Final Report for "Design calculations for high-space-charge beam-to-RF conversion".

RF sources are used in many applications, including high-power radar, communications, cancer therapy and diagnosis, electronic countermeasures, and high-energy accelerators. Many of these sources are located in well controlled environments where high-quality coolant fluids are readily available. Others, however, are located on platforms where harsh environmental conditions exist, and high-quality coolants are not easily provided. This includes ships at sea and remote transmitters in the second-world and third-world countries. Corrosive ethylene glycol mixtures may be required where extreme temperatures are encountered. Essentially, all high-power RF sources are fabricated from brazed copper structures, which are particularly susceptible to corrosion from corrosive coolants. This paper demonstrates using atomic layer deposition (ALD) to deposit nanometer-scale ceramic coatings inside coolant passages to separate the coolant from the underlying metal surface. This effectively reduces or eliminates corrosion and erosion, thereby increasing lifetime and reliability of RF sources. We describe ALD equipment and process specifically focused on RF sources and related components. The results of numerous experiments are reported, confirming the efficacy of the technology.

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This paper provides an overview of the tools used to conduct independent modeling and simulation of Artemis 1 separation events and highlights challenges and lessons learned across a decade of analysis. An overview of each of the tools along with the process flow and key components of maintaining independence from the Program’s baseline tool set will be provided. In addition, each of the separation analyses conducted will be discussed, making note of key models incorporated for each event and unique challenges that were addressed. The technical work described in this paper is part of an on-going technical assessment conducted by the NASA Engineering and Safety Center to develop and maintain an independent modeling and simulation capability in support of Space Launch System (SLS) and Multi-Purpose Crew Vehicle (MPCV) throughout design, verification, and flight readiness cycles. For Artemis 1, an end-to-end analysis approach was taken with events simulated from launch to re-entry. This paper will focus on the separation events, including liftoff clearance analysis, solid rocket booster (SRB) separation from Core Stage (CS), Service Module (SM) panel jettison, ICPS / Core Stage Separation, and MPCV/ ICPS separation. To assess separation clearances, the NESC has developed a high fidelity, independent 6-degree of freedom (DOF) simulation of the separation events. The simulation was developed using the Program to Optimize Simulated Trajectories II (POST2). POST2 is an industry standard flight trajectory optimization and simulation tool that is Class D compliant and follows NASA NPR 7150.2, which includes regression testing, unit testing, and configuration control of the simulation. The primary objective of the SLS separation and clearance analyses was to estimate the clearances between different vehicle components and/or ground support equipment (GSE) during separation events. The clearance is defined as the minimum distance from any point on one 3D geometry model associated with one vehicle component (or GSE) to any point on another 3D geometry model associated with a vehicle component separating from the main body. The vehicle geometry models used by the NESC to compute the separation clearances were derived from CAD models. POST2 simulation results were post processed in the Exploration Visualization Environment (EVE) tool to compute clearances, and detect recontact occurrences, if any, between the separating stages. EVE is a simulation, visualization, and analysis system designed to integrate time-based dynamics data with detailed graphical models in a full-scale virtual environment. EVE was used to simultaneously calculate more than 30 different separation clearances by “driving” detailed vehicle geometry models with position and attitude time history data obtained from the POST2 simulation. EVE displays each clearance distance visually, at any given instant of time, with a straight line connecting the two closest points on each object and records the time history of the clearance distance in a file. If the clearance reduces to zero, then EVE reports it as a recontact. To perform separation analyses, POST2 Monte Carlo analyses were performed by running 2000 dispersed cases that included over hundreds of uncertainties and dispersions for a range of simulation models including aerodynamics, propulsion, navigation sensors, control actuators, slosh and flexible body dynamics, and winds and atmosphere. Clearances were computed for each of the dispersed Monte Carlo trajectories and overall separation performance was statistically assessed. Over the past decade the POST2 simulation and EVE visualization tools have been used to perform detailed assessments of the separation events that occur during the Artemis 1 ascent. To support these assessments, numerous unique simulation models were developed and integrated into the standard suite of POST2 simulation models to address and resolve key concerns specific to each of the Artemis 1 separation events. In addition to presenting an overview of the multi-body 6-DOF simulation and separation analysis capability, the paper will discuss the integration of key simulation models such as flexible body dynamics, slosh dynamics, separation mechanisms, multi-body aerodynamics, and environmental models. Additional details are provided about other challenges that were addressed, including the use of a convex hull to reduce computational time for clearance calculations, the modeling of incidental recontact during separation events, and the capability to simulate failure scenarios.

Energy recovery linacs (ERLs) have the potential to be superb coherent hard x-ray sources. ERLs are described with reference to a 5 GeV ERL design being studied at Cornell University. The properties of this ERL, and the x-ray beams that may be produced, are described and spectral curves are calculated and compared to other existing and future x-ray sources. It is shown that ERL and x-ray free electron laser (X-FEL) sources are complementary in terms of the experiments they may optimally serve. ERLs will be especially advantageous in a variety of coherent and nanobeam experiments where the sample must be repetitively probed and in cases where the samples are unique and the requisite scattering information cannot be obtained with a single X-FEL pulse. ERL strengths are elaborated relating to the very high coherent flux, inherently round beams, flexibility and quasi-continuous time structure of the sources. Examples are given where these x-ray characteristics will facilitate advancement of important ‘big challenge’ areas of science.

In a high average power free-electron laser (FEL) the wall-plug efficiency is of critical importance in determining the size, complexity, and cost of the overall system. The wall-plug efficiency for the FEL oscillator and amplifier (uniform and tapered wiggler) is strongly dependent on the energy recovery process. A theoretical model for electron beam dynamics in the energy recovery linac is derived and applied to the acceleration and deceleration of nano-Coulomb electron bunches for a tapered FEL amplifier. For the tapered amplifier, the spent electron beam exiting the wiggler consists of trapped and untrapped electrons. Decelerating these two populations using different phases of the radio-frequency wave in the recovery process enhances wall-plug efficiency. For the parameters considered here, the wall-plug efficiency for the tapered amplifier can be ~10% using this approach.

Summary form only given. Recent advances and implementations of new "cut-cell" algorithms now permits very accurate 2nd-order geometry representation in modeling tools which have the required self-consistent treatment of space-charge and mode fields, necessary for design of the high-space-charge beam-to-RF conversion subcomponents. This development significantly reduces design and modeling effort, while simultaneously improving precision. We demonstrate this new capability in the VORPAL software framework for devices which convert bunched beam kinetic energy to RF energy, including output cavity components of klystrons and IOT's in accelerator upgrade programs. In addition, we test the ability of this new modeling capability to accurately predict the energy recovery process in a superconducting rf cavity. In this latter design calculation, we require an order-or-magnitude greater precision than typically required for similar calculations of RF sources. In all situations, we also require accurate information of the induced particle spread and velocity shears on exit from the cavities. Time permitting, we also investigate the feasibility of the modeling capability to be applied to other complex-geometry aspects of RF sources, such as the gun and collectors.

This letter presents a new solid-state RF source generating kW-level output power at HF band.Four power transistors are driven in a doubly-differential way, operated in a switching mode, and combined together to produce high output power. A multilayered input feed network is carefully designed, such that an accurate doubly-differential signal is provided to the transistors. The output matching and combining circuitry is optimized toward a high efficiency. The implemented RF source produces a CW signal of 1.5 kW at 29 MHz with a drain efficiency of 85%. The measured high efficiency is attributed to the well-balanced and symmetric switching-mode operation of each transistor, which is verified experimentally by a thermal image. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1489–1492, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25230

Energy recovery linacs (ERLs) are potential candidates for the high power and high brightness electron beams sources. The main advantages of ERL are that electron beam is generated at relatively low energy, injected and accelerated to the operational energy in a linac, and after the use is decelerated in the same linac down to injection energy, and, finally, dumped. A merging system, i. e. a system merging together high energy and low energy beams, is an intrinsic part of any ERL loop. One of the challenges for generating high charge, high brightness electron beams in an ERL is development of a merging system, which provides achromatic condition for space charge dominated beam and which is compatible with the emittance compensation scheme. In this paper we present principles of operation of such merging systems. We also describe an example of such system, which we call a Zigzag or a Z-system. We use a specific implementation of the Z-system for R&D ERL at Brookhaven as the illustration.

Electron-Ion colliders such as the Medium energy Electron Ion Collider (MEIC) being developed by JLAB require high current electrons with low energy spread for electron cooling of the collider ring. Accelerator techniques for improving bunch charge, average current, emittance, and energy spread are required for Energy Recovery Linacs (ERLs) and Circulator Rings (CR) for next generation colliders for nuclear physics experiments. Example candidates include thermionic-cathode electron guns with RF accelerating structures. Thermionic cathodes are known to produce high currents and have excellent lifetime. The success of the IR and THz Free-Electron Laser (FEL) designed and installed by Advanced Energy Systems at the Fritz Haber Institute (FHI) of the Max Planck Society in Berlin [1,2] demonstrates that gridded thermionic cathodes and rf systems be considered for next generation collider technology. In Phase 1 Advanced Energy Systems (AES) developed and analyzed a design concept using a superconducting cavity pair and gridded thermionic cathode. Analysis included Beam Dynamics and thermal analysis to show that a design of this type is feasible. The latest design goals for the MEIC electron cooler were for electron bunches of 420 pC at a frequency of 952.6 MHz with a magnetic field on the cathode of 2kG. This field magnetizes the beam imparting angular momentum that provides for helical motion of the electrons in the cooling solenoid. The helical motion increases the interaction time and improves the cooling efficiency. A coil positioned around the cathode providing 2kG field was developed. Beam dynamics simulations were run to develop the particle dynamics near the cathode and grid. Lloyd Young added capability to Tstep to include space charge effects between two plates and include image charge effects from the grid. He also added new pepper-pot geometry capability to account for honeycomb grids. These additions were used to develop the beam dynamics for this gun. The general design is a modified ballistic compression cavity pair with two independently powered cells [3]. The first is a cathode cell that includes the thermionic cathode and grid to provide for beam bunching. The second is a full cell with independent phasing and field levels designed to minimize energy spread. The primary goal for Phase II is to manufacture a superconducting gun with a thermionic cathode and imbedded coil. The system developed here is applicable to many high current electron accelerators. The analysis and design constraints imposed by the magnetized cathode make the cathode system developed here more complicated and limited than one without the magnetized beam constraints. High power ERLs would benefit by a gun with the capabilities shown here, 400 mA or more of current. ERLs hold great promise for electron cooling experiments, advanced light sources and Free Electron Lasers. This high current electron injector is a technological advance that will place the requirements for an ERL capable of providing quality bunches needed for cooling within the MEIC circulator ring within reach. This injector would have application to future ERLs around the world.

A proposed Energy Recovery Linac (ERL) x-ray source at Comell would be a 5-7 GeV synchrotron facility based on the energy-recovery principle with a superconducting linac. Because of its ultra-small round electron source and ultra-short but flexible bunch structure, the ERL has the potential to produce subpicosecond ultra-bright diffraction-limited hard x-ray beams that are superior to those from existing storage rings, and would enable new scientific experiments that are difficult or impossible to perform today. In this paper, we discuss the transverse coherence properties of the ERL source and show how these properties could benefit various x-ray microscopy applications that require a highly coherent and intense x-ray beam. These applications include full coherent illumination of wide-aperture zone-plate optics for scanning x-ray microscopy, diffraction-limited coherent phase-contrast microscopy and tomography, fully coherent hard x-ray diffraction microscopy, and large-coherence-area x-ray holography techniques.

It is important to develop the high power EUV light source up to 1 kW to realize the 3nm node to reduce stochastic variation and achieve a higher through put. To this end, we have proposed an energy recovery linac (ERL)-based free electron laser (FEL) ,which will produce more than 10 kW EUV light to provide the light into several scanners. We studied the feasibilities to reduce the size of the accelerator system itself, and the part of them was presented at the previous symposium last year. And we also gave an idea to develop the POC of the ERL-FEL by using compact ERL (cERL) as a first stage of the EUV-FEL. In this paper, we present the upgrade plan of cERL for the POC. The technologies to be clarified are both of to realize the SASE-FEL based on the ERL and to achieve short electron bunch around 100fs for EUV-FEL light generation. The former is that the energy of the electron beams after the FEL generation can be recovered with the ERL accelerator systems with the high repetition rate such as more than 100MHz. The POC will be completed at the wavelength of near infrared, because of the size limitation of the cERL. The latter will be also realized at the cERL by the bunch compression scheme using a combination of electron beams with a momentum chirp and magnet assemblies with a non-zero longitudinal dispersion. The design values of the upgrade are as follows; the energy of the recirculation electron is 80 MeV, energy of the injection electron is 5 MeV, wavelength of the FEL is 1.35 micron, electron beam current is 10mA, bunch charge is 60 pC/bunch, repetition rate is 162.5 MHz. We have already studied the electron trajectory by using simulation, and checked the feasibilities. We will give detailed studies on the POC.

Research is in progress on a TeV-scale linear collider that will operate at 5-10 times the energy of present-generation accelerators. This will require development of high power RF sources generating of 50-100 MW per source. Transmission of power at this level requires overmoded waveguide to avoid breakdown. In particular, the TE{sub 01} circular waveguide mode is currently the mode of choice for waveguide transmission at Stanford Linear Accelerator Center (SLAC) in the Multimode Delay Line Distribution System (MDLDS). A common device for protecting an RF source from reflected power is the waveguide circulator. A circulator is typically a three-port device that allows low loss power transmission from the source to the load, but diverts power coming from the load (reflected power) to a third terminated port. To achieve a low loss, matched, three port junction requires nonreciprocal behavior. This is achieved using ferrites in a static magnetic field which introduces a propagation constant dependent on RF field direction relative to the static magnetic field. Circulators are currently available at X-Band for power levels up to 1 MW in fundamental rectangular waveguide; however, the next generation of RF sources for TeV-level accelerators will require circulators in the 50-100 MW range. Clearly,more » conventional technology is not capable of reaching the power level required. In this paper, we discuss the development of an X-Band circulator operating at multi-megawatt power levels in overmoded waveguide. The circulator will employ an innovative coaxial geometry using the TE{sub 01} mode. Difficulties in maintaining mode purity in oversized rectangular guide preclude increasing guide area to allow increasing the power level to the desired 50-100 MW range. The TE{sub 01} mode in circular waveguide is very robust mode for transmission of high power in overmoded waveguide. The mode is ideal for transmission of high power microwaves because of its low-losses, zero tangential field on the guide (which minimizes arcing problems) and reduced propensity for mode conversion compared to non-asymmetric circular waveguide modes. Unfortunately, no current designs exist for circulators using the circular TE{sub 01} mode. The basic building block for all low-loss circulators and isolators is a nonreciprocal element with a phase shift dependent on the propagation direction in the guide. Such an element can be constructed by placement of a hollow ferrite rod in a cylindrical waveguide. An inner conductor placed inside the ferrite rod conducts a current pulse that induces an azimuthal magnetic field inside the ferrite. This configuration is depicted in Figure 1a. An alternate configuration using permanent magnets is shown in Figure 1b. Either of these configurations will create a different phase shift for waves propagating in opposite directions along the waveguide axis. This feature can be used to develop a high power circulator. We are currently testing a TE{sub 01} nonreciprocal phase shifter in a 50 MW test stand. This device is in the configuration shown in Figure 1a. The induced differential phase shift and loss will be measured and compared to calculations.« less

The maximum beam current that can be accelerated in an energy recovery linac (ERL) can be severely limited by the transverse multi-pass beam breakup instability (BBU), especially in future ERL light sources with multi-GeV high energy beam energy and more than 100 mA average current. In this paper, the multi-pass BBU of such a high energy ERL is studied based on the simulation of a 3-GeV ERL light source that is proposed by KEK. This work is expected to provide a reference for future high energy ERL projects.

We propose inverse Compton scattering (ICS) of coherent synchrotron radiation (CSR) from a relativistic short electron bunch in energy recovery linacs (ERL) as a high-flux subpicosecond x-ray and $\ensuremath{\gamma}$-ray source. An advantage of the CSR scheme over a conventional ICS source is that no external laser is required, and synchronization between CSR pulses and electron bunches is obtained automatically. Moreover, higher-flux x rays can be generated from the ICS of CSR in an ERL operated at a high repetition rate, 100 MHz to 1.3 GHz. Using parameters of the Compact ERL at KEK, $1\ifmmode\times\else\texttimes\fi{}{10}^{13--14}\text{ }\text{ }\mathrm{phs}/\mathrm{s}$ b.w. 10% (the number of photons ${\mathrm{pulse}}^{\ensuremath{-}1}$ bandwidth ${\mathrm{unit}}^{\ensuremath{-}1}$) x ray with a 100 fs--1 ps pulse duration can be obtained, for an energy range from 0.04 to 4 keV. In the case of a 5-GeV ERL, $\ensuremath{\gamma}$ rays with energy around tens of MeV are generated with $1\ifmmode\times\else\texttimes\fi{}{10}^{8}\text{ }\text{ }\mathrm{phs}/\mathrm{pulse}$ b.w. 10% at a repetition rate of several hundreds of MHz.

In this paper we describe the unique features and analysis techniques used on the magnets for a R&D Energy Recovery Linac (ERL) [1] under construction at the Collider Accelerator Department at BNL. The R&D ERL serves as a test-bed for future BNL ERLs, such as an electron-cooler-ERL at RHIC [2] and a future 20 GeV ERL electron-hadron at eRHIC [3]. Here we present select designs of various dipole and quadrupole magnets which are used in Z-bend merging systems [4] and the returning loop, 3-D simulations of the fields in aforementioned magnets, particle tracking analysis, and the magnet's influence on beam parameters. We discuss an unconventional method of setting requirements on the quality of magnetic field and transferring them into measurable parameters as well as into manufacturing tolerances. We compare selected simulation with results of magnetic measurements.