High RF Fields Research Articles

The use of high RFQ fields to manipulate ions

Recent tests of high dc and RF fields in helium at up to several tens of Pascals have shown that electric fields of tens of kilovolts can be used to manipulate ions under RFQ confinement in buffer gas. The theoretical and practical considerations of such confinement are considered and the results of a prototype linear RFQ ion trap designed for examining typical ion clouds under such confinement are presented. Operation at quadrupole field strengths of an order of magnitude greater than currently used is proven to be feasible. The ion temperature achieved is also of an order of magnitude greater than that in lower field devices but this higher temperature is believed to be due to distortions in the RF waveform from the purely simple harmonic rather than just due to the higher RF field.

NMR/ON of 60Co Experiments on the Neutron Activated Fe-Co Alloy

The irradiation by thermal neutrons is a commonly used procedure for the preparation of radioactive samples for low-temperature nuclear orientation (LTNO) experiments. Such an irradiation is a potential source of irradiation damage. The ferromagnetic alloy Fe+0.5 Co was selected for this kind of radiation damage study by LTNO and the nuclear magnetic resonance of oriented nuclei (NMR/ON), respectively. It was found by LTNO that in an as-irradiated sample, smaller amount of 60Co nuclei is subject to regular hyperfine field Bhf=29.01 T in comparison with the annealed sample. The same LTNO single resonance line on the frequency f=165.6(2) MHz was observed with both as-irradiated and annealed samples, respectively. The resonant destruction of γ-ray anisotropy is significantly smaller in as-irradiated sample and higher rf field is to be used to obtain a sizable NMR/ON effect. The experimental data indicates a great spread of hyperfine field values in non-regular positions of 60Co nuclei without contribution to the NMR/ON.

Implementing solenoid microcoils for wide-line solid-state NMR

Solid-state nuclear magnetic resonance (NMR) probeheads using solenoid microcoils with an inner diameter of 300–400 μm are developed for the study of mass-limited solid samples. Some attention is paid to the mechanical ruggedness of the probes allowing sample changing. The performance, in terms of sensitivity and RF-characteristics, of these probeheads is studied for 1H, 31P, and 27Al in different model compounds in view of the feasibility of specific applications. The results show that the sensitivity is high enough to detect approximately 10 14 spins/√Hz with a signal-to-noise ratio of 1 in a single scan. A specific advantage of microcoils for solid-state NMR applications is that they can generate extremely high RF-fields if implemented in appropriate circuits. Using RF-powers in the hundreds of Watts range, RF-fields well in excess of 1 MHz can be made. This allows the excitation of spectra of nuclei whose resonance lines are dispersed of several megahertz. This is particularly useful for quadrupolar nuclei experiencing large quadrupolar interactions as is demonstrated for 27Al in single crystal and powdered minerals.

Heteronuclear spin decoupling in solid-state NMR under magic-angle sample spinning

Achieving high spectral resolution is an important prerequisite for the application of solid-state NMR to biological molecules. Higher spectral resolution allows to resolve a larger number of resonances and leads to higher sensitivity. Among other things, heteronuclear spin decoupling is one of the important factors which determine the resolution of a spectrum. The process of heteronuclear spin decoupling under magic-angle sample spinning is analyzed in detail. Continuous-wave RF irradiation leads only in a zeroth-order approximation to a full decoupling of heteronuclear spin systems in solids under magic-angle spinning (MAS). In a higher-order approximation, a cross-term between the dipolar-coupling tensor and the chemical-shielding tensor is reintroduced, providing a scaled coupling term between the heteronuclear spins. In strongly coupled spin systems this second-order recoupling term is partially averaged out by the proton spin-diffusion process, which leads to exchange-type narrowing of the line by proton spin flips. This process can be described by a spin-diffusion type superoperator, allowing the efficient simulation of strongly coupled spin systems under heteronuclear spin decoupling. Low-power continuous-wave decoupling at fast MAS frequencies offers an alternative to high-power irradiation by reversing the order of the averaging processes. At fast MAS frequencies low-power continuous-wave decoupling leads to significantly narrower lines than high-power continuous-wave decoupling while at the same time reducing the power dissipated in the sample by several orders of magnitude. The best decoupling is achieved by multiple-pulse sequences at high RF fields and under fast MAS. Two such sequences, two-pulse phase-modulated decoupling (TPPM) and X-inverse-X decoupling (XiX), are discussed and their properties analyzed and compared.

Rf Field Dependence of Surface Resistance for a–b Plane-Textured YBa2Cu3O7–δ Films Deposited on Copper Substrate

Low rf loss at high rf field levels should be realized in order to apply high-Tc films to accelerator cavities. It is well known that c-axis perpendicular to the surface is essential to reduce Rs and its field dependence However, the effect of a–b plane texturing on Rs is not so clear because lack of experiments, especially for films deposited on metallic substrates. We developed a deposition technique that enables a–b plane texturing as well as c-axis orientation of YBCO films on a copper substrate. We prepared four samples with c-axis normal to the surface: two of them were a–b plane well textured films and the other two were a–b plane weakly textured films. The a–b plane well-textured films in low rf field exhibited a Rs higher than that of weakly textured films below 80 K. The rf field dependence measurements showed that the increase of Rs with increasing rf field for the a–b plane well-textured films was slower as compared with that of the weakly textured films.

Comprehensive diagnostic suite for a magnetically insulated transmission line oscillator

The magnetically insulated transmission line oscillator (MILO) is a gigawatt-class cross-field microwave tube that requires no external magnetic field due to inherent self-magnetic insulation. The tube operates with a 500 kV, 60 kA electron beam which, along with high rf fields, poses quite a challenge for diagnosing the device. We report on the comprehensive set of experimental diagnostics (both beam and microwave) employed in the MILO experiment, and show how these diagnostics, teamed with particle-in-cell computer simulations, have been instrumental in discovering problems with the tube.

Multiple nuclear spin echo in thin polycrystalline ferromagnetic films

The formation of multiple nuclear spin echo signals has been studied in thin ferromagnetic polycrystalline films of 3d-metals and their alloys with induced anisotropy at temperatures between 2.2 and 300 K using two-pulse and three-pulse excitation. A method is proposed for the experimental determination of the contributions made by different mechanisms to the formation of spin-echo signals in magnets with strongly inhomogeneous Zeeman and quadrupole interactions. It is shown that in ferromagnets with a high rf field gain at the nucleus, the frequency modulation mechanism has a substantial influence in observations of nuclear spin-echo signals at nuclei with a high magnetic moment, even at liquid-helium temperatures.

Triple, quintuple and higher order multiple quantum MAS NMR of quadrupolar nuclei

The optimization of the coherence transfers involved in five, seven and nine-quantum versions of the recently discovered MQMAS technique, is analysed numerically. Data reported in this paper may serve as starting parameters for the experiment set up. An analysis of the intensity and resolution given by each type of experiment is performed, which confirms the need to use very high rf fields for MQ transfers. It follows that five-quantum is achievable rather easily but the use of seven and nine-quantum MAS experiments becomes increasingly difficult due to the demand for high rf power and decreasing sensitivity. The advantages of using the z-filter MQMAS method with respect to a two-pulse sequence are analysed. The method for qualitatively and quantitatively interpret the MQMAS spectra is described. The nature of the spinning side bands along the multiple quantum dimension is explained. It is shown that the rotor synchronization can be conveniently used to eliminate these side bands, but only for 3QMAS experiments. The use of the multiple-quantum method in combination with static samples and VAS, DAS and DOR techniques is finally discussed.

Study of luminous phenomena observed on contaminated metallic surfaces submitted to high RF fields

The primary limitation to the increase of the attainable accelerating gradient in superconducting cavities is the strong field emission (FE) of electrons from the RF surface. Many studies devoted to the DC field emission have shown that the electron emission on a broad metallic surface stems from a few very localized sources, called emitters. Superficial dust particles (metallic or dielectric) or semiconducting impurities embedded in the surface can originate the emission. Different experimental results have led to several theoretical models explaining the physical mechanisms of the field emission. We have investigated the RF field emission from a sample subjected to high RF fields ( E peak > 40 MV/m) in a copper cavity. Our study is focused on the luminous emissions occurring on the RF surface simultaneously with the electron emission. The optical apparatus attached to the cavity permits one to observe the evolution of the emitters and the direct effect of the surface conditioning. Also, the parameters of the emitted radiation (intensity, glowing duration, spectral distribution) may provide additional information on the field emission phenomena. This paper relates some results concerning samples intentionally contaminated with particles (metallic or dielectric).

An Improved TM110 ENDOR Cavity for the Investigation of Transition Metal Complexes

An improved, high-sensitivity X-band cavity for ENDOR measurements using high RF fields is described. The advantages of this cavity are stable microwave coupling using an iris/wave-guide, high quality factor (Q = 5000), high field-modulation amplitude, light irradiation capability, and wide temperature range from 1.8 to 300 K using an Oxford liquid helium flow cryostat.

The microwave surface impedance of high-T/sub c/ superconductors

The available data on the microwave surface impedance Z/sub s/ of high-T/sub c/ superconductors from more than 30 laboratories around the world are reviewed to show the frequency, temperature, and field dependence of the surface resistance R/sub s/ and the temperature dependence of the field penetration depth lambda . The data are discussed on the basis of a two-fluid model and are compared with results from the classical superconductors Nb and Nb/sub 3/Sn. At temperatures above 0.8 T/sub c/, Z/sub s/ values of classical and high-T/sub c/ superconductors show strong similarities and are in close agreement with the Mattis-Bardeen theory. The comparatively very high R/sub s/ of the cuprates at 4.2 K, however, is indicative of a high density of unpaired charge carriers. The lowest values for R/sub s/ and lambda obtained until now with epitaxially grown YBa/sub 2/Cu/sub 3/O/sub 7- delta / films are 16 mu Omega at 4.2 K and 10 GHz and 140 nm, respectively. Such films are required for high RF field applications. Polycrystalline samples show a strong field dependence of R/sub s/. The highest magnetic surface field without R/sub s/ deterioration achieved so far on single crystals of YBa/sub 2/Cu/sub 3/O/sub 7- delta / is about 100 G.

Measurements of the surface resistance of high-T c superconductors at high RF fields

We report measurements of the r.f. surface resistance of a wide variety of high-Tc superconductors in bulk form and as films on silver substrates. Several apparatuses have been constructed and used for measurements at frequencies from 0.15 to 40 GHz and r.f. surface magnetic fields as high as 640 G. In every case in which the field dependence of the surface resistance was measured, the surface resistance increased monotonically with field amplitude through a transition region characterized by a strong field dependence. It then saturated at high field at a value of a few percent of the normal-state surface resistance just aboveTc. In the presence of this field dependence, the frequency dependence of the surface resistance changed from quadratic to less than linear.

Electron nuclear double resonance near the phase transitions of the KH2PO4-type of ferroelectrics

Using high rf fields, a low-loss microwave cavity/ENDOR coil system, and the AsO4- 4 centre as a probe, we have detected ENDOR signals from 1H. 39K, 31P, 85Rb, 87Rb and 133Cs in the close vicinity of paraelectric-ferroelectric transitions of the KH2PO4-type of compounds. This methodology enhances E.P.R. spectral resolution and motional sensitivity by almost two orders of magnitude, and appears to be a valuable complement to spin-echo techniques, even for samples exhibiting high dielectric losses.

Power efficient double-resonance NMR probe for electromagnets

A NMR probe design of high efficiency is presented for a double-resonance spectrometer. The design uses easily constructed transmission line filters and a bridge box duplexing switch to reduce probe ringing. Circuits are used which allow independent tuning of the 100- and 25.1-MHz circuits in the probe. High strength rf fields are produced in the probe using low power transmitters, and good sensitivity is obtained on carbon.

Superconducting Cavities and Modulated RF

If a cavity has an infinite Qo, 81.5% of the energy contained in a pulse incident upon the cavity is transferred into the cavity by the end of the pulse if the cavity Qe is chosen so that the cavity time constant is 0.796 pulse width (Ta). As Qo decreases, the energy in the cavity at the end of the pulse decreases very slowly as long as Ta is much less than the unloaded cavity time constant, Tco. SC cavities with very high Qo enable us to obtain very high gradients with a low power cw source. At high gradients, however, we often do not attain the high Qo predicted by theory. Therefore, if we are interested in attaining maximum energy in the cavity, as is the case for RF processing and diagnostics, for a given available source energy there is no point in keeping the power on for longer than 0.1 Tco because the energy expended after 0.1 Tco is wasted. Therefore, to attain high fields at moderate Qo, pulsed operation is indicated. This note will derive the fields and energy stored and dissipated in the cavity when Qe is optimized for a given Ta. It will show how to use this data to measure Qo of an SC cavity as a function of field level, how to process the cavity with high RF fields, how to operate SC cavities in the pulsed mode to obtain higher efficiencies and gradients. Experimental results will also be reported.

Microwave surface resistance of Nb films

The surface resistance, R s , of niobium (Nb) films has been experimentally investigated as a function of thickness, preparation technique and substrate material at 8.86 GHz. Nb films were prepared by either sputtering or evaporation in the thickness range Between 0.1 μm and 3.0 μm on either copper (Cu) or sapphire substrate. R s was determined using a cylindrical TE 011 mode resonant cavity with one removable end-plate which was utilized as the test substrate. The low field R s at 4.2 K is lower than that of bulk Nb and shows good agreement with BCS calculation which takes into account the effects of mean free path. The temperature dependence of R s indicates a normalized film gap parameter, Δ(0)/KT c , nearly equivalent to the bulk value for most of the films. At low temperatures, R s is dominated by residual resistance (R 0 ) which approaches 1 μΩ. The overall characteristics of Nb on Cu (Nb/Cu) indicate that this composite material is potentially useful in applications requiring high rf field as well as high thermal stability.

ENDOR cell with high RF fields for steady state experiments

A new ENDOR cell is described which serves both microwave and RF irradiating functions. The strong RF magnetic field is created by a relatively small current and allows the study of free radicals in solution. This field is continuous and consequently the analysis of ENDOR spectra is facilitated. Another feature is that no modulation of the static magnetic field is required.

Plasma instabilities excited by high RF field near the lower hybrid frequency

Plasma instabilities excited by high RF field near the lower hybrid frequency

NMR line shapes for exchanging systems at high rf field

A method is described by which NMR line shapes at high rf may be calculated for chemically exchanging systems. First order exchange results in a set of manifestly linear density matrix equations that are solved with standard methods. Second order exchanging systems, a model example of which is considered here, give rise to second order density matrix equations. Inspection of sample values of density matrix elements reveals that products of off-diagonal elements are small enough to drop from the equations, while products of off-diagonal and diagonal elements, respectively, may be linearized as ρi jAρk,kB=ρi jA/n B. This selective neglect of bilinear terms (SNOB) leads to a set of first order density matrix equations. Inspection of the calculated line shapes reveals that NMR at high rf in exchanging systems allows more precise determination of rate constants for exchange (compared to low power) in the slow, and, in a limited sense, in the fast exchange limits.

On surface preparation and measurement of niobium used in high-frequency cavities

The influence of several surface preparation techniques, such as electropolishing, chemical polishing, anodizing, and firing in an UHV furnace with different cool-down cycles, on Q0 values, peak rf fields, and surface mean free path ls have been studied in a welded S-band cavity having large beam tubes. With special treatments, peak fields up to 810 G at Q0≈1011 have been obtained, whereas the surface mean free paths stayed fairly short (≲300 Å ) not changing systematically with (surface) treatment. Hence, to obtain low rf losses and high rf fields, the purity is not important, but, as we will explain, the spatial homogeneity is very important. Measurements with Nb cylinders, processed like the cavities, showed that the variation of flux penetration depth λ(Hdc) with a dc magnetic field Hdc parallel to the cylinder surface changed markedly with surface treatment. This variation of λ(Hdc) shows several features, which are related to surface properties, namely, the reversibility near Hc1 and the signal at Hc2 as indications for roughness or oxygen in the surface sheath, the irreversibility between Hc1 and Hc2 as an indication for inhomogeneities, especially NbO precipitates, in a Nb surface layer. For example, by this method we observed that the usual cool down from 400 to 50°C in the UHV furnace deteriorates a Nb surface layer of several μ thickness. These surface properties obtained with λ(Hdc) are not only important for achieving better rf cavities but also for ac power transmission lines.