Abstract
In the framework of the upgrade of the SPARC_LAB facility at INFN-LNF (EuSPARC proposal) and of the
EuPRAXIA@SPARC_LAB design study, it is foreseen an high gradient LINAC. One of the most suitable option is to
realize it in X-Band. A preliminary design study of the accelerating structures and power distribution system has
been performed. It is based on short travelling wave (TW) accelerating structures operating on the 2π/3 mode
and feed by klystron and pulse compressor systems. The main parameters of the structures and LINAC are
presented with the basic RF LINAC layout.
EuSPARC and EuPRAXIA projects
In the framework of the Horizon 2020, a Design Study called EuPRAXIA (“European Plasma Research Accelerator
with eXcellence In Applications“), has been funded to bring together for the first time novel acceleration
schemes, based for instance on plasmas, modern lasers, the latest correction/feedback technologies and largescale user areas (INFRADEV-1-2014). Such a research infrastructure would achieve the required quantum leap in
accelerator technology towards more compact and more cost-effective accelerators, opening new horizons for
applications and research. In this scenario the SPARC_LAB upgrade, named EuSPARC, is one of the possible
candidates to host EuPRAXIA@SPARC_LAB. High brightness photo-injectors are fundamental for the successful
development of plasma-based accelerators whereas external injection schemes are considered, i.e. particle
beam driven and laser driven plasma wakefield accelerators (PWFA and LWFA, respectively), since the ultimate
beam brightness and its stability and reproducibility are strongly influenced by the RF-generated electron beam.
To take profit of the compactness of these novel acceleration techniques, a X-Band LINAC is under studying.
LINAC layout and parameters
S-Band
RF Gun
2 S-Band TW
structures
EL=580 MeV (Phase 1)
EL=1060 MeV (Phase 2)
E0~100 MeV
LINAC 2
LINAC 1
Bunch
Compressor
The preliminary LINAC layout foresees an S-Band Gun, two S-Band TW structures (3 m long) and an XBand booster with a bunch compressor. The beam energy after the S-band injector is 100 MeV. Two
cases of X-Band LINAC energy gain have been studied for two different phases: Phase 1, Egain=480 MeV
and Phase 2, Egain=960 MeV.
Our wishes
Present status
X-Band LINAC parameters
X-Band LINAC parameters
Lt
12 m
Lt
16 m
E0
100 MeV
E0
100 MeV
Phase 1
Phase 2
Egain
480 MeV
960 MeV
<G>
40 MV/m
EL
580 MeV
Phase 1
Phase 2
Ultimate
Egain
480 MeV
960 MeV
1280 MeV
80 MV/m
<G>
30 MV/m
60 MV/m
80 MV/m
1060 MeV
EL
580 MeV
1060 MeV
1380 MeV
LINAC DESIGN GOALS
The Linac design goals to be reported in the CDR are the optimized design of the Xband TW structures on the base of the parametrized performances of the basic
accelerating cell obtained with simulations, and the proposed RF layout for the
machine. This means:
• Define the optimal filling time and length of the structures;
• Make our choice between Constant Gradient or Constant Impedance options (or
eventually define a suitable trade off in between);
• Define the layout of the complete RF system on the base of:
 the TW section optimized parameters
 the chosen scenario (available space for the Linac RF)
 the available RF power sources (klystrons)
Preliminary SLED and X-Band LINAC parameters
Example: compressed pulse of 100 ns
3 dB coupler
Ek
From Klystron
Eout
To accelerator
The RF LINAC layout is based on klystrons with SLED that
feed several TW accelerating structures operating on the
2π/3 mode at a frequency of 11,9942 GHz.
SLED parameters
frequency
11.9942 GHz
tk
1.5 μs
Q0
180000
Qe
20000
Effective shunt impedance and accelerating gradient
CI
(Constant
Impedance)
CG
(Constant
Gradient)
Fixing the quality factor Q of the cells equal to 6500 it is possible to calculate the normalized effective
shunt impedance Rs/R as a function of the section attenuation τs. The choice of τs fixes the filling time of
the structure and hence the compressed pulse length after the SLED. If we consider the optimum τs value
(τs0) it is possible to calculate the accelerating gradient profile after one filling time.
Single cell study with HFSS
r
t
b
Geometrical parameters
t/2
a
a [mm]
3÷6
b [mm]
10,075 ÷ 11,480
d [mm]
8,332 (2π/3 mode)
r [mm]
1
t [mm]
2.5
d
The single cell parameters (α, R, vg/c, Q, Scmax/E2acc) have been calculated with HFSS as a
function of the iris radius.
Single cell study with HFSS
Field attenuation constant /
Normalized group velocity
Maximum value of normalized
modified Poynting vector
Shunt impedance
per unit length
Quality factor
Constant Impedance X-Band LINAC parameters
The final LINAC parameters as a function of the iris radius have been calculated using the simulated single
cell parameters. The minimum iris radius has to be fixed according to the beam dynamics calculations and
single bunch beam break up limits. Similar results can be obtained considering CG structures, in this case
a is the average iris radius <a>. CG structures exhibit lower Scmax.
KLYSTRON
MODULATOR
HALL
MODE
CONVERTER
CIRCULAR WAVEGUIDE
SLED
MODE
CONVERTER
LINAC
HALL
LINAC MODULE FOR PHASE 1
Results and RF layout
(12 m available / 24 TW sections)
According to the beam dynamics calculations, <a> has
been fixed to 3,2 mm obtaining 24 structures (8 in LINAC
1 and 16 in LINAC 2). Each structure is 500 mm long.
Considering a waveguide attenuation of 20%, for a 50
MW klystron the net available power is 40 MW. For Phase
1 we propose 1 klystron every 8 structures, for Phase 2
we propose 1 klystron every 4 structures. For phase 1 we
have also extra power to reach an higher gradient.
LINAC MODULE FOR PHASE 2
Constant Gradient LINAC parameters
<a> [mm]
3.2
Ls [mm]
500
Qe
21400
vg/c [%]
2.5 – 0.77
Tp [ns]
121
Ns
24
Phase 1
Phase 2
Egain [MeV]
480 (680)
960
Ptot [MW]
56
223
N° klystrons
3 (1 every 8)
6 (1 every 4)
KLYSTRON
MODULATOR
HALL
MODE CONVERTER
CIRCULAR WAVEGUIDE
MODE CONVERTER
SLED
LINAC
HALL
LINAC MODULE FOR PHASE 2
Constant Gradient LINAC parameters
LINAC MODULE FOR PHASE 1
Results and RF layout
(16 m available / 32 TW sections)
For Phase 1 we propose 1 klystron every 8
structures, for Phase 2 we propose 1
klystron every 4 structures. For phase 1 we
have also extra power to reach an higher
gradient.
<a> [mm]
3.2
Ls [mm]
500
Qe
21400
vg/c [%] /Tp [ns]
2.5 – 0.77 / 121
Ns
32
Phase 1
Phase 2
ultimate
Egain [MeV]
640
905
1280
Ptot [MW]
74
149
297
N° klystrons
2 (1 every 16)
4 (1 every 8)
8 (1 every 4)