures more compact than UHF or L-band
(200–800 MHz) commonly employed in high-power beam
generation and with smaller bore holes.
Previous designs of RF proton accelerators for medical applications used a sequence of S-band coupled cavity linacs (CCL) only for the high-energy section (from
60–70 MeV up to the final energy of 230–250 MeV). They
are conceptually similar to the ones employed in conventional electron clinical machines for radio-therapy with
photons and electrons. Their cell length that is equal to
βλ/2 (β = relative particle velocity, λ = RF wavelength)
becomes prohibitively short (a few mm) at energies below
(a) E-mail:
[email protected]
60 MeV (particularly below 30 MeV) with a drastic drop
in efficiency.
Previous designs used two distinct approaches for the
low-medium energy section (injector) of the accelerator: highly invasive injectors such as long UHF linacs or
commercial cyclotrons [3] typically used for radioisotope
production. Although proposed and sometimes strongly
supported by several papers, these systems have not been
realized nor fully tested yet. Our group introduced and
patented in Europe [4] a 3 GHz standing wave accelerating structure named SCDTL (Side Coupled Drift Tube
Linac) characterized by a high shunt impedance in the
low-energy range (0.1 < β < 0.35) [5–8]. This structure, composed by small Drift Tube Linac (DTL) tanks
coupled by side coupling cavities, with inter-cavity focusing Permanent Magnet Quadrupoles (PMQs), can be included in the same development stream as the CCDTL
(Cavity Coupled Drift Tube Linac) firstly proposed for
high-current linacs and patented by Los Alamos [9], but
has been sized for low-intensity beams therefore allowing
a more relaxed FODO lattice with more space between
PMQs, larger number of cells/tank, smaller bore holes
and therefore suitable for high RF frequency operation.
SCDTL indeed allows using 3 GHz frequency for the whole
accelerator, down to an energy range of 4–7 MeV. The
14002-p1
C. Ronsivalle et al.
Fig. 1: (Colour on-line) Sketch of the interior of a SCDTL
structure with 4 gaps/tank.
dedicated S-band linear accelerator is considered a major
improvement for proton therapy, with respect to existing commercial facilities based on circular machines (cyclotrons and synchrotrons). The main advantages of this
technology are: modularity of the structure, standard and
cost effective RF technology, low weight, beam losses only
at low energies, pulsed operation at high repetition rate
and rapid energy variation, well suited to support scanning applications. The low transverse beam emittance
(typically 0.2π · mm · mrad rms normalized) allows tiny
gaps in the dipole magnets of the transport lines, thus
considerably reducing their weight and cost. Moreover,
the construction procedures (machining and brazing) are
well known and technology transfer from research centres
to industry can be performed effectively. In this context,
the good performance of the SCDTL medium energy part
of the linac is a key issue.
This structure (fig. 1) consists of short DTL tanks with
a small number of cells (4–7) resonantly coupled by side
cavities.
The operating mode of the total structure is π/2 with
coupling cavities field vanishing because of the excitation
with opposite fields by the neighboring tanks. The tanks
operate in phase opposition with a relative distance of
(2n+1)βλ/2 (n = integer). The cells inside a tank operate
in zero mode with a length βλ. The bore hole radius is
very small (2–2.5 mm) and also the drift tubes are very
small (12 mm in diameter), and therefore the peak power
consumption can be drastically reduced with respect to
an equivalent CCL structure. Figure 2 compares CCL
and SCDTL structure dimensions for different energies:
at low energy CCL efficiency is dramatically affected by
the losses on the walls separating the very tiny βλ/2 long
cells, whereas DTL tanks, containing several cells, can be
built longer providing high RF shunt impedance, reduced
by the stem supporting the drift tubes by 15–20%. The
only drawback is the reduction of the average gradient due
to the larger cell length (βλ instead of βλ/2).
A comparison between the shunt impedance of SCDTL
and CCL structures vs. particle relative velocity is reported in [5]. Unlike the low-frequency DTL in which the
focusing is provided by quadrupoles placed inside the drift
tubes, in this type of structure, where the small dimension
Fig. 2: Comparison of CCL and SCDTL at different energies.
of drift tubes forbids this arrangement, short PMQs (3 cm
long, max gradient ∼ 200 T/m) are accommodated in the
inter-tank space. No degradation of PMQ magnetic field is
expected because of the very low accelerated current and
consequently negligible stray radiation field all around the
machine. The fixed gradient is not a problem for the beam
dynamics if the tolerances in terms of strength (±4%) and
alignment (±50 μm) are satisfied. However, the use of external PMQs allows an easy exchange both for reparation
and for optimization of beam dynamics.
The side coupling cavities have to be long enough to
permit the insertion of PMQs on the axis. Their length
range between 70 and 100 mm, being very close to one
wavelength, brings the TM011 mode into the pass-band
of the system [10]. A central axial post coupler, whose
dimensions are optimized by 3D electromagnetic analysis,
is used to move it out of the operational pass-band.
Beam dynamics design for the SCDTL is accomplished
in two steps. First, the DESIGN code [11] performs
single-particle dynamics that receives as input the values
of the synchronous phase, the maximum PMQs gradient
and the dependence of transit time factors on particle
β in the DTL tanks (as retrieved by the SUPERFISH
code [12]), and determines the main parameters of the
stand-alone structure in terms of tanks length, number of cells/tank, quadrupoles gradient. Then, by the
LINAC code [11], multi-particle dynamics calculations in
the DESIGN-generated structure are done in order to
check the quality of the design, to optimize the matching with the injector and to perform error and tolerances
studies. Both codes DESIGN and LINAC were specifically developed by Crandall on the basis of the models implemented in the well-known PARMILA code [13] largely
used for DTLs design. The absence of space charge effects,
due to the very low currents required by proton therapy,
simplifies the dynamics, but other problems arise because
of the use of high frequency at very low energy, specifically the coupling of longitudinal and transverse motion
that can increase the beam emittance. The dependence
on phase of the RF defocusing force given by [14]
14002-p2
Fr =
E0 I1 ωr sin φ
βcγ
(1)
First acceleration of a proton beam in a side coupled drift tube linac
6000
5000
4000
3000
2000
1000
0
5
Fig. 3: (Colour on-line) Layout of the 150 MeV TOPIMPLART proton linear accelerator at ENEA-Frascati.
is stronger at low energies. In addition, an unsustainable emittance growth, that would eventually produce a
complete loss of the beam could be induced by parametric resonances [15] if the longitudinal phase advance in a
period L
2πqE0 T sin(−ϕs )L
(2)
σl =
mc2 (βγ)3 λ
is a multiple of the transverse phase advance that is typically around 70◦ . This means that a particular attention
is required in the choice of the number of cells/tank, tank
inter-distance and average gradient. In the initial part of
the SCDTL we limit the average gradient and the number of cells per tank, and gradually increase both of them
with the energy. This choice maximizes the transverse acceptance minimizing the FODO period compatibly with
the space required for the allocation of external PMQs
(>68.5 mm, considering also other mechanical constraints)
and the maximum gradient of PMQs (200 T/m). The synchronous phase has been chosen as ϕs = −19.5◦ corresponding to a phase acceptance of 3|ϕs | = 58.5◦ . This is
a good compromise between the need to get a good phase
acceptance and to limit the RF defocusing in the tanks.
The resulting 100% transverse unnormalized acceptance of
the SCDTL structure for a bore hole radius of r = 2 mm
is given by
A=
r2
βTwiss
max
= 8.7π · mm · mrad,
(3)
where βTwiss max is the maximum value of the Twiss parameter in the middle of the first PMQ where αTwiss = 0.
The above criteria constitute the basis of the SCDTL
structure design whose first operation is described in this
letter.
The experiment has been done in the framework of
the TOP-IMPLART (Oncological Therapy with ProtonsIntensity Modulated Proton Therapy Linear Accelerator
for Radio-Therapy) project aiming at realizing a compact
high-frequency 230 MeV linac for proton therapy [16]. The
IMPLART segment up to 150 MeV, whose basic layout is
shown in fig. 3, is under construction and installation at
ENEA-Frascati, chosen as test site for its validation before
the transfer to IRE-IFO-Rome hospital, that will be the
clinical user.
6
7
8
9
energy(MeV)
10
11
12
Fig. 4: (Colour on-line) Computed energy spectrum at the
SCDTL-1 exit.
Table 1: TOP-IMPLART SCDTL parameters.
Module Module Module Module
1
2
3
4
Number of Tanks
9
7
5
5
Gaps/tank
4
5
6
6
Bore hole radius
2
2
2.5
3
Distance between
5.5
4.5
3.5
3.5
tanks/βλ
Energy out, MeV
11.6
18
27
35
Power, MW
1.3
1.6
2.2
2
Length, m
1.109
1.082
1.353
1.105
The proton beam is delivered by a commercial injector (AccSys-HITACHI Model PL-7) operating at 425 MHz
consisting in a 30 keV duoplasmatron source, a 3 MeV
RFQ and a 7 MeV DTL [17] followed by a sequence of
2997.92 MHz linear modules boosting the beam up to the
final energy. In the current configuration the injector is
able to deliver pulses with currents up to 160 μA. The
injector is followed by a Low-Energy Beam Transport
(LEBT) consisting of four electromagnetic quadrupoles
matching the beam to the high-frequency booster. The
7–35 MeV section of the high-frequency linac employs four
SCDTL modules whose main parameters are reported in
table 1.
The first module SCDTL-1 consists in 9 DTL tanks
coupled by 8 side cavities with an array of 9 removable
PMQs with a gradient around 197 T/m [18] and an effective length of 30 mm arranged in a DOFO-like scheme
aligned with a precision greater than ±50 μm: the first
PMQ is placed at the entrance of the structure and the
other eight are in the inter-tank space. The proton beam
energy spectrum at the SCDTL-1 exit computed by beam
dynamics simulations is shown in fig. 4. Up to 42% of the
exit beam is effectively accelerated to the design average
energy of 11.63 MeV, while the remaining portion is a tail
at low energy composed by off-phase protons that will be
lost in the next module.
14002-p3
C. Ronsivalle et al.
Fig. 6: (Colour on-line) (Left) PMQ mounted in the inter-tank
space; (right) beam spot at the SCDTL-1 exit: the pink area
is a 10 mm diameter alumina disk.
Accelerated charge normalized
to maximum value (%)
120
100
Fig. 5: (Colour on-line) SCDTL-1 module.
The computed transmission, defined as the ratio between the accelerated and the injected particles in the
SCDTL, is 33%, 16%, 13%, 12% at the end of each
SCDTL module. Beam dynamics simulations show that
the beam can be transported without no further losses
up to 150 MeV. The low value of transmission is caused
by the longitudinal mismatch between the injector and
the high-frequency booster. It is produced by the large
frequency difference between the two structures and is increased by the lengthening of the injector bunch caused by
the velocity spread in the 2.5 m long LEBT. The resulting bunch length at the SCDTL-1 entrance covers three
periods of 2997.92 MHz frequency. Since the two frequencies (425 MHz and 2997.92 MHz) are not in harmonic relation and therefore are not locked in phase, the quite
long injector bunch allows to limit the jitter of the charge
accelerated by the SCDTL structure. The drawback is a
strong reduction of the capture efficiency. However, this
issue is not critical because the required output current is
very low for the injector current capabilities: average currents of 2–5 nA required by proton therapy are achieved
by 2.5–6.5 μA in a 4 μs long pulse at a repetition frequency
of 200 Hz.
The SCDTL-1 module (fig. 5) has been mounted,
aligned and tested with protons. Figure 6 shows on the
left one of the PMQs mounted in the inter-tank space
and on the right the beam spot on a fluorescent screen
at the exit. The elliptical beam shape is determined by
the last PMQ traveled that focuses the protons in vertical
direction.
The transmitted proton charge was measured using a
Faraday cup placed 22.8 mm from the exit of the last tank
of SCDTL-1. The output design energy of 11.63 MeV is
achieved for an input power of 1.3 MW, corresponding to
the maximum beam transmission according to simulations
(fig. 7).
The energy has been measured from the proton range
in aluminum. The measurement is based on the detection
of the transmitted charge through aluminum targets of
80
60
40
Simulaon
20
Measurements
0
0.8
0.9
1
1.1 1.2 1.3
Power (MW)
1.4
1.5
1.6
Fig. 7: (Colour on-line) Charge at the SCDTL-1 exit after an
aluminum foil 700 μm thick vs. RF power.
different thicknesses. Protons gradually lose their energy
due to interactions with atomic electrons and nuclei and
stop if the absorber is thick enough. The beam average
energy is related to the projected range defined as the
absorber thickness corresponding to a transmission of
50% (R50). We used a stack of different aluminum foils
acquired from LEBOW Company [19] with nominal
thickness of 500 μm, 100 μm, 30 μm, 7 μm, 4 μm and a
certified purity of 99.95% for the first three and 99% for
the last two. The effective thickness was measured with a
balance from the weight/area ratio with a precision within
1%. In the experimental setup the beam exiting from
SCDTL-1 passes through a 50 μm thick Kapton window,
6 mm of air, the variable thickness of aluminum target and
22.5 mm of air before being collected by the Faraday cup.
Beam dynamics calculations show that the PMQ array is able to transport a fraction of the injected protons even in the absence of acceleration, so two measurement sets were taken, with and without RF power applied to the structure. The results are reported in fig. 8
and are compared with SRIM [20] code simulations of
the experimental setup. In the absence of accelerating
field, the beam is completely absorbed after 315 μm of aluminum. With acceleration, the measurements start from
an aluminum thickness of 660 μm in order to completely
absorb the low-energy tail and select the exiting beam
portion consisting of effectively accelerated protons. As
many as 810 μm are needed to stop the beam completely.
The experimental data for a 36 μA pulsed beam without
14002-p4
First acceleration of a proton beam in a side coupled drift tube linac
Transmied current (µ
µ A)
45
40
measurements RF off
35
30
measurements RF on
25
SRIM 11.63 MeV
20
SRIM 7 MeV
15
10
5
0
100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850
Aluminium thickness (µA)
Fig. 8: (Colour on-line) Energy measurement: charge transmission curves with and without acceleration.
acceleration and a 11 μA beam when 1.3 MW RF power
is put into the structure are in agreement with simulated
curves corresponding respectively to an average energy of
7 MeV (the injector energy) and 11.63 MeV (the design
energy).
In this letter we have reported the experimental demonstration of the acceleration capability of the 3 GHz SCDTL
structure of low β protons.
An experimental test of proton acceleration by 3 GHz
structures was already pursued in the past [21] but on
CCL structures and with an energy jump of 11 MeV, much
lower than the injection energy (62 MeV), with a really
negligible current (a few nA, pulsed), and with no transverse focusing lattice installed. Here the demonstration is
complete and effective from the point of view of the further
acceleration stages.
Further activities will concern the operation of the following SCDTL modules up to 35 MeV and on the study of
a simpler injection subsystem consisting of a lower-energy
sub-harmonic injector followed by a short matching section in order to enhance the transmission from the injector also up to more than 90% with a better longitudinal
matching [6] to the high-frequency booster.
Moreover, the low beam losses that occur after the
SCDTL second module (1–2%) and the low average energy of the lost particles mainly around 7 MeV suggest a
possible evolution to a locally shielded accelerator with
heavy shielding limited only to the treatment room. Since
the impact of shielding is heavy on installation costs, we
consider this arrangement an improvement for a low-cost,
compact, single-room proton therapy facility.
∗∗∗
This work has been granted by Regione Lazio under the
agreement “TOP-IMPLART Project”.
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