1. Abstract EuSPARC and EuPRAXIA projects
In the framework of the upgrade of the SPARC_LAB facility at INFN-LNF (EuSPARC proposal) and of the 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 large-scale user areas (INFRADEV ). 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 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.

2. X-Band LINAC parameters X-Band LINAC parameters
LINAC layout and parameters
E0~100 MeV
EL=580 MeV (Phase 1)
EL=1060 MeV (Phase 2)
LINAC 1
LINAC 2
S-Band
RF Gun
2 S-Band TW
structures
Bunch
Compressor
The preliminary LINAC layout foresees an S-Band Gun, two S-Band TW structures (3 m long) and an X-Band 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.
Present status
Our wishes
X-Band LINAC parameters Lt
12 m E0
100 MeV
Phase 1
Phase 2
Egain
480 MeV
960 MeV
<G>
40 MV/m
80 MV/m EL
580 MeV
1060 MeV
X-Band LINAC parameters Lt
16 m E0
100 MeV
Phase 1
Phase 2
Ultimate
Egain
480 MeV
960 MeV
1280 MeV
<G>
30 MV/m
60 MV/m
80 MV/m EL
580 MeV
1060 MeV
1380 MeV

3. LINAC DESIGN GOALS
The Linac design goals to be reported in the CDR are the optimized design of the X- band 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)

4. Preliminary SLED and X-Band LINAC parameters
Example: compressed pulse of 100 ns
3 dB coupler
SLED parameters
frequency
GHz tk
1.5 μs Q0
180000 Qe
20000 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.

5. Effective shunt impedance and accelerating gradientCI
(Constant
Impedance) CG
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.

6. Geometrical parameters
Single cell study with HFSS d b a t r
t/2
Geometrical parameters
a [mm]
3 ÷ 6
b [mm]
10,075 ÷ 11,480
d [mm]
8,332 (2π/3 mode)
r [mm] 1
t [mm]
2.5
The single cell parameters (α, R, vg/c, Q, Scmax/E2acc) have been calculated with HFSS as a function of the iris radius.

7. 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

8. 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.

9. Constant Gradient LINAC parameters (12 m available / 24 TW sections)
KLYSTRON
SLED
MODE
CONVERTER
CIRCULAR WAVEGUIDE
LINAC MODULE FOR PHASE 1
MODULATOR
HALL
LINAC
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)
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.

10. Constant Gradient LINAC parameters (16 m available / 32 TW sections)
LINAC MODULE FOR PHASE 1
LINAC MODULE FOR PHASE 2
KLYSTRON
MODULATOR
HALL
MODE CONVERTER
CIRCULAR WAVEGUIDE
MODE CONVERTER
SLED
LINAC
HALL
Constant Gradient LINAC parameters
<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)
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.