The LANSCE RM project is restoring the linac to it’s original high power capability after the power grid tube manufacturer could no longer provide triodes that could consistently meet our power requirements. High duty factor Diacrodes® now supply RF power to the largest DTL tank. These tetrodes reuse the existing infrastructure including water-cooling systems, coaxial transmission lines, high voltage power supplies and capacitor banks. The power amplifier system uses a combined pair of LANL-designed cavity amplifiers using the TH628L Diacrode® to produce as much as 3.5 MW peak and 420 kW of mean power. A digital low-level RF control system was developed to complement these new linear amplifiers. Design and testing was completed in 2012, with commercialization following in 2013. The first installation is commissioned. The two remaining high power RF systems for tanks 3 and 4 will be replaced in subsequent years using a hybrid old/new RF system until the changeover is complete. Features and operating results of the replacement system are summarized, along with observations from the rapid-paced installation project.

RESULTS FROM THE INSTALLATION OF A NEW 201 MHz RF SYSTEM AT LANSCE

The LANSCE RM project has restored the proton linac to high power capability after the RF power tube manufacturer could no longer provide devices that consistently met the high average power requirement. Diacrodes® now supply RF power to three of the four DTL tanks. These tetrodes reuse the existing infrastructure including water-cooling systems, coaxial transmission lines, high voltage power supplies and capacitor banks. Each final power amplifier (FPA) system uses a combined pair of LANL-designed cavity amplifiers using the TH628L Diacrode® to produce up to 3.5 MW peak and 420 kW of average power. A new intermediate power amplifier (IPA) was developed using a TH781 tetrode. These amplifiers are the first production of new high power 200 MHz RF sources at accelerators in three decades. Design and prototype testing of the high power stages was completed in 2012, with commercialization following in 2013. Each installation was accomplished during a 4 to 5 month beam outage each year starting in 2014. Simultaneously, a new digital low-level RF control (dLLRF) system was designed and tested, and placed into operation this year, meeting the stringent field control requirements for the linac. The rapid-paced installation project changed over from old to new RF systems while minimizing beam downtime to the user facility schedule.

Design, Fabrication, Installation and Operation of New 201 MHz RF Systems at LANSCE

The LANSCE RM project has restored the linac to high power capability after the power tube manufacturer could no longer provide triodes that consistently met our high average power requirement. Diacrodes® now supply RF power to two of the four DTL tanks. These tetrodes reuse the existing infrastructure including water-cooling systems, coaxial transmission lines, high voltage power supplies and capacitor banks. The power amplifier system uses a combined pair of LANL-designed cavity amplifiers using the TH628L Diacrode® to produce up to 3.5 MW peak and 420 kW of mean power. Design and prototype testing was completed in 2012, with commercialization following in 2013. The first installation was completed in 2014 and a second installed system is ready to test. The remaining replacement will follow in 2016. Meanwhile, there is a hybrid of old/new amplifiers until the changeover is complete. Operating results of the replacement system are summarized, along with observations from the rapid-paced installation project. RF SYSTEM IMPROVEMENTS The LANSCE drift tube linac (DTL) uses four Alvarez cavities powered at 201.25 MHz, to accelerate both protons (H) and negative hydrogen ions (H) from 0.75 to 100 MeV before injection into a coupled-cavity linac (CCL). Pulsed RF power must be capable of 12% duty factor (DF) and as high as 3.5 MW of peak RF power, with corresponding average power capability of 420 kW per cavity. This is in contrast to the high-peak/lowaverage power machines at 200 MHz proton injector linacs at Fermilab, CERN, RAL and BNL. Over the past 25 years, manufacture of reliable RF amplifier triodes operating at this high average power has been unpredictable. Both premature loss of cathode emission and ceramic cracking have occurred in some tubes when operated at LANL. In 2006, the operating point of the power amplifiers (PA) had to be reduced in order to hold operating costs on budget (for all-too-frequent tube replacements) and prevent excess downtime. This led to the decision to operate LANSCE at half of its design original duty factor. A primary goal of the LANSCE Risk Mitigation project has been to double the linac duty factor by replacing the original 201.25 MHz amplifiers with modern power amplifier circuits with new generation tetrodes. Another goal has been to modernize the low level RF controls. Finally, end-of-life klystrons for the CCL are being replaced with forty-five new CPI VA862A1 1.3 MW klystrons [1]. Gridded Tube Cavity Amplifier A previous report [2] explained the reasoning behind the choice of the TH628L Diacrode from Thales Electron Tubes as the active device for this application. Combining the outputs of two PAs (Fig. 1) provides suitable headroom in peak and average power, allowing the tubes to operate well within their rating. Increased amplifier reliability and tube lifetime results from this pairing. The caveat for gridded tubes is that a matching cavity amplifier circuit must be developed around a chosen device. The solution was to develop a commercially buildable PA design by our team, with technical assistance from the Thales tube product engineering team. The common-grid PA configuration uses a full wavelength double-ended coaxial line output circuit, in order to double the RF power available over a traditional singleended tetrode. Progress on the mechanical and electrical design of the PA, supporting electronics and intermediate power amplifier (IPA) are discussed elsewhere [3][4][5]. Months of testing in 2012-13 ran up to 2.5 MW peak power at 12% duty factor and up to 3 MW into water loads to demonstrate design capability and to test the cathode emission capabilities of the tube. Each PA operates at < 1.85 MW at the DTL. A tender for manufacturing the LANL-designed PA was issued in 2012 and the work was subsequently awarded to Continental Electronics Corporation [6]. Five PAs were delivered in 2013-2014 and tested at LANL. Two additional units are being manufactured for delivery in late 2015. Six TH628L Diacrodes from Thales Electron Tubes have been received and tested. Two more are coming in late 2015. Three intermediate power amplifiers (IPA) have been produced by Betatron Electronics, Inc. with a fourth to be delivered in May. This amplifier assembly uses a Thales TH781 tetrode and matching TH18781 cavity amplifier. One IPA drives two final PAs (FPA) at each DTL RF station. All amplifiers conveniently reuse the same cooling water plant, the HV power supplies, capacitor banks and the 35.5 cm diameter coaxial transmission lines of the old RF powerplant. Coaxial Transmission Lines The 7.9 cm (3 1/8 inch) diameter coaxial transmission line from the 175 kW IPA is split by a /4 hybrid into two —————————— * Work supported by the United States Department of Energy, National Nuclear Security Agency, under contract DE-AC52-06NA25396 # jtml@lanl.gov 6th International Particle Accelerator Conference IPAC2015, Richmond, VA, USA JACoW Publishing ISBN: 978-3-95450-168-7 doi:10.18429/JACoW-IPAC2015-WEPWI002 7: Accelerator Technology T08 RF Power Sources WEPWI002 3485 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 15 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I.

Installation and Operation of Replacement 201 MHz High Power RF System at LANSCE

LANSCE has restored the proton drift-tube linac (DTL) to high-power capability after the original RF-power tube manufacturer could no longer supply devices that consistently met our high-average power requirement. Thales TH628L Diacrodes® now supply RF power to three of the four DTL tanks. These tetrodes reused the existing infrastructure including water-cooling systems, coaxial transmission lines, high-voltage power supplies and capacitor banks. Each transmitter uses a combined pair of power amplifiers to produce up to 3 MW peak and 360 kW of mean power. A new intermediate power amplifier (IPA) was simultaneously developed using a TH781 tetrode. Design and prototype testing of the high-power stages was completed in 2012, with commercialization following in 2013. Each installation was accomplished during a 4 to 5 month beam outage each year from 2014-2016. A new digital low-level RF control system was designed, built and placed into operation in 2016. The interaction of the dual power amplifiers, the I/Q LLRF, and the DTL cavities provided many challenges that were overcome. The replacement RF systems have completely met our accelerator requirements. Lessons learned from our experience are shared here. COMPONENTS AND INTERACTIONS The LANSCE DTL uses four Alvarez cavities powered at 201.25 MHz, to accelerate both protons (H+) and hydrogen ions (H-) from 0.75 to 100 MeV before injection into a coupled-cavity linac (CCL). Pulsed RF systems are capable of 12% duty factor and nominally 3 MW of peak RF power, with corresponding average power capability of 360 kW per cavity. Previous reports [1, 2, 3] discussed the benefits of using the TH628L Diacrode® from Thales Electron Tubes as the active device for this application. Combining the outputs of two PAs provided ample headroom in peak and average power, allowing the tubes to operate safely within their rating [4]. Amplifier reliability and tube lifetime are both improved from these new systems. In April of 2018, there were 6 Diacrodes® operating with a range of 11,363-23,824 hours without needing filament power adjustment. The integration of a new digital low-level RF (dLLRF) controller for each transmitter was completed in 2016. Experience from this work identified improvements that were made to the RF transmission infrastructure, amplifier protection logic, and dLLRF algorithms and timing. Coaxial Transmission Lines The IPA for each transmitter nominally operates from 80-120 kW as needed for the final stage power of 2-3 MW driving each DTL cavity. Figure 1 is a block diagram of the cascade of amplifiers. The IPA feeds power to a /4 coupled-line hybrid to develop power for both final power amplifiers (FPA). One path has a passive high-power phase shifter with +/10 degrees of mechanical adjustment and the other has a fixed length to compensate for the shifter's midpoint delay. The outputs from the two FPA are combined in a /4 branch-line hybrid made from 30.5 cm (12 in) diameter coaxial line. Matched directional couplers feed phase-stable coax to a custom-pulsed phase meter. This novel vector voltmeter uses the AD8302 phase detector integrated circuit along with a FPGA and a digital display of +/10 degrees. This diagnostic allows for real time adjustment of intraamplifier phasing using the passive phase adjuster. At 200 MHz, 1 cm of coax length is equivalent to 2.4 degrees of phase. These devices allow optimizing the power combining to account for mechanical tolerances of the transmission lines at each transmitter. There are no circulators used in the new RF plant. The hybrids are reciprocal devices, so the power combiner also splits reflected power from the DTL into quadrature components at the two Diacrodes®. This causes difficulty in maintaining equal power balanced between amplifiers as each tube operates into a different phase of mismatch. To resolve this problem, a separate /4 electrical delay is placed in the 23.3 cm (9 in) diameter coaxial line from one FPA to force the two tubes to drive identical complex impedances. This re-phasing would upset the combiner unless a similar delay is placed in the input line of the opposite amplifier. It re-establishes the quadrature relationship at the two inputs at the branch-line combiner. Mega Industries provided the custom components and largest diameter coaxial lines. Myat and Connecticut Microwave provided the balance of the items that had EIA standard flanges. The original 35.5 cm (14 in) diameter transmission line is connected to the output of the branch-line combiner to drive the DTL power coupler. Re-phasing between the amplifiers is not sufficient to ensure satisfactory system operation with reflected power. It is well known that a tube amplifier should be positioned at multiples of /2 from the power coupler. This ensures that the RF voltage inside the active region of the tube does not rise when there is a spark inside the DTL, from impedance inversion. The correct choice of line length reduces tube gain during reflected power events from _______________________ * Work supported by the United States Department of Energy, National Nuclear Security Agency, under contract DE-AC52-06NA25396 # jtml@lanl.gov 9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW Publishing ISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-THPAL025

http://jacow.org/ipac2018/papers/thpal025.pdf

New Drift-Tube Linac RF Systems at LANSCE

Two new 201.25 MHz RF Final Power Amplifiers (FPAs) have been designed, assembled, installed and successfully tested at the Los Alamos Neutron Science Center (LANSCE), in Module 2 of the Drift Tube Linac (DTL). These production units were fabricated at Continental Electronics Corporation. In this paper, we summarize the FPA air and water cooling requirements and cooling systems [1]. Description of Systems Figure 1 shows a CAD model of the FPA installation. Figure 1: Model of FPA Installation. Three separate water systems were required to fully cool each FPA and its auxiliary components. Two existing systems were modified and a new system was installed to meet the cooling requirements. A deionized water system (A01), cools the two power-combined FPAs and Intermediate Power Amplifier (IPA) directly. A chilled water system (A05), cools the RF reference source for the LANSCE accelerator and ignitrons in each of the four capacitor rooms. A treated water system (A06), cools the solid state driver amplifier (Rack 5), FPA and IPA water loads. Flow requirements for the three water systems were developed for combined amplifier power, 3.4 MW at 13% duty factor or 442 kW each [2]. In addition, air cooling was implemented where water cooling was not practical. The A01 System The A01 system is an existing deionized water system that served the obsolete power amplifier that was previously located at Module 2 of the DTL. Centrifugal pumps, located in an adjacent mechanical room, deliver 88 psi, 1,250 gpm for the four DTL modules. One pump operates while the other serves as a backup. Adaptation of this water system consists of demolishing the existing plumbing at the module and installing the new plumbing as depicted in Fig. 2. Deionized water is required to eliminate the potential electrical path to ground in an ionized system. The A01 system is adapted to cool both FPAs and the IPA which is not shown in Fig. 2. Figure 2: A01 Water System Adapted to the FPA Installation. Note that the gray-colored piping and associated components in Fig. 2 are the distribution hardware for the A01 water system, and the brown colored piping is the 3 1/8 inch coax RF transmission lines. The A01 piping is mounted to the legs of the platform, and the legs of the platform are anchored to the concrete. Figure 3: Side View of A01 Water System Mounted to Platform. *Work supported by the United States Department of Energy, National Nuclear Security Agency, under contract DE-AC52-06NA25396 #barkley@lanl.gov Proceedings of IPAC2015, Richmond, VA, USA WEPTY084 7: Accelerator Technology T08 RF Power Sources ISBN 978-3-95450-168-7 3479 Co py rig ht © 20 15 CC -B Y3. 0 an d by th er es pe ct iv ea ut ho rs Figure 4: Rear View of A01 Water System Mounted to Platform. Figures 3 and 4 are photos of the A01 system’s distribution plumbing from the side and rear of the platform. The piping and piping components are preassembled before the maintenance outage in a separate building to help minimize the schedule impact of assembling this hardware in situ. This preassembly saves two weeks in the outage schedule. To accommodate the relocation of the assembled A01 system to the accelerator equipment aisle, the platform and water system are disassembled into thirds making it easier to transport and fit into the doorways. Skilled technicians facilitate the disassembly, transport and reassembly of this important hardware using forklifts and a flat-bed truck. Table 1 is a summary of the A01 cooling requirements for the Module 2 amplifier installation. The flow rates were specified to control the temperature rise to between 5 and 10°C for the multi-phase (Hypervapotron) cooled anode of the Thales model TH628 electron tube [3]. Note that there are 2 FPAs per module and one IPA, so the total flow rate required for this system is 206.1 gpm. By comparison, the old system required 323 gpm per RF module. Table 1: A01 Water Cooling Requirements Unit Description Flow Rate

Cooling Systems for the New 201.25 MHz Final Power Amplifiers at Los Alamos Neutron Science Center (LANSCE)

Acceleration of long pulses of proton current at high repetition rates (1 mS, 120 pps) at LANSCE have been impacted by reduced capability of the original 201.25 MHz RF system driving the 100 MeV DTL. The present operating point of the existing triode power amplifiers doesn’t allow margin for tuning excursions with voltage standing wave effects or for tube manufacturing variations. This has led to the undesirable consequence of operating LANSCE at half of its original duty factor for the past seven years to maintain acceptable beam availability. A recent program has been initiated to restore the accelerator to its original high current operation. Investigated alternatives for high power RF included the development of physically large klystrons, solid-state and new power grid tube (tetrode) amplifiers. The power grid tube was favored as it reused much of the existing infrastructure including water-cooling systems, coaxial transmission lines, high voltage power supplies and capacitor banks. High duty factor tetrodes called Diacrodes ® , are capable of supplying the DTL requirements with ample headroom. A LANL-designed power amplifier using the TH628L Diacrode® has produced 3 MW peak and 300 kW of mean power with > 14 dB of power gain and > 60% efficiency. It has been operating for over seven thousand hours generating RF power with a test load. Installation of the first new system begins in January of 2014.

SYSTEM CONSIDERATIONS FOR 201.25 MHz RF SYSTEM FOR LANSCE

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SYSTEM CONSIDERATIONS FOR 201.25 MHz RF SYSTEM FOR LANSCE