Construction and RF Test of the Compactlight Accelerating Structure Prototype

I.FAST Innovation Fostering in Accelerator Science and Technology Horizon 2020 Research Infrastructures GA n° 101004730 MILESTONE REPORT CONSTRUCTION AND RF TESTS OF THE COMPACTLIGHT ACCELERATING STRUCTURE PROTOTYPE MILESTONE: MS30 Document identifier: IFAST-MS30 Due date of deliverable: 31/12/2023 Report release date: 22/09/2025 Work package: WP7: High Brightness Accelerators for Light Sources Lead beneficiary: ELETTRA Document status: Final ABSTRACT This milestone (MS30) confirms the successful construction of the CompactLight accelerating structure prototype within the I.FAST WP7. This achievement marks a key step towards Deliverable D7.6 and the realization of the full accelerating structure. CONSTRUCTION AND RF TESTS OF THE COMPACTLIGHT ACCELERATING STRUCTURE PROTOTYPE Milestone: MS30 Date: 22/09/2025 Grant Agreement 101004730 PUBLIC 2 / 31 IFAST Consortium, 2025 For more information on IFAST, its partners and contributors please see https://ifast-project.eu/ This project has received funding from the European Union’s Horizon 2020 Research and Innovation programme under Grant Agreement No 101004730. IFAST began in May 2021 and will run for 4 years. Delivery Slip Name Partner Date Authored by G. D’Auria ELETTRA 09/09/25 Edited by G. D’Auria ELETTRA 09/09/25 Reviewed by M. Vretenar and L. Celona on behalf of the Steering Committee CERN 22/09/25 Approved by Steering committee CERN 22/09/25 CONSTRUCTION AND RF TESTS OF THE COMPACTLIGHT ACCELERATING STRUCTURE PROTOTYPE Milestone: MS30 Date: 22/09/2025 Grant Agreement 101004730 PUBLIC 3 / 31 Contents 1. Introduction ...................................................................................................................... 4 2. The CompactLight accelerating structure ........................................................................ 4 3. First accelerating structure stack assembly .................................................................... 7 4. Metrology and vacuum tests ............................................................................................ 9 5. Low level RF measurements and RF characterization................................................... 11 6. Conclusions .................................................................................................................. 13 7. References ................................................................................................................... 13 8. Annex 1 .......................................................................................................................... 16 CONSTRUCTION AND RF TESTS OF THE COMPACTLIGHT ACCELERATING STRUCTURE PROTOTYPE Milestone: MS30 Date: 22/09/2025 Grant Agreement 101004730 PUBLIC 4 / 31 1. Introduction This report describes the X-band accelerating structure and the RF tests for the CompactLight project (https://www.compactlight.eu/). The detailed construction phases of the first prototype of the structure are reported in the D7.5, hereinafter the CERN metrology report are analyzed together with the RF tests. CompactLight was a a Design Study funded by the European Union under the Horizon 2020 Research and Innovation programme, completed in 2021 with the design of an innovative, compact, and costeffective hard X-ray FEL facility, using the most advanced technologies developed for accelerator cmponents, in terms of high brightness photo-injectors, compact and very high-gradient X-band accelerating structures, as well as state-of-the-art undulators. The CompactLight linac consists of approximately one hundred 0.9m long X-band accelerating structures that can operate both in a high-gradient mode, at 65 MV/m and 100 Hz pulse repetition rate, and high-repetition rate mode, at 30 MV/m and 1 kHz pulse repetition rate. The structure parameters have been optimized for both radio frequency and beam dynamics performance and have higher order transverse mode suppression for stable two-bunch operation. The cooling circuit has been designed to accommodate the high average power of the high-repetition rate mode and optimized for minimum difference between operating modes. Finally, the mechanical design of the structures has also been analyzed and optimized for industrial production. 2. The CompactLight accelerating structure The design and optimization of a linear accelerator based on traveling-wave structures involve several critical steps [1] and the design process typically involves several iterations among them to achieve an optimal balance between performance, efficiency, and reliability. These processes are detailed reported in the D7.5. With the average iris radius of the structure defined, the first step in designing the entire structure was to identify the optimal lengths for both Constant Impedance (CI) and Constant Gradient (CG) structures. The effective shunt impedance, as a function of the accelerating structure attenuation, is shown in Fig. 1a for both CI and CG structures, while the optimal structure length as a function of the average iris aperture is depicted in Fig. 1b. For the CI structure, the optimal length has been fixed at 0.890 m, and for the CG structure, 0.818 m. These values have been used as the basis for a numerical optimization of iris tapering. Two-bunch operations in CompactLight is essential for pump-and-probe experiments. However, this operational mode introduces a critical design challenge: addressing the long-range transverse wakefield behavior of the accelerating structure. The wakefield excited by the leading bunch can influence the trajectory of the trailing bunch, leading to emittance growth. To mitigate this, longrange transverse wakefield suppression is necessary. This suppression can be achieved by varying the iris diameter along the structure's length, which detunes the synchronous frequencies of the most significant transverse modes. This detuning induces decoherence in the transverse wakefield, reducing its amplitude. CONSTRUCTION AND RF TESTS OF THE COMPACTLIGHT ACCELERATING STRUCTURE PROTOTYPE Milestone: MS30 Date: 22/09/2025 Grant Agreement 101004730 PUBLIC 5 / 31 Figure 1: (a) : Effective shunt impedance as a function of the section attenuation for CI and CG structures , (b) Optimal structure length as function of the average iris radius for CI and CG structures. Through a multi-parameter optimization of the tapering parameters, the final iris aperture and thickness profiles were determined, as illustrated in Fig. 2. The blue line represents the dimensions of the linear iris design, while the red line shows the Gaussian-like design. Magnetically coupled input and output power couplers were chosen for their compact design. Key design considerations for these couplers include achieving impedance matching, minimizing peak surface electric fields, and reducing the quadrupolar component of the accelerating field within the structure. Figure 2: The iris dimension of the RF structure. Left figure is the distribution of the iris aperture. Right figure is the distribution of the iris thickness. At the end of optimizations the Table 1 lists the main parameters of the structures that can operate both in a high-gradient mode, at 65 MV/m and 100 Hz pulse repetition rate, and high-repetition rate mode, at 30 MV/m and 1 kHz pulse repetition rate.and the RF module. CONSTRUCTION AND RF TESTS OF THE COMPACTLIGHT ACCELERATING STRUCTURE PROTOTYPE Milestone: MS30 Date: 22/09/2025 Grant Agreement 101004730 PUBLIC 6 / 31 Table 1: Main parameters of the RF structures and modules Parameter Units Value Frequency GHz 11.994 Peak Klystron power (100-250 Hz) MW 50 Peak klystron power (1kHz) MW 10 RF pulse length (250 Hz) 𝜇𝑠 1.5 (0.15) Waveguide power attenuation % ≈10 Average iris radius < 𝑎 > mm 3.5 Iris radius 𝑎 mm 4.3-2.7 Iris thickness 𝑡 mm 2-2.24 Structure length 𝐿𝑠 m 0.9 Unloaded SLED Q-factor 𝑄0 18000 External SLED Q-factor 𝑄𝑒𝑥𝑡 23300 Shunt impedance R 𝑀Ω/𝑚 85-111 Peak modified Poynting vector 𝑊/𝜇𝑚2 3.4 Group velocity 𝑣𝑔/𝑐 % 4.7-0.9 Filling time 𝑡𝑓 ns 146 Repetition rate Hz 100 250 10000 Pulse compressor ON OFF ON Required klystron power MW 44 44 9 Average accelerating gradient MV/m 65 30 30 The gradient profile along the structure (after one filling time) is shown in Fig. 3, while the power distribution (averaged over the pulse length and normalized to the peak value) is illustrated in Fig. 3. Figure 3: Accelerating gradient profile along the structure after one filling time. CONSTRUCTION AND RF TESTS OF THE COMPACTLIGHT ACCELERATING STRUCTURE PROTOTYPE Milestone: MS30 Date: 22/09/2025 Grant Agreement 101004730 PUBLIC 7 / 31 Figure 4: Power distribution (averaged over the pulse length and normalized to the peak value). 3. First accelerating structure stack assembly The following pictures show the partially constructed accelerating structure at the CPI TMD factory (detailed construction process is reported in D7.5). The whole structure, was brazed mid of February 2025. Figure 5 shows the entire structure assembled before brazing. Figure 5. Entire structure before brazing CONSTRUCTION AND RF TESTS OF THE COMPACTLIGHT ACCELERATING STRUCTURE PROTOTYPE Milestone: MS30 Date: 22/09/2025 Grant Agreement 101004730 PUBLIC 8 / 31 Prior to running this braze, a test furnace run was performed to ensure that the braze profile chosen would reach the desired brazing temperature. A thermocouple was attached to the centre of the test mass and the results showed that the braze profile was adequate. Following the braze run, the structure was removed from the furnace and placed into CPI TMD’s cleanroom for vacuum leak testing, see Figure 6. The structure was tested using ultra-high vacuum leak checking equipment and was unfortunately shown to have a gross leak. Figure 6. RF structure after brazin CPI TMD attempted to pinpoint the source of the leak, but the structure appeared to be leaking from several sources, and the size of the leak was too large to be able to accurately identify the sources. In addition, mechanical damage was noted on the outside of the RF coupler, probably due to the thermal expansion of the structure into its braze jig during the brazing cycle. Prior to furnace, CPI TMD had put the brazing jig through a wet hydrogen greening process, which forms a green oxide layer on the stainless steel. This green oxide layer is applied to prevent the brazing parts from sticking to the braze jig. In addition to this, the braze jig was loosened once the structure was positioned in the furnace, to allow room for thermal expansion of the copper structure. Despite this, copper did stick to the stainless steel braze jig and expanded into it, causing damage to at least the outside of the stack. Following preliminary tests at CPI TMD, at the beginning of April 2025 the structure has been sent to CERN for low level RF measurements, mechanical tests (to assess any damage on the RF couplers) and for further evaluation to identify the cause of the vacuum leaks. In particular CERN Metrology will check the alignment of the structure and, using an endoscope, will also check the brazing alloy penetration on the beam axis of the structure. In addition, in order to identify the sources of vacuum leaks, the vent holes of the brazing channel will be sealed. Finally, if necessary, to fully understand the cause of the vacuum leaks, the section will be cut and the brazed areas are analysed. CONSTRUCTION AND RF TESTS OF THE COMPACTLIGHT ACCELERATING STRUCTURE PROTOTYPE Milestone: MS30 Date: 22/09/2025 Grant Agreement 101004730 PUBLIC 9 / 31 4. Metrology and vacuum tests Below the metrology report carried out at CERN to verify the alignment of the structure (the full report is in Annex 1). Figure 7a, shows the structure under measurement with the reference axes. Measurements taken: 1. The diameters of all the cells in the structure (102 in total), evaluating their deviations from the nominal values. Fig. 7b shows the measured values for the first 10 cells. 2. The value of a reference location for each cell in the structure (102 in total), evaluating all deviations from the nominal values. Fig. 7c shows the measured values for the first 10 cells. 3. The structure axe, assessing any deviations in X+, X-, Y+, Y-, see Fig. 8 All the values shown in Fig. 7 and Fig. 8 are in mm. CONSTRUCTION AND RF TESTS OF THE COMPACTLIGHT ACCELERATING STRUCTURE PROTOTYPE Milestone: MS30 Date: 22/09/2025 Grant Agreement 101004730 PUBLIC 16 / 31 8. Annex 1 ANNEXE 1 CERN METROLOGY REPORT CONSTRUCTION AND RF TESTS OF THE COMPACTLIGHT ACCELERATING STRUCTURE PROTOTYPE Milestone: MS30 Date: 22/09/2025 Grant Agreement 101004730 PUBLIC 17 / 31 CONSTRUCTION AND RF TESTS OF THE COMPACTLIGHT ACCELERATING STRUCTURE PROTOTYPE Milestone: MS30 Date: 22/09/2025 Grant Agreement 101004730 PUBLIC 18 / 31 CONSTRUCTION AND RF TESTS OF THE COMPACTLIGHT ACCELERATING STRUCTURE PROTOTYPE Milestone: MS30 Date: 22/09/2025 Grant Agreement 101004730 PUBLIC 19 / 31 CONSTRUCTION AND RF TESTS OF THE COMPACTLIGHT ACCELERATING STRUCTURE PROTOTYPE Milestone: MS30 Date: 22/09/2025 Grant Agreement 101004730 PUBLIC 20 / 31 CONSTRUCTION AND RF TESTS OF THE COMPACTLIGHT ACCELERATING STRUCTURE PROTOTYPE Milestone: MS30 Date: 22/09/2025 Grant Agreement 101004730 PUBLIC 21 / 31 CONSTRUCTION AND RF TESTS OF THE COMPACTLIGHT ACCELERATING STRUCTURE PROTOTYPE Milestone: MS30 Date: 22/09/2025 Grant Agreement 101004730 PUBLIC 22 / 31 CONSTRUCTION AND RF TESTS OF THE COMPACTLIGHT ACCELERATING STRUCTURE PROTOTYPE Milestone: MS30 Date: 22/09/2025 Grant Agreement 101004730 PUBLIC 23 / 31 CONSTRUCTION AND RF TESTS OF THE COMPACTLIGHT ACCELERATING STRUCTURE PROTOTYPE Milestone: MS30 Date: 22/09/2025 Grant Agreement 101004730 PUBLIC 24 / 31 CONSTRUCTION AND RF TESTS OF THE COMPACTLIGHT ACCELERATING STRUCTURE PROTOTYPE Milestone: MS30 Date: 22/09/2025 Grant Agreement 101004730 PUBLIC 25 / 31