Xband RF Linac technology Alessandro Gallo on behalf
X-band RF Linac technology Alessandro Gallo on behalf of the Eu. PRAXIA@SPARC_Lab Linac Team Eu. PRAXIA@SPARC_LAB CDR Review Committee Meeting 27 -28 November 2018 INFN Frascati 1
Eu. PRAXIA@SPARC_LAB CDR Review Committee Meeting 27 -28 November 2018 INFN Frascati Why X-band? A. Gallo: X-band RF Linac technology Compactness! only ≈ 25 m physically available for the main linac modules, then ≈ 16 m of available active length baseline operational RF gradient beyond any other existing facility Accelerator+plasma 55 m Lasers FEL+experiment 40 m Experiments with beam and lasers Users 27 m 35 m Þ 500 Me. V by RF Linac + 500 Me. V by Plasma (Eu. PRAXIA@SPARC_LAB) Þ 1 Ge. V by high gradient RF Linac only (Eu. SPARC) 130 m 2
Eu. PRAXIA@SPARC_LAB CDR Review Committee Meeting 27 -28 November 2018 INFN Frascati A. Gallo: X-band RF Linac technology Technology choice X-band (≈12 GHz): • • • X-band klystron R&D effort led by CERN as baseline for the multi-Te. V collider CLIC project; relevant progress in the last years allows exceeding 100 MV/m on many prototype sections with tolerable breakdown rates; intrinsic higher efficiency and lower tendency to discharge; no facility based on this technology operating yet; cost of components still quite high, but decreasing with standardization C-band(≈6 GHz): • • baseline adopted by some modern facilities (Spring 8 SACLA and PSI SWISSFEL); lower industrial costs; no experimental demonstration of accelerating gradients in the 50÷ 100 MV/m range no specific R&D efforts to reach ultra-high gradients; X-band adopted and INFN – CERN official partnership established 3
Eu. PRAXIA@SPARC_LAB CDR Review Committee Meeting 27 -28 November 2018 INFN Frascati rd a o B N F IN 2017 y d b. 22, e v o Dec r p ap s on y l l ia ector c i f of f Dir o A. Gallo: X-band RF Linac technology INFN – CERN official partnership on X-band RF development 4
Eu. PRAXIA@SPARC_LAB CDR Review Committee Meeting 27 -28 November 2018 INFN Frascati A. Gallo: X-band RF Linac technology As part of the CERN-INFN collaboration agreement, CERN will support the construction of a X-band high power test stand (the Frascati X-box, duplication of the CERN X-box #2) INFN Frascati will get one klystron, one pulse compressor, plus some other components, on loan. SPARC_LAB Building #7 4. 4 m 5 m it will be located in LNF building #7, very close to the SPARC_LAB area, formerly used for testing and conditioning of the DAFNE RF power plants and cavities 5
Eu. PRAXIA@SPARC_LAB CDR Review Committee Meeting 27 -28 November 2018 INFN Frascati A. Gallo: X-band RF Linac technology Design of the Eu. PRAXIA@SPARC_Lab X-band Accelerating Structure: Workflow • Baseline accelerating gradient: ≈ 60 MV/m (to > 80 MV/m doubling RF power) • RF system and pulse compressor characteristics • Average iris radius: 3. 2 mm Beam dynamics requirements (BBU threshold) • Electromagnetic parametric study of the TW cell (main cell specs vs. iris aperture) • Effective shunt impedance optimization by a 2 D numerical scan of the total length and the iris tapering • Check of expected Breakdown rate (modified Poynting vector values @ nominal gradient) • Design a realistic RF module including power distribution network • Finalize the electromagnetic (input and output couplers) and mechanical design Iterations among these various steps are typically required. 6
Eu. PRAXIA@SPARC_LAB CDR Review Committee Meeting 27 -28 November 2018 INFN Frascati A. Gallo: X-band RF Linac technology RF power source and pulse compressor CPI VKX-8311 50 MW, 2 µs, 50 Hz The total RF pulse duration (1. 5 µs) and the SLED cavity Q 0 (180. 000) are input data for the structure optimization process, while the compressed pulse duration and the SLED cavity Qext (i. e. the cavity input coupling) are part of the optimization process. The amount of RF power available from the klystron define the configuration of the RF distribution waveguide network. Pulse compressor cavities 7
Eu. PRAXIA@SPARC_LAB CDR Review Committee Meeting 27 -28 November 2018 INFN Frascati A. Gallo: X-band RF Linac technology Breakdown rate and modified Poynting vector The major obstacle to high gradient is RF breakdown. It is a phenomenon that abruptly changes transmission and reflection RF power directed towards the structure. A local field quantity which predicts the high gradient performance of an accelerating structure is the modified Poynting vector Sc: The dependence of the modified Poynting vector on RF pulse length tp at a fixed breakdown rate (BDR) has a well established scaling law observed in many experiments: The BDR is defined as the probability of having a breakdown and it is typically measured in breakdown per pulse for 1 m long structure. As design guideline for a new RF structure, Sc should not exceeds 4 MW/mm 2 if the structure is supposed to operate at a breakdown rate smaller than 10 -6 bpp/m and a pulse length of 200 ns. A. Grudiev, S. Calatroni, and W. Wuensch, Phys. Rev. STAB. 12. 102001 (2009) K. Sjobak, E. Adli, A. Grudiev, MOPP 028, Proc. of LINAC 2014 (2014) 8
Eu. PRAXIA@SPARC_LAB CDR Review Committee Meeting 27 -28 November 2018 INFN Frascati A. Gallo: X-band RF Linac technology Electromagnetic parametric study of the TW cell accelerating cell shape adopted for CDR a = 2 ÷ 5 mm b = 9. 828 ÷ 10. 917 mm d = 8. 332 mm (2π/3 mode) r 0 = 1 mm t = 2. 5 mm A scan of the iris radius from 2 mm to 5 mm a has been performed with HFSS in order to obtain the single cell parameters (R, vg/c, Q, Scmax/E 2 acc) as a function of the iris radius. Also the related polynomial fits have been derived. accelerating cell furtherly optimized (latest results) a = 2 ÷ 5 mm b = 10. 155 ÷ 11. 215 mm d = 8. 332 mm (2π/3 mode) r 0 = 2. 5 mm; t = 2 mm r 1/r 2 = 1. 3 (Min Sc max for a=3. 2 mm) Q - factor Shunt Impedance Group velocity Max. value of normalized modified Poynting vector 9
Eu. PRAXIA@SPARC_LAB CDR Review Committee Meeting 27 -28 November 2018 INFN Frascati A. Gallo: X-band RF Linac technology Accelerating structure numerical optimization R/Q variation with iris aperture is not negligible and CG concept does not apply for not-flat RF pulses (SLED). For this reason we have developed a numerical tool able to calculate the main structure parameters (effective shunt impedance, modified Poynting vector, field profile) with an arbitrary cell-by-cell iris modulation along the structure. We have considered linear iris tapered structures. The optimization process aims at maximizing the structure effective shunt impedance defined as: Integrated gradient Structure length Klystron power (upstream the SLED ) 10
Eu. PRAXIA@SPARC_LAB CDR Review Committee Meeting 27 -28 November 2018 INFN Frascati A. Gallo: X-band RF Linac technology Accelerating structure numerical optimization working point As a reference design of the structure, we have considered 0. 5 m long structures with a tapering angle of 0. 1° as a good compromise between modularity, Rs and iris tapering. In particular Ls = 0. 5 m has been chosen to avoid weird splitting factors for the RF power (Ls = 0. 5 m → 8 structures per klystrons → good!). 11
Eu. PRAXIA@SPARC_LAB CDR Review Committee Meeting 27 -28 November 2018 INFN Frascati A. Gallo: X-band RF Linac technology Eu. PRAXIA@SPARC_LAB <a> = 3. 2 mm Ls=0. 5 m Freq. [GHz] 11. 9942 RF pulse [µs] 1. 5 Average gradient <G> [MV/m] Best eff shunt impedance <a> = 3. 2 mm Ls=0. 67 m 62. 5 MV/m Linac Energy gain Egain [Ge. V] 1 Linac active length Lact [m] 16 Unloaded SLED Q-factor Q 0 180000 External SLED Q-factor QE 19300 22600 Iris radius a [mm] 3. 6 -2. 8 3. 8 -2. 6 Group velocity v g [%] 2. 8 -1 3. 2 -0. 8 Effective shunt Imp. R s [M /m] 410 418 Filling time t f [ns] 100 140 Input power per structure P k_s [MW] 4. 8 6. 3 Structures per module Nm (input power per module Pkm [MW]) 8 (38. 4) 6 (37. 8) Total number of structures N tot 32 24 Total number of klystrons N k 4 4 12
Eu. PRAXIA@SPARC_LAB CDR Review Committee Meeting 27 -28 November 2018 INFN Frascati A. Gallo: X-band RF Linac technology X-BAND LINAC DESIGN WP 1: particle driven plasma acceleration WP 2: laser driven plasma acceleration WP 3: no plasma acceleration, only RF X-Band LINAC parameters Lt [m] 16 WP 1 WP 2 WP 3 Ultimate E 0 [Me. V] 100 170 170 Egain [Me. V] 450 380 890 1280 20(L 1)-36(L 2) 20(L 1)-27(L 2) 57 80 550 1060 1450 <G> [MV/m] EL [Me. V] 1 klystron x LINAC Module <Eacc> = 62. 5 MV/m 2 klystrons x LINAC Module <Eacc> = 88 MV/m CDR layout 1 + ½ X-band modules 2 + ½ X-band modules Design under revision (2 RF modules in both linac #1 and #2). Work is well advanced. 13
Eu. PRAXIA@SPARC_LAB CDR Review Committee Meeting A. Gallo: X-band RF Linac technology 27 -28 November 2018 INFN Frascati COUPLERS DESIGN As first case, we have considered a z-type coupler because of its compactness with respect to the waveguide and mode launcher ones. Racetrack geometry has been implemented in order to compensate the residual quadrupole field components. The calculated pulsed heating on the input coupler is <25 °C (@ Eacc = 80 MV/m), the obtained reflection coefficient is <− 30 d. B. 7 cells model 14
Eu. PRAXIA@SPARC_LAB CDR Review Committee Meeting 27 -28 November 2018 INFN Frascati A. Gallo: X-band RF Linac technology RF MODULE LAYOUT 2. 2 m Preliminary layout of the RF module (collaboration with CERN): 8 structures, 1 SLED, 1 or 2 Klystrons per module. Estimated waveguide attenuation (including circular waveguide): 10% WR-90 total length [mm] WC-50 circular wg length [mm] WR-90 loss [d. B] WC-50 loss [d. B] total loss [%] 1. 2 m 3758 3674 -0. 368 -0. 0456 -0. 414 -9. 09 15
Eu. PRAXIA@SPARC_LAB CDR Review Committee Meeting 27 -28 November 2018 INFN Frascati A. Gallo: X-band RF Linac technology CONCLUSIONS • X-band technology has been chosen for the linac as the most viable option to provide accelerating gradients of the order of 60 MV/m and beyond. A specific collaboration between INFN and CERN has been set up on this topic. • A dedicated TW accelerating structure 0. 5 m long has been designed through a complex numerical process to define the right tapering profile of the iris, including the optimization of the pulse compressor system. An effective impedance > 400 MΩ/m has been obtained. • The RF power station is designed around a 50 MW, 50 Hz CPI VKX-8311 klystron. The basic RF module including 1 klystron, 1 pulse compressor and 8 accelerating cavities can provide accelerating gradients > 60 MV/m. As an option, RF power in one or more modules can be doubled by adding a second tube, bringing the deliverable gradient to > 80 MV/m. • Even at the extreme gradients the expected BDR is below the threshold of 1∙ 10 -6 breakdowns per pulse per meter, according to the modified Poynting vector indicator. • Linac design is very well advanced but not considered frozen yet. Some readjustments of the structure design, RF module composition and linac layout are still under consideration. 16
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