Cornell Energy Recovery Linac Research Articles

Impact of nitrogen doping of niobium superconducting cavities on the sensitivity of surface resistance to trapped magnetic flux

Future particle accelerators such as the SLAC “Linac Coherent Light Source-II” (LCLS-II) and the proposed Cornell Energy Recovery Linac require hundreds of superconducting radio-frequency (SRF) niobium cavities operating in continuous wave mode. In order to achieve economic feasibility of projects such as these, the cavities must achieve a very high intrinsic quality factor (Q0) to keep cryogenic losses within feasible limits. To reach these high Q0's in the case of LCLS-II, nitrogen-doping of niobium cavities has been selected as the cavity preparation technique. When dealing with Q0's greater than 1 × 1010, the effects of ambient magnetic field on Q0 become significant. Here, we show that the sensitivity to RF losses from trapped magnetic field in a cavity's walls is strongly dependent on the cavity preparation. Specifically, standard electropolished and 120 °C baked cavities show a sensitivity of residual resistance from trapped magnetic flux of ∼0.6 and ∼0.8 nΩ/mG trapped, respectively, while nitrogen-doped cavities show a higher sensitivity of residual resistance from trapped magnetic flux of ∼1 to 5 nΩ/mG trapped. We show that this difference in sensitivities is directly related to the mean free path of the RF surface layer of the niobium: shorter mean free paths lead to less sensitivity of residual resistance to trapped magnetic flux in the dirty limit (ℓ ≪ ξ0), while longer mean free paths lead to lower sensitivity of residual resistance to trapped magnetic flux in the clean limit (ℓ ≫ ξ0). These experimental results are also shown to have good agreement with recent theoretical predictions for pinned vortex lines oscillating in RF fields.

Demonstration of cathode emittance dominated high bunch charge beams in a DC gun-based photoinjector

We present the results of transverse emittance and longitudinal current profile measurements of high bunch charge (≥100 pC) beams produced in the DC gun-based Cornell energy recovery linac photoinjector. In particular, we show that the cathode thermal and core beam emittances dominate the final 95% and core emittances measured at 9–9.5 MeV. Additionally, we demonstrate excellent agreement between optimized 3D space charge simulations and measurement, and show that the quality of the transverse laser distribution limits the optimal simulated and measured emittances. These results, previously thought achievable only with RF guns, demonstrate that DC gun based photoinjectors are capable of delivering beams with sufficient single bunch charge and beam quality suitable for many current and next generation accelerator projects such as Energy Recovery Linacs and Free Electron Lasers.

Design, conditioning, and performance of a high voltage, high brightness dc photoelectron gun with variable gap.

A new high voltage photoemission gun has been constructed at Cornell University which features a segmented insulator and a movable anode, allowing the cathode-anode gap to be adjusted. In this work, we describe the gun's overall mechanical and high voltage design, the surface preparation of components, as well as the clean construction methods. We present high voltage conditioning data using a 50 mm cathode-anode gap, in which the conditioning voltage exceeds 500 kV, as well as at smaller gaps. Finally, we present simulated emittance results obtained from a genetic optimization scheme using voltage values based on the conditioning data. These results indicate that for charges up to 100 pC, a 30 mm gap at 400 kV has equal or smaller 100% emittance than a 50 mm gap at 450 kV, and also a smaller core emittance, when placed as the source for the Cornell energy recovery linac photoinjector with bunch length constrained to be <3 ps rms. For 100 pC up to 0.5 nC charges, the 50 mm gap has larger core emittance than the 30 mm gap, but conversely smaller 100% emittance.

High-power fiber lasers for photocathode electron injectors

Many new applications for electron accelerators require high-brightness, high-average power beams, and most rely on photocathode-based electron injectors as a source of electrons. To achieve such a photoinjector, one requires both a high-power laser system to produce the high average current beam, and also a system at reduced repetition rate for electron beam diagnostics to verify high beam brightness. Here we report on two fiber laser systems designed to meet these specific needs, at 50 MHz and 1.3 GHz repetition rate, together with pulse pickers, second harmonic generation, spatiotemporal beam shaping, intensity feedback, and laser beam transport. The performance and flexibility of these laser systems have allowed us to demonstrate electron beam with both low emittance and high average current for the Cornell energy recovery linac.

Expected performance of cryogenic silicon monochromator for high power density and high coherence ERL beamlines

With small emittance and a high degree of X-ray coherence, the Energy Recovery Linac beamlines, proposed at Cornell, need excellent optics to preserve the high quality of X-ray beam. In this paper, thermal deformation of cryogenic silicon monochromators and its effect on wavefront deformation are studied. For crystal thermal deformation analysis, we introduce the so-called modified linear power density as a universal parameter of heat load. For the wavefront simulation, using synchrotron radiation workshop, we study the focusing of source under one-to-one demagnification ratio, for the full central cone of undulator radiations. The studies conclude that cryogenic Si monochromators, in general, are suitable for Cornell energy recovery linac beamlines.

Expected thermal deformation and wavefront preservation of a cryogenic Si monochromator for Cornell ERL beamlines

Cornell energy-recovery linac (ERL) beamlines will have higher power density and higher fractional coherence than those available at third-generation sources; therefore the capability of a monochromator for ERL beamlines has to be studied. A cryogenic Si monochromator is considered in this paper because the perfect atomic structure of Si crystal is needed to deliver highly coherent radiation. Since neither the total heat load nor the power density alone can determine the severity of crystal deformation, a metric called modified linear power density is used to gauge the thermal deformation. For all ERL undulator beamlines, crystal thermal deformation profiles are simulated using the finite-element analysis tool ANSYS, and wavefront propagations are simulated using Synchrotron Radiation Workshop. It is concluded that cryogenic Si monochromators will be suitable for ERL beamlines in general.

The main linac cavity for Cornell's energy recovery linac: Cavity design through horizontal cryomodule prototype test

Future particle accelerators will require continuous wave operation of SRF cavities capable of supporting high beam currents. An example of this is the Energy Recovery Linac (ERL) at Cornell University, a next generation light source designed to run high currents (100mA) with a high bunch repetition rate (1.3GHz). Obtaining the beam emittance necessary to meet design specification requires strong damping of higher-order modes that can lead to beam breakup. We discuss the optimization and verification of the accelerating cavity. Next we show that an ERL constructed from the optimized cavity geometry – including realistic shape errors – can support beam currents in excess of 300mA while still maintaining beam stability. A niobium prototype 7-cell cavity was fabricated and tested in a horizontal cryomodule. We show that the prototype cavity exceeds quality factor and gradient specifications of 2×1010 at 16.2MV/m at 1.8K by 50%, reaching Q=(3.0±0.3)×1010. The prototype cavity also satisfies all design constraints and has a higher order mode spectrum consistent with the optimized shape geometry. At 1.6K, the cavity set a record for quality factor of a multicell cavity installed in a horizontal cryomodule reaching Q=(6.1±0.6)×1010.

Demonstration of low emittance in the Cornell energy recovery linac injector prototype

We present a detailed study of the six-dimensional phase space of the electron beam produced by the Cornell Energy Recovery Linac Photoinjector, a high-brightness, high repetition rate (1.3 GHz) DC photoemission source designed to drive a hard x-ray energy recovery linac (ERL). A complete simulation model of the injector has been constructed, verified by measurement, and optimized. Both the horizontal and vertical 2D transverse phase spaces, as well as the time-resolved (sliced) horizontal phase space, were simulated and directly measured at the end of the injector for 19 and 77 pC bunches at roughly 8 MeV. These bunch charges were chosen because they correspond to 25 and 100 mA average current if operating at the full 1.3 GHz repetition rate. The resulting 90% normalized transverse emittances for $19\text{ }(77)\text{ }\text{ }\mathrm{pC}/\mathrm{bunch}$ were $0.23\ifmmode\pm\else\textpm\fi{}0.02$ $(0.51\ifmmode\pm\else\textpm\fi{}0.04)\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ in the horizontal plane, and $0.14\ifmmode\pm\else\textpm\fi{}0.01$ $(0.29\ifmmode\pm\else\textpm\fi{}0.02)\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ in the vertical plane, respectively. These emittances were measured with a corresponding bunch length of $2.1\ifmmode\pm\else\textpm\fi{}0.1$ $(3.0\ifmmode\pm\else\textpm\fi{}0.2)\text{ }\text{ }\mathrm{ps}$, respectively. In each case the rms momentum spread was determined to be on the order of ${10}^{\ensuremath{-}3}$. Excellent overall agreement between measurement and simulation has been demonstrated. Using the emittances and bunch length measured at $19\text{ }\text{ }\mathrm{pC}/\mathrm{bunch}$, we estimate the electron beam quality in a 1.3 GHz, 5 GeV hard x-ray ERL to be at least a factor of 20 times better than that of existing storage rings when the rms energy spread of each device is considered. These results represent a milestone for the field of high-brightness, high-current photoinjectors.

Thermal analysis of LN2-cooled silicon crystals for Energy Recovery Linac monochromators

We discuss major features of the predicted power distribution in undulator beams at the proposed Cornell Energy Recovery Linac, and the anticipated consequences for monochromator performance. Finite element analysis of thermal deformation of LN2-cooled Si crystal monochromators for the ERL is summarized. A new metric, the modified linear power density, is found to provide a clear description of crystal thermal deformation.

Frequency control in the process of a multicell superconducting cavity production

Modifications in the geometry of a superconducting RF cavity due to various processing procedures are presented in a convenient matrix formulation. Specifically, the effect of chemical etching, cooling down, and preloading are characterized, while the corresponding frequency shifts are calculated with a reliable software. This matrix method was used in the fabrication of the first cornell energy recovery linac (ERL) 7-cell cavity. Cavity fabrication can be broken down into three main stages: deep-drawing cups, welding the cups in pairs to obtain "dumbbells" and end groups, and, finally, welding the obtained components into a completed cavity. Frequency measurements and precise machining were implemented after the second stage. A custom RF fixture and data acquisition system were designed and validated for this purpose. The system comprised of a mechanical press with RF contacts, a network analyzer, a load cell and custom LABVIEW and MATLAB scripts. To extract the individual frequencies of the cups from these measurements, the established algorithm of calculations was analysed and corrected. Corrections for the ambient environment were also incorporated into the measurement protocol. Using the procedure presented, the frequency deviation of the completed 1.3 GHz 7-cell cavity was 360 kHz, corresponding to an average error about 75 μm in length for every cell.

Mechanical optimization of superconducting cavities in continuous wave operation

Several planned accelerator facilities call for hundreds of elliptical cavities operating cw with low effective beam loading, and therefore require cavities that have been mechanically optimized to operate at high ${Q}_{L}$ by minimizing $df/dp$, the sensitivity to microphonics detuning from fluctuations in helium pressure. Without such an optimization, the facilities would suffer either power costs driven up by millions of dollars or an extremely high per-cavity trip rate. ANSYS simulations used to predict $df/dp$ are presented as well as a model that illustrates factors that contribute to this parameter in elliptical cavities. For the Cornell Energy Recovery Linac (ERL) main linac cavity, $df/dp$ is found to range from 2.5 to $17.4\text{ }\text{ }\mathrm{Hz}/\mathrm{mbar}$, depending on the radius of the stiffening rings, with minimal $df/dp$ for very small or very large radii. For the Cornell ERL injector cavity, simulations predict a $df/dp$ of $124\text{ }\text{ }\mathrm{Hz}/\mathrm{mbar}$, which fits well within the range of measurements performed with the injector cryomodule. Several methods for reducing $df/dp$ are proposed, including decreasing the diameter of the tuner bellows and increasing the stiffness of the enddishes and the tuner. Using measurements from a Tesla Test Facility cavity as the baseline, if both of these measures were implemented and the stiffening rings were optimized, simulations indicate that $df/dp$ would be reduced from $\ensuremath{\sim}30\text{ }\text{ }\mathrm{Hz}/\mathrm{mbar}$ to just $2.9\text{ }\text{ }\mathrm{Hz}/\mathrm{mbar}$, and the power required to maintain the accelerating field would be reduced by an order of magnitude. Finally, other consequences of optimizing the stiffening ring radius are investigated. It is found that stiffening rings larger than 70% of the iris-equator distance make the cavity impossible to tune. Small rings, on the other hand, leave the cavity susceptible to plastic deformation during handling and have lower frequency mechanical resonances, which is undesirable for active compensation of microphonics. Additional simulations of Lorentz force detuning are discussed, and the results are compared to measurements on the ERL injector cavities.

Photocathode behavior during high current running in the Cornell energy recovery linac photoinjector

The Cornell University energy recovery linac (ERL) photoinjector has recently demonstrated operation at 20 mA for approximately 8 hours, utilizing a multialkali photocathode deposited on a Si substrate. We describe the recipe for photocathode deposition, and will detail the parameters of the run. Post-run analysis of the photocathode indicates the presence of significant damage to the substrate, perhaps due to ion back-bombardment from the residual beam line gas. While the exact cause of the substrate damage remains unknown, we describe multiple surface characterization techniques (x-ray fluorescence spectroscopy, x-ray diffraction, atomic force, and scanning electron microscopy) used to study the interesting morphological and crystallographic features of the photocathode surface after its use for high current beam production. Finally, we present a simple model of crystal damage due to ion back-bombardment, which agrees qualitatively with the distribution of damage on the substrate surface.

Optimal choice of cell geometry for a multicell superconducting cavity

An algorithm for optimization of the multicell cavity cells is proposed. Inner cells are optimized for minimal losses or minimal magnetic field, when the aperture diameter, ${E}_{pk}/{E}_{\mathrm{acc}}$---the ratio of peak electric field to the accelerating field, and the wall slope angle are given. Optimization of the end cells is done for minimal losses or maximal acceleration in them. Two shapes of the end cells---with and without the end irises---are analyzed. This approach facilitates further optimization for higher order modes extraction because it permits keeping the achieved optimal values nearly the same while changing some dimensions of the cells. Comparison of the proposed cavity geometry with the TESLA cavity geometry illustrates the traits of the presented approach. It is also shown that lower values of the wall slope angle, which lead to the reentrant shape for the inner cells, are also beneficial for the end cells. For the Cornell Energy Recovery Linac most dangerous are dipole modes causing the beam breakup (BBU). Minimization of power of higher order modes (HOMs) in a multicell cavity was done using derivatives of the BBU parameter with respect to geometric parameters of the cavity cells. As a starting point of optimization, the shape with minimal losses at the fundamental mode was taken. Further changing the shape for better propagation of HOMs was done with degradation of the fundamental mode loss parameter $G\ifmmode\cdot\else\textperiodcentered\fi{}{R}_{\mathrm{sh}}/Q$ within 1% while decrease of the BBU parameter was nearly 3 orders of magnitude. The BBU threshold current tends to be inversely proportional to this parameter.

Delta undulator for Cornell energy recovery linac

In anticipation of a new era of synchrotron radiation sources based on energy recovery linac techniques, we designed, built, and tested a short undulator magnet prototype whose features make optimum use of the unique conditions expected in these facilities. The prototype has pure permanent magnet (PPM) structure with 24 mm period, 5 mm diameter round gap, and is 30 cm long. In comparison with conventional undulator magnets it has the following: (i) full x-ray polarization control.---It may generate varying linear polarized as well as left and right circular polarized x rays with photon flux much higher than existing Apple-II--type devices. (ii) 40% stronger magnetic field in linear and approximately 2 times stronger in circular polarization modes. This advantage translates into higher x-ray flux. (iii) Compactness.---The prototype can be enclosed in a $\ensuremath{\sim}20\text{ }\text{ }\mathrm{cm}$ diameter cylindrical vacuum vessel. These advantages were achieved through a number of unconventional approaches. Among them is control of the magnetic field strength via longitudinal motion of the magnet arrays. The moving mechanism is also used for x-ray polarization control. The compactness is achieved using a recently developed permanent magnet soldering technique for fastening PM blocks. We call this device a ``Delta'' undulator after the shape of its PM blocks. The presented article describes the design study, various aspects of the construction, and presents some test results.

Equilibrium ion distribution in the presence of clearing electrodes and its influence on electron dynamics

Here we compute the ion distribution produced by an electron beam when ion-clearing electrodes are installed. This ion density is established as an equilibrium between gas ionization and ion clearing. The transverse ion distributions are shown to strongly peak in the beam's center, producing very nonlinear forces on the electron beam. We will analyze perturbations to the beam properties by these nonlinear fields. To obtain reasonable simulation speeds, we develop fast algorithms that take advantage of adiabatic invariants and scaling properties of Maxwell's equations and the Lorentz force. Our results are very relevant for high current energy recovery linacs, where ions are produced relatively quickly, and where clearing gaps in the electron beam cannot easily be used for ion elimination. The examples in this paper therefore use parameters of the Cornell Energy Recovery Linac project. For simplicity we only consider the case of a circular electron beam of changing diameter. However, we parametrize this model to approximate nonround beams well. We find suitable places for clearing electrodes and compute the equilibrium ion density and its effect on electron-emittance growth and halo development. We find that it is not sufficient to place clearing electrodes only at the minimum of the electron beam potential where ions are accumulated.