The LINAC4 Power Coupler

2006

Linac4 is a new 160 MeV H − linac proposed at CERN to replace the 50 MeV Linac2 as injector to the PS Booster, with the goal of doubling its brightness and intensity. The present design foresees after RFQ and chopping line a sequence of three accelerating structures: a Drift Tube Linac (DTL) from 3 to 40 MeV, a Cell-Coupled DTL (CCDTL) to 90 MeV and a Side Coupled Linac (SCL) up to the final energy. The DTL and CCDTL operate at 352 MHz, while in the SCL the frequency is doubled to 704 MHz. Although the injection in the PS Booster requires only a low duty cycle, the accelerating structures are designed to operate at the high duty cycle required by a possible future extension to a high-power linac driver for a neutrino facility. This paper presents the different accelerating structures, underlining the progress in the design of critical resonator elements, like post-couplers in the DTL, coupling slots in the CCDTL and bridge couplers for the SCL. Prototyping progress for the different structures is reported, including the RF design of a DTL tank prototype and results of lowand high-power tests on a CCDTL prototype.

The review of the cell geometries, of the coupling modes between cells in accelerating sections, of the sections combination relative to a common RF source shows new possibilities as strongly reentrant cell geometries, SW at the 3pi/4 coupling mode, TW with slot coupling backwardly for v/c near unity or forwardly for lower v/c values, new combinations in serie of sections.

A Cell-Coupled Drift Tube Linac (CCDTL) accelerating structure at 352 MHz has been adopted for the energy range 40 to 90 MeV of Linac4, the new 160 MeV injector linac for the CERN accelerator complex. With regard to a conventional DTL in this energy range this structure presents the advantages of lower construction cost and easier access, cooling and alignment of the focusing quadrupoles placed between tanks. A full-scale high-power prototype representing 1/3 of a complete module has been designed and built at CERN. It is fed by a waveguide input coupler of novel conception. This paper summarizes the main mechanical features of the prototype and reports the RF tuning procedure and the results of high-power RF testing.

Journal of the Korean Physical Society, 2009

A conventional 20-MeV drift tube linac (DTL) for the Proton Engineering Frontier Project (PEFP) has been developed as part of the low-energy section of the 100-MeV proton accelerator. The 20-MeV DTL consists of four tanks, and the RF power generated by a klystron is coupled to the DTL tanks through RF power couplers. By introducing analytical and numerical methods to correlate the coupling coefficient with the coupling hole size in a dumbbell shaped iris coupler, we optimized the coupling coefficient to minimize the required power. The results from the two approaches are in excellent agreement with each other and with the experimental measurements. The coupling coefficient is shown to be proportional to the third power of the coupling hole diameter.

The high-energy section of Linac4, between 100 and 160 MeV, will be made of a sequence of 12 seven-cell ac- celerating cavities of the Pi-Mode Structure (PIMS) type, resonating at 352 MHz. The cell length is the same within a cavity, but changes from cavity to cavity according to the beam velocity profile. Compared to other structures used in this energy range, π-mode cavities with a low number of cells have the advantage of simplified construction and tun- ing, compensating for the fact that the shunt impedance is about 10% lower because of the lower frequency. Field sta- bility in steady state and in presence of transients is assured by the low number of cells and by the relatively high cou- pling factor of 5%. Standardising the linac RF system to a single frequency is considered as an additional economical and operational advantage. The mechanical design of the PIMS will be very sim- ilar to that of the 352 MHz normal conducting 5-cell LEP (Large Electron Proton collider at CERN)...

2011

As the first step of a long-term programme aiming at an increase in the LHC luminosity, CERN is building a new 160 MeV H¯ linear accelerator, Linac4, to replace the ageing 50 MeV Linac2 as injector to the PS Booster (PSB). Linac4 is an 86-m long normal-conducting linac made of an H¯ source, a Radio Frequency Quadrupole (RFQ), a chopping line and a sequence of three accelerating structures: a Drift-Tube Linac (DTL), a Cell-Coupled DTL (CCDTL) and a Pi-Mode Structure (PIMS). The civil engineering has been recently completed, and construction of the main accelerator components has started with the support of a network of international collaborations. The low-energy section up to 3 MeV including a 3-m long 352 MHz RFQ entirely built at CERN is in the final construction phase and is being installed on a dedicated test stand. The present schedule foresees beam commissioning of the accelerator in the new tunnel in 2013/14; the moment of connection of the new linac to the CERN accelerator chain will depend on the LHC schedule for long shut-downs.

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

The energy upgrade of the SPARC_LAB photo-injector at LNF-INFN (Frascati, Italy) has been originally conceived replacing one low gradient (13 MV/m) 3 m long SLAC type S-band traveling wave (TW) section with two 1.4 m long C-band accelerating sections. Due to the higher gradients reached by such structures, a higher energy beam can be obtained within the same accelerator footprint length. The use of C-band structures for electron acceleration has been adopted in a few FEL linacs in the world, among others, the Japanese Free Electron Laser at SPring-8 and the SwissFEL at Paul Scherrer Institute (PSI). The C-band sections are traveling wave, constant impedance structures with symmetric input and output axial couplers. Their design has been optimized for the operation with a SLED RF pulse compressor. In this paper we briefly review their design criteria and we focus on the construction, tuning, low and high-power RF tests. We also illustrate the design and realization of the dedicated low level RF system that has been done in collaboration with PSI in the framework of the EU TIARA project. Preliminary experimental results appear to confirm the operation of such structures with accelerating gradients larger than 35 MV/m.

… of EPAC 2006, …, 2006

The TOP Linac (Oncological Therapy with Protons), under development by ENEA and ISS is a sequence of three pulsed (5 microseconds, 300 Hz) linear accelerators: a 7 MeV, 425 MHz RFQ+DTL (AccSys Model PL-7), a 7-65 MeV, 2998 MHz Side Coupled Drift Tube Linac (SCDTL) and a 65-200 MeV, variable energy 2998 MHz Side Coupled Linac (SCL). The first SCDTL module structure, composed by 9 DTL tanks coupled by 8 side cavities, has been built. Low power RF measurements have shown good field uniformity and stability along the axis. The structure has been tested with a 1 -4 MW power RF. Results of low and high power tests are reported and discussed.

HHH- …, 2009

Linac4 is the new 160 MeV, 40 mA Haccelerator which will be the source of particles for all proton accelerators at CERN from 2013. Its construction has started in 2008, as part of a programme for the progressive replacement or upgrade of the LHC injectors during the next decade. Linac4 will initially inject into the PS Booster and at a later stage into a 4 GeV Superconducting Proton Linac (SPL), which could ultimately be upgraded to high duty cycle operation. For this reason accelerating structures, RF infrastructure and shielding of Linac4 are dimensioned for higher duty cycle from the initial phase.

2006

A conventional 20-MeV drift tube linac (DTL) for the Proton Engineering Frontier Project (PEFP) has been developed as part of the low-energy section of the 100-MeV proton accelerator. The 20-MeV DTL consists of four tanks, and the RF power generated by a klystron is coupled to the DTL tanks through RF power couplers. By introducing analytical and numerical methods to correlate the coupling coecient with the coupling hole size in a dumbbell shaped iris coupler, we optimized the coupling coecient to minimize the required power. The results from the two approaches are in excellent agreement with each other and with the experimental measurements. The coupling coecient is shown to be proportional to the third power of the coupling hole diameter.