High RF Fields Research Articles

Design of injectors and stay-alone cryostats with superconducting cavities for high RF powers applications

Injectors and stay-alone cryostats with superconducting (sc) cavities are sometimes applied at modern particle accelerators. Typically, very stringent requirements on beam quality are foreseen at injectors, and application of sc cavities is advantageous, particularly if operation at high current densities and high accelerating gradients is foreseen. Stay-alone cryostats with one or few sc cavities also find applications, e.g. at ELBE, DIAMOND, S-DALINAC accelerators, and accelerate particle beams in High Frequency fields. In the future, very high RF fields will be needed, and improvement of the cooling of cavity surfaces will improve cavity performance. Application of a sub-cooled superfluid helium gives several advantages, e.g. higher heat flux densities, longer time for onset of a film boiling regime and shorter recovery time, reduced Kapitza resistances, etc [2]. In the present paper, an application of sub-cooled superfluid helium for injector and stay alone cryostats is considered. Design of the cryogenic system is presented in some details and possibility of adapting of such injector for future applications is also discussed.

Radio frequency He4 and sonoluminescence photons are combined

Two systems are presented in this paper: 1) describes the Radio Frequency, RF, reactor that produces alphas and 2) the polycarbonate reactor that produced sonoluminescence, SL or gammas several years earlier. During experiments, dV/dt rapid changes were noted in volumes of proton and deuteron capturing an electron as the initial temperature cools. This is a very basic event at the atomic level. The D+ + e−D* will produce a shockwave that is only compressive for the elemental hydrogen isotopes. The shock wave that destroys the MC cluster of deuterons in a squeezing process, ends with an explosion that excites the MC debris. An excited state occurs where a free MC deuteron combines with the D*forming an alpha as part of the debris system for that cycle. He4 was collected in the Ar gas phase for a mass spectroscopy analysis, MS. Only the 3 hydrogen isotopes have the potential of the large dV/dt compression. Old measurements of the sonoluminescence (SL) photons emitted during DOD cavitation fits RF experiments, and it was never thought He4 might be associated with SL. But a closer look makes a case that SL and gamma photons were the same. The RF system during DOD cavitation could be associated with the improbable equation 2D He4 + g. The measured SL fits the RF He4 MS measurements. The advantage of the RF amplifier driven system was its freedom of a locked frequency of the oscillator driven systems and can be frequency tuned while running. The system could be tuned off for 5 s temperature measurement in the 30 s run mode, thus avoiding the high RF field interference to thermocouple measurements. This SW system was impressed with Lawandy’s like charge attractive forces system that was compatible with MC SW system making the experiments that yield other He4 paths. These were single events of a few atoms, with high activity electrons for a short time, during a 2-MHz cycle.

Electrodeposition of copper applied to the manufacture of seamless superconducting rf cavities

Niobium thin film coated copper superconducting radio frequency elliptical cavities have demonstrated for many years their strong potential as an alternative to bulk niobium cavities. The thin film lower performance at high rf field is often attributed to the defects observed in the elaborated Nb layer, sometimes originated from defects inherited from the substrate itself. The currently used methods of manufacturing the copper elliptical substrates include several steps of electron-beam welding in order to join the half cells and the cutoffs which can contribute to defects and porosities. Seamless methods are nowadays developed in order to avoid welding steps and to decrease the global manufacturing cost of the cavities. We propose in this study an innovative alternative route in which the cavity is formed by electrodeposition of copper on a sacrificial aluminum mandrel. The strength of the process relies on the total absence of welding joints. Two different electroforming techniques using either direct current or pulsed plating have been investigated. The electroformed copper exhibited similar mechanical robustness, cryogenic properties and purity as the oxygen-free copper. In addition, the fabrication process was validated on test mandrels which mimic the geometry of 1.3 GHz cavities.

Ambient Pressure Ion Funnel: Concepts, Simulations, and Analytical Performance.

In mass spectrometry (MS), a major loss of ions can readily occur during their transfer from atmospheric pressure to a lower pressure, which limits performance. Here, we report an ion funnel that can be used to effectively focus ions at ambient pressure (∼777 Torr) to significantly enhance performance in electrospray ionization (ESI) MS. For seven singly charged test ions (m/z 124-1131), the ambient pressure ion funnel (APIF) is demonstrated to improve ion abundances, sensitivity, and detection limits by up to factors of ∼17, ∼16, and ∼3, respectively, compared to the operation of conventional ESI-MS. Simulated ion trajectories were used to rationalize the enhanced performance of the APIF, which is attributed primarily to using a relatively high RF field amplitude to radially confine ions, a high DC field, and a wide exit ring electrode. The effective focusing of ions at ambient pressures should be beneficial in the future for improving the performance of (i) additional methods that ionize molecules at atmospheric pressure, (ii) ambient pressure ion mobility-based instruments, and (iii) high flow rate liquid chromatography mass spectrometry platforms.

High performance InAlN/GaN high electron mobility transistors for low voltage applications**Project supported by the China Postdoctoral Science Foundation (Grant No. 2018M640957), the Fundamental Research Funds for the Central Universities, China (Grant No. 20101196761), the National Natural Science Foundation of China (Grant No. 61904135), the National Defense Pre-Research Foundation of China (Grant No. 31513020307), and the Natural Science Foundation of Shaanxi

A high performance InAlN/GaN high electron mobility transistor (HEMT) at low voltage operation (6–10 V drain voltage) has been fabricated. An 8 nm InAlN barrier layer is adopted to generate large 2DEG density thus to reduce sheet resistance. Highly scaled lateral dimension (1.2 μm source–drain spacing) is to reduce access resistance. Both low sheet resistance of the InAlN/GaN structure and scaled lateral dimension contribute to an high extrinsic transconductance of 550 mS/mm and a large drain current of 2.3 A/mm with low on-resistance (Ron) of 0.9 Ω⋅mm. Small signal measurement shows an fT/fmax of 131 GHz/196 GHz. Large signal measurement shows that the InAlN/GaN HEMT can yield 64.7%–52.7% (Vds = 6–10 V) power added efficiency (PAE) associated with 1.6–2.4 W/mm output power density at 8 GHz. These results demonstrate that GaN-based HEMTs not only have advantages in the existing high voltage power and high frequency rf field, but also are attractive for low voltage mobile compatible rf applications.

Studies of space charge dominated electron photoemission at PITZ

Photoemission in modern high brightness electron sources is under studies at the PITZ photo injector. Space charge dominated photoemission in the presence of high RF field at the semiconductor photocathode is studied. By utilizing core and halo particle distributions based on measured radial laser profiles, simulations reproduce the behaviour of the measured emission curves for a wide range of RF gun parameters within the measurement uncertainties for Gaussian laser pulses. But applying this model to the case of long flattop photocathode laser pulses revealed discrepancies between experimental data and simulation results. Corresponding emittance simulations have been compared to measurements for both temporal profiles of the photocathode laser.

Terahertz-based subfemtosecond metrology of relativistic electron beams

Photons, electrons, and their interplay are at the heart of photonic devices and modern instruments for ultrafast science [1-10]. Nowadays, electron beams of the highest intensity and brightness are created by photoemission with short laser pulses, and then accelerated and manipulated using GHz radiofrequency electromagnetic fields. The electron beams are utilized to directly map photoinduced dynamics with ultrafast electron scattering techniques, or further engaged for coherent radiation production at up to hard X-ray wavelengths [11-13]. The push towards improved timing precision between the electron beams and pump optical pulses though, has been stalled at the few tens of femtosecond level, due to technical challenges with synchronizing the high power rf fields with optical sources. Here, we demonstrate attosecond electron metrology using laser-generated single-cycle THz radiation, which is intrinsically phase locked to the optical drive pulses, to manipulate multi-MeV relativistic electron beams. Control and single-shot characterization of bright electron beams at this unprecedented level open up many new opportunities for atomic visualization.

High radio-frequency field strength nutation NMR of quadrupolar nuclei

Owing to the introduction of microcoils, high RF field strength nutation NMR is a viable candidate for the study of quadrupolar nuclei with strong quadrupolar couplings, not accessible using contemporary NMR techniques. We show powder 23Na nutation spectra on sodium nitrite for RF field strengths of up to 1170kHz, that conform to theoretical predictions. For lanthanum fluoride powder, 139La nutation spectra taken at elevated RF field amplitudes show clear discrepancies when compared to the theory. These errors are shown to be mainly caused by pulse transients at the end of the pulse, which proved to be detrimental to the shape of the nutation spectra. Using a nutation pulse which ends in a sudden frequency jump, we show that these errors can be reduced, and nutation spectra that conform to theory can be readily acquired. This enables nutation NMR for the study of quadrupolar nuclei with a strong quadrupolar coupling, bridging the gap between NMR, which can only analyse nuclei with a weak to medium quadrupolar coupling, and NQR, were extensive searching for the right quadrupolar frequency is the limiting factor.

The dust nature of micro field emitters in accelerators

Field emission currents emitted by micro-emitters are a limiting factor for the operational gradients of accelerating radio frequency (rf) cavities. Within the rf field emission theory the existence of needle like micro field emitters with very high length relative to the radius and corresponding high enhancement factor (β) is assumed. In this article the hypothesis that micro field emitters consists of long chains of conductive micro-particles is considered. Five different forces acting onto the particles in a high rf field are considered and the respective equations are derived. Some experimental observations and their explanation within this hypothesis are discussed.

A cross-polarization based rotating-frame separated-local-field NMR experiment under ultrafast MAS conditions

Rotating-frame separated-local-field solid-state NMR experiments measure highly resolved heteronuclear dipolar couplings which, in turn, provide valuable interatomic distances for structural and dynamic studies of molecules in the solid-state. Though many different rotating-frame SLF sequences have been put forth, recent advances in ultrafast MAS technology have considerably simplified pulse sequence requirements due to the suppression of proton–proton dipolar interactions. In this study we revisit a simple two-dimensional 1H–13C dipolar coupling/chemical shift correlation experiment using 13C detected cross-polarization with a variable contact time (CPVC) and systematically study the conditions for its optimal performance at 60kHz MAS. In addition, we demonstrate the feasibility of a proton-detected version of the CPVC experiment. The theoretical analysis of the CPVC pulse sequence under different Hartmann–Hahn matching conditions confirms that it performs optimally under the ZQ (w1H−w1C=±wr) condition for polarization transfer. The limits of the cross polarization process are explored and precisely defined as a function of offset and Hartmann–Hahn mismatch via spin dynamics simulation and experiments on a powder sample of uniformly 13C-labeled L-isoleucine. Our results show that the performance of the CPVC sequence and subsequent determination of 1H–13C dipolar couplings are insensitive to 1H/13C frequency offset frequency when high RF fields are used on both RF channels. Conversely, the CPVC sequence is quite sensitive to the Hartmann–Hahn mismatch, particularly for systems with weak heteronuclear dipolar couplings. We demonstrate the use of the CPVC based SLF experiment as a tool to identify different carbon groups, and hope to motivate the exploration of more sophisticated 1H detected avenues for ultrafast MAS.

Effect of Electron Emission on Microparticle Heating and Melting in High-Power Microwave Systems

In the circuits of high-power microwave (HPM) devices, such as HPM sources or high-gradient accelerating structures, small quantities of metallic dust may exist. These metallic particles of micrometer size immersed in high-RF fields can absorb enough energy for significant heating and melting. The heating effect of an RF magnetic field was analyzed in our previous papers. In this paper, we analyze the role of the RF electric field which may cause field emission from a microparticle. Our consideration is restricted by ellipsoidal metallic particles. It is shown that the heating effect becomes significant when such microparticles have a needlelike shape, and therefore, the electric field amplification at the ends is high. In this case, the field-emitted current may cause the heating of the microparticles. The heating in single shots and in repetition-rate regimes is studied for the case of the combined effect of both the RF magnetic and electric fields.

MgB2nonlinear properties investigated under localized high rf magnetic field excitation

The high transition temperature and low surface resistance of MgB2 attracts interest in its potential application in superconducting radio frequency accelerating cavities. However, compared to traditional Nb cavities, the viability of MgB2 at high rf fields is still open to question. Our approach is to study the nonlinear electrodynamics of the material under localized rf magnetic fields. Because of the presence of the small superconducting gap in the $\pi$ band, the nonlinear response of MgB2 at low temperature is potentially complicated compared to a single-gap s-wave superconductor such as Nb. Understanding the mechanisms of nonlinearity coming from the two-band structure of MgB2, as well as extrinsic sources of nonlinearity, is an urgent requirement. A localized and strong rf magnetic field, created by a magnetic write head, is integrated into our nonlinear-Meissner-effect scanning microwave microscope [Tamin Tai, X. X. Xi, C. G. Zhuang, D. I. Mircea, and S. M. Anlage, IEEE Trans. Appl. Supercond. 21, 2615 (2011)]. MgB2 films with thickness 50 nm, fabricated by a hybrid physical-chemical vapor deposition technique on dielectric substrates, are measured at a fixed location and show a strongly temperature-dependent third harmonic response. We propose that several possible mechanisms are responsible for this nonlinear response.

Superconducting Radio-Frequency Fundamentals for Particle Accelerators

An overview of fundamentals of superconductors under radio-frequency electromagnetic fields in particle accelerators is given, with emphasis on intrinsic physics and materials mechanisms which limit the performance of the superconducting radio-frequency (SRF) resonator cavities. Multiscale mechanisms which control the surface resistance and the quality factor of the SRF cavities at low and high rf fields are discussed. We also discuss possible ways of pushing the limit of the SRF performance by materials impurities and multilayer nanostructuring which may open up opportunities of using materials other than Nb to significantly increase the maximum accelerating fields and improve the performance of the SRF cavities operating at 4.2 K.

EASY-GOING DUMBO on-spectrometer optimisation of phase modulated homonuclear decoupling sequences in solid-state NMR

A one-step many-parameter optimisation scheme for phase modulated proton homonuclear decoupling sequences in solid-state NMR is presented. Phase modulations, parameterised by DUMBO Fourier coefficients, were optimised using a Covariance Matrix Adaptation Evolution Strategies algorithm. Our method, denoted EASY-GOING DUMBO, starts with featureless spectra and optimises proton–proton decoupling, during either proton or carbon signal detection. Optimisations at moderate sample magic angle spinning (MAS) frequencies and medium radio-frequency (rf) field strengths resulted in solutions closely resembling (e)DUMBO. Application of EASY-GOING DUMBO for optimisation at very high 680kHz rf field strength, 12.5kHz MAS on a 400MHz NMR spectrometer resulted in a new solution, with competitively resolved proton spectra.

High-resolution solid-state13C μMAS NMR with long coherence life times

Longer coherence life times (i.e. smaller homogeneous linewidths) can be achieved for carbon resonances which are strongly coupled to protons with high rf field heteronuclear decoupling in micro magic angle spinning NMR. Better proton decoupling enhances the sensitivity and resolution of two-dimensional through-bond correlation experiments for mass-limited samples with uniform carbon labeling.

Comparison of Breakdown Characteristics in Copper and Stainless Steel under High RF Field with Narrow Waveguide

To study the characteristics of breakdowns in different materials under a high RF field, a high-gradient experiment was proposed with using narrow waveguides having a field of around 140 MV/m at 50 MW power. Two experiments, one on copper (OFC) and another on stainless-steel (AISI-316L), were done. The breakdown rate of the copper waveguide was found to be larger than that of the stainless-steel waveguide at fields higher than several tens of megavolts per meter. The surfaces of both materials were observed after high-field experiments. The area suffering from heavy breakdowns was found to be rugged for the copper waveguide, while that of the stainless-steel smoother than the copper case, even though the heights of dips and bumps were almost the same at 20∼30 microns for the two materials. From these results, it was proven that the breakdown study using narrow waveguide under the high gradient field offered great potential.

A solid-state NMR and DFT study of compositional modulations in AlxGa1−xAs

We have conducted (75)As and (69)Ga Nuclear Magnetic Resonance (NMR) experiments to investigate order/disorder in Al(x)Ga(1-x)As lift-off films with x∼ 0.297 and 0.489. We were able to identify all possible As(Al(n)Ga(4-n)) sites with n = 0-4 coordinations in (75)As NMR spectra using spin-echo experiments at 18.8 Tesla. This was achieved by employing high rf field strengths using a small solenoid coil and an NMR probe specifically designed for this purpose. Spectral deconvolution, using an evolutionary algorithm, complies with the absence of long-range order if a CuAu based order parameter is imposed. An unconstrained fit shows a deviation of the statistics imposed by this type of ordering. The occupational disorder in the Ga and Al positions is reflected in a distribution of the Electric Field Gradients (EFGs) experienced at the different arsenic sites. We established that this can be modelled by summing the effects of the first coordination sphere and a Czjzek type distribution resulting from the compositional variation in the Al/Ga sub-lattice in the higher coordination spheres. (69)Ga 3QMAS and nutation data exclude the presence of highly symmetric sites and also show a distribution in EFG. The experimentally obtained quadrupolar interactions are in good agreement with calculations based on Density Functional Theory (DFT). Using additivity of EFG tensors arising from distant charge perturbations, we could use DFT to model the EFG distributions of the n = 0-4 sites, reproducing the Czjzek and extended Czjzek distributions that were found experimentally. On the basis of these calculations we conclude that the (75)As quadrupolar interaction is sensitive to compositional modulations up to the 7th coordination shell in these systems.

High-resolution liquid- and solid-state nuclear magnetic resonance of nanoliter sample volumes using microcoil detectors

The predominant means to detect nuclear magnetic resonance (NMR) is to monitor the voltage induced in a radiofrequency coil by the precessing magnetization. To address the sensitivity of NMR for mass-limited samples it is worthwhile to miniaturize this detector coil. Although making smaller coils seems a trivial step, the challenges in the design of microcoil probeheads are to get the highest possible sensitivity while maintaining high resolution and keeping the versatility to apply all known NMR experiments. This means that the coils have to be optimized for a given sample geometry, circuit losses should be avoided, susceptibility broadening due to probe materials has to be minimized, and finally the B(1)-fields generated by the rf coils should be homogeneous over the sample volume. This contribution compares three designs that have been miniaturized for NMR detection: solenoid coils, flat helical coils, and the novel stripline and microslot designs. So far most emphasis in microcoil research was in liquid-state NMR. This contribution gives an overview of the state of the art of microcoil solid-state NMR by reviewing literature data and showing the latest results in the development of static and micro magic angle spinning (microMAS) solenoid-based probeheads. Besides their mass sensitivity, microcoils can also generate tremendously high rf fields which are very useful in various solid-state NMR experiments. The benefits of the stripline geometry for studying thin films are shown. This geometry also proves to be a superior solution for microfluidic NMR implementations in terms of sensitivity and resolution.

Theory of Thermal Fatigue Caused by RF Pulsed Heating

The normal conducting electron-positron Linear Collider projects imply that accelerating structures and other RF components will undergo an action of extremely high RF fields. Except for the RF breakdown threat, there is an effect of the copper surface being damage due to multi-pulse mechanical stress caused by Ohmic losses in the skin layer.

Broadband homonuclear chemical shift correlation at high MAS frequencies: a study of tanh/tan adiabatic RF pulse schemes without $${^{{\bf 1}}\hbox{{\bf H}}}$$ decoupling during mixing

At high magic angle spinning (MAS) frequencies the potential of tanh/tan adiabatic RF pulse schemes for 13C chemical shift correlation without 1H decoupling during mixing has been evaluated. It is shown via numerical simulations that a continuous train of adiabatic 13C inversion pulses applied at high RF field strengths leads to efficient broadband heteronuclear decoupling. It is demonstrated that this can be exploited effectively for generating through-bond and through-space, including double-quantum, correlation spectra of biological systems at high magnetic fields and spinning speeds with no 1H decoupling applied during the mixing period. Experiments carried out on a polycrystalline sample of histidine clearly suggest that an improved signal to noise ratio can be realised by eliminating 1H decoupling during mixing.