Field-free Region Research Articles

Improving the spatial resolution of soft X-ray detection using an Electron-Multiplying Charge-Coupled Device

The Super Advanced X-ray Emission Spectrometer (SAXES) isan instrument at the Swiss Light Source designed for Resonant InelasticX-ray Scattering with an energy resolution (E/ΔE) better than 12000at 930 eV. Improvements to the instrument have been predicted that couldallow the energy resolution to be improved by a factor of two. To achievethis, the spatial resolution of the detector (currently a Charge-CoupledDevice, CCD) over which the energy spectrum is dispersed would have to beimproved to better than 5 μm.X-ray photons with energies between a few hundred to a few thousand electronvolts primarily interact within the field-free region of back-illuminatedCCDs, where each photon forms an electron cloud that diffuses isotropicallybefore reaching the depleted region close to the electrodes. Each photon'selectron cloud is likely to be detected as an event with signal split acrossmultiple pixels. Analysing these split events using centroiding techniquesallows the photon's interaction position to be determined to a sub-pixellevel.PolLux is a soft X-ray microspectroscopy endstation at the Swiss LightSource that can focus 200 eV to 1200 eV X-rays to a spot size of approximately20 nm. Previous studies using data taken with a linear scan across thecentre of a pixel in 3 μm steps predicted an improved resolution byapplying centroiding techniques and using an Electron-Multiplying CCD(EM-CCD). In this study, a full 2D map of the centroiding accuracy in thepixel is presented, formed by rastering in two dimensions across the imageplane in single micron steps. The improved spatial resolution fromcentroiding events in the EM-CCD in all areas of the pixel over the standardCCD is attributed to the improved signal to noise ratio provided by themultiplication register even at high pixel readout speeds (tens of MHz).

Phototransistor effects in Cu(In,Ga)Se2 solar cells

Cu(In,Ga)Se2 solar cells exhibit non-idealities with respect to a standard diode especially at low temperatures: a blocking behavior of the dark current in forward direction, a crossover of the dark and illuminated current–voltage-characteristics and a saturation of the open circuit voltage with respect to temperature and illumination intensity. It will be demonstrated that at low temperatures a Cu(In,Ga)Se2 solar cell with a back barrier essentially behaves as a phototransistor with a ZnO-emitter and Schottky collector. Hereby, the base current is provided by photoexcited holes in the field-free region of the absorber. Comparing simulations with measurements it will be illustrated that these non-idealities can be quantitatively explained. It will be shown that depending on the barrier height of the back diode and the doping profile these characteristics even occur at room temperature. This back barrier might be due to a Schottky contact or to a low doping at the back contact and a higher doping level close to the heterointerface.

Understanding the Thickness-Dependent Performance of Organic Bulk Heterojunction Solar Cells: The Influence of Mobility, Lifetime, and Space Charge

We investigate the reasons for the dependence of photovoltaic performance on the absorber thickness of organic solar cells using experiments and drift-diffusion simulations. The main trend in photocurrent and fill factor versus thickness is determined by mobility and lifetime of the charge carriers. In addition, space charge becomes more and more important the thicker the device is because it creates field free regions with low collection efficiency. The two main sources of space-charge effects are doping and asymmetric mobilities. We show that for our experimental results on Si-PCPDTBT:PC71BM (poly[(4,40-bis(2-ethylhexyl)dithieno[3,2-b:20,30-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,50-diyl]:[6,6]-phenyl C71-butyric acid methyl ester) solar cells, the influence of doping is most likely the dominant influence on the space charge and has an important effect on the thickness dependence of performance.

Design and Analysis of Novel Focused Hyperthermia Devices

Magnetic fluid hyperthermia (MFH) is a promising cancer therapy by virtue of its good depth penetration. As the injected magnetic fluids of MFH are inevitably distributed in the tumor as well as in normal tissues nearby, it is difficult to accurately heat and ablate the targeted tumor without damaging the surrounding healthy tissues. Based on the observation that magnetic nanoparticles are only heated by alternating fields and not by static dc fields, it is possible to exploit such findings in MFH therapy to provide an efficient cure for cancer patients. To avoid unnecessary heating of healthy tissues, a novel system is proposed to set up a dc magnetic field around the tumor target. At the same time, the system creates a field-free region (FFR) with negligible static magnetic field in the target area. The target area can then be fully heated by an alternating field whereas the nearby tissues are not heated. In this paper, a novel ac and dc magnetic field combined MFH system, targeting the treatment area, is presented to produce an accurately focused magnetic field to abate the tumor cells. The proposed method is validated using finite element analysis.

How can large-scale twisted magnetic structures naturally emerge from buoyancy instabilities?

We consider the three-dimensional instability of a layer of horizontal magnetic field in a polytropic atmosphere where, contrary to previous studies, the field lines in the initial state are not unidirectional. We show that if the twist is initially concentrated inside the unstable layer, the modifications of the instability reported by several authors (see e.g. Cattaneo et al. (1990)) are only observed when the calculation is restricted to two dimensions. In three dimensions, the usual interchange instability occurs, in the direction fixed by the field lines at the interface between the layer and the field-free region. We therefore introduce a new configuration: the instability now develops in a weakly magnetised atmosphere where the direction of the field can vary with respect to the direction of the strong unstable field below, the twist being now concentrated at the upper interface. Both linear stability analysis and non-linear direct numerical simulations are used to study this configuration. We show that from the small-scale interchange instability, large-scale twisted coherent magnetic structures are spontaneously formed, with possible implications to the formation of active regions from a deep-seated solar magnetic field.

A design for a compact time-of-flight mass spectrometer

The design of a prototype, compact time-of-flight (TOF) mass spectrometer is described. The system primarily consists of an ion acceleration∕focusing∕steering assembly (AFSA), an 8 cm field-free region, a 4 cm dual-stage reflectron, and a miniature microchannel plate detector. Consequently, the resulting flight length of the system is 12 cm. The system has been designed with the capability to sample directly from atmosphere at ambient pressures. This is accomplished through the use of an electrodynamic ion funnel, housed in an intermediate-vacuum chamber that is coupled to the inlet of the TOF chamber. TOF spectra were obtained using noble gases (Ar, Kr, and Xe) as test chemicals. These measured flight times were used to probe the performance of the instrument. A temporal resolution (t(flight)∕Δt) of approximately 125, acquired using (129)Xe(+), has been measured for the system.

Plasma source development for the NDCX-I and NDCX-II neutralized drift compression experiments

AbstractCompressed ion beams are being studied as a driver for inertial confinement fusion energy and for the creation of matter in the high-energy-density regime. In order to facilitate compression of a positive ion charge bunch longitudinally and transversely beyond the limit determined by the space-charge field of the bunch, a source of charge-neutralizing electrons must be provided. Plasma sources have been developed for the NDCX-I and NDCX-II experimental facilities, both for the 2-m-long, field-free drift regions, and for the small-diameter interior of the multi-Tesla final focus solenoid. Barium titanate based cylinders with a high dielectric coefficient are used to line the wall of the 2-m-long drift region and by applying a 9 kV pulse between the inner and outer surfaces of the cylinders, plasma with a density in the 1010 cm−3 range is formed. Results are presented from experiments using this plasma source on NDCX-I. A compact plasma source 5.1 cm long and 3.8 cm in diameter, also made using the barium titanate based material, has been developed for use in the bore of the final focus solenoid. Plasma generated near the wall of the plasma source will follow the fringing magnetic field lines of the solenoid and help to fill the bore of the magnet with plasma. Improved designs for the barium titanate plasma sources are being considered that use different inner-surface electrode materials and structures, and also use a modified electrical driver employing a spark gap crowbar switch. In addition, plasma source designs using so-called flashboard technology have been developed. In the flashboard plasma source, high density plasma is formed when the applied high voltage pulse causes a series of breakdowns between isolated copper patches aligned in rows along the surface of the 0.2 mm thick flashboard.

Biased internal potential distributions in a bulk-heterojunction organic solar cell incorporated with a TiOx interlayer

External-biased potential distributions of a polymer bulk-heterojunction (BHJ) solar cell, incorporated with electron/hole transporting layers, were directly obseved through a cross-sectional Kelvin probe force microscopy. The bulk electric field of BHJ was found to be nearly field-free even under reverse biases, and the field-free region was probed to expand with the incorporation of TiOx electron transporting layer; as a result, inducing a decrease of quasi-Fermi level splitting region in obtaining a high fill factor in the TiOx-interlayered junction photodiodes.

Gas-Phase Ion Chemistry and Organic Chemistry: The Story of a Hybrid Six-Sector Mass Spectrometer—The “AutoSpec 6F”

The AutoSpec 6F mass spectrometer is a large, floor standing instrument comprising a pair of commercial EBE geometry (AutoSpec) mass spectrometers coupled in series to provide an hybrid EBE-EBE configuration, (E and B being respectively electrostatic and magnetic sectors.) It was designed in close collaboration between Professor R. Flammang and VG Analytical in Manchester, UK. It was equipped with five collision cells and allowed the recording of high energy CID (collision induced dissociation), MIKES (mass analyzed ion kinetic energy spectrometry) and NRMS (neutralization re-ionization mass spectrometry) data as well as consecutive MSn analyses. The field-free regions between sectors allowed the study of unimolecular decomposition products from long-lived metastable ions. The mass spectrometer became even more versatile when an RF-only quadrupole collision cell was installed between the second and the third electric sector. This allowed the study of associative ion/molecule reactions in the low kinetic energy regime. Bimolecular chemical reactions were performed inside the quadrupole cell when a neutral reagent was introduced and the reaction products were analyzed by high energy CID in the downstream sectors. This paper tells the history and summarizes the capabilities of this versatile instrument.

Observation and Identification of Metastable Excited States in Ultrafast Laser-Ionized Pyridine

We report on the fragmentation of ionized pyridine (C(5)H(5)N) molecules by focused 50 fs, 800 nm laser pulses. Such ionization produces several metastable ionic states that fragment within the field-free drift region of a reflectron-type time of flight mass spectrometer, with one particular metastable dissociation being the leading fragmentation process. Because the time of flight is no longer dependent in a simple way on the mass of the ion, the metastable decay is manifested as an unfocused peak on the mass spectrum that appears at a time of flight not corresponding to an integer mass. A previously-developed method is used to identify the precursor and final masses of these ions. The metastable process that creates the most prevalent peak is shown to be C(5)H(5)N(+) → C(4)H(4)(+) + HCN. Simulations confirm this result and place restrictions on the processes for several other observed metastable reactions.

Investigation of ion–ion-recombination at atmospheric pressure with a pulsed electron gun

For future development of simple miniaturized sensors based on pulsed atmospheric pressure ionization as known from ion mobility spectrometry, we investigated the reaction kinetics of ion-ion-recombination to establish selective ion suppression as an easy to apply separation technique for otherwise non-selective ion detectors. Therefore, the recombination rates of different positive ion species, such as protonated water clusters H(+)(H(2)O)(n) (positive reactant ions), acetone, ammonia and dimethyl-methylphosphonate ions, all recombining with negative oxygen clusters O(2)(-)(H(2)O)(n) (negative reactant ions) in a field-free reaction region, are measured and compared. For all experiments, we use a drift tube ion mobility spectrometer equipped with a non-radioactive electron gun for pulsed atmospheric pressure ionization of the analytes. Both, ionization and recombination times are controlled by the duty cycle and repetition rate of the electron emission from the electron gun. Thus, it is possible to investigate the ion loss caused by ion-ion-recombination depending on the recombination time defined as the time delay between the end of the electron emission and the ion injection into the drift tube. Furthermore, the effect of the initial total ion density in the reaction region on the ion-ion-recombination rate is investigated by varying the density of the emitted electrons.

Mass shift in the mass spectra of TOF-SIMS and the analysis of kinetic energies of the ions

The apparent mass shift of the CsI cluster ions (CsI) n Cs + and (CsI) n I − observed in high resolution mass spectra by TOF-SIMS of CsI crystal has been investigated in terms of the kinetic energy of the ions. The sharp peak of CsI cluster ion showed apparent mass shifts, which cannot be corrected by usual method. The kinetic energy analysis has shown that the initial kinetic energy depends on the cluster size, and this is the reason for apparent mass shifts. The unimolecular dissociation of CsI cluster ion in field-free drift region that gives the broad peaks has been assigned by the analysis of the kinetic energies. The precursor ion (CsI) p Cs + or (CsI) p I − emits one or two CsI units at a time and the branching ratio R p = k p→ p−2 / k p→ p−1 strongly depends on the cluster size. It becomes large when p − 2 is the magic number (6, 9, 13) of the cluster ion, and small when p = 7. The result was compared with the DFT calculations.

Measurement of low mass dielectrons with the HBD upgrade of PHENIX

The PHENIX experiment was upgraded with the Hadron Blind Detector (HBD) for the measurement of low-mass electron pairs. The HBD reduces the combinatorial background by providing additional electron identification in a field-free region close to the vertex and by exploiting the small opening angle of the π0 Dalitz and conversion pairs. The HBD was successfully operated during Run 9 and Run 10 at the RHIC in the measurements of e+e− pairs in p+p and Au+Au collisions, respectively. This paper reports on the present status of the dielectron analysis in Au+Au collisions at = 200 GeV.

Selective ion suppression as a pre-separation method in ion mobility spectrometry using a pulsed electron gun

Ion mobility spectrometry is a well-known method for fast trace gas detection. Detection limits in the very low ppb- and even ppt-range, fast response times down to a second and good separation power combined with a reasonable instrumental effort make ion mobility spectrometry more and more attractive. Aiming for higher separation power we investigate the ion specific lifetime of different ion species in a field free reaction region of a drift tube ion mobility spectrometer equipped with a pulsed non-radioactive electron gun. When turning off the electron gun ionization stops and the total ion concentration in the reaction region starts to decrease, while different ion species have different decay times. By varying the time delay between the end of the ionization and the injection pulse transferring all remaining ions of one polarity from the reaction region into the drift region the individual decay times can be measured. Our experimental data show that the lifetime of ion species in a field free reaction region mainly depends on ion-ion-recombination and charge transfer reactions leading to significant lifetime differences. Therefore, short-lived ions can be effectively suppressed in the reaction region by introducing a sufficient time delay between the end of the ionization and the injection pulse. This allows detecting even smallest concentrations of long-lived ions in a complex short-lived background. From our experimental data it can be also concluded that wall losses and the ion transport within the sample gas stream out of the reaction region just play a minor role in the ion loss.

A time of flight mass spectrometer with field free interaction region for low energy charged particle-molecule collision studies

A new design of a linear time of flight mass spectrometer (ToFMS) is implemented that gives nearly field-free interaction region without compromising on the mass resolution. The design addresses problems that would arise in a conventional Wiley-McLaren type of ToFMS: (i) field leakages into the charged particle-molecule interaction region from various components of the mass spectrometer, including that through the high transparency mesh used to obtain evenly distributed electric fields; (ii) complete collection and transportation of the ions produced in the interaction region to the detector, which is essential for high sensitivity and cross section measurements. This ToFMS works over a wide range of masses from H(+) to a few hundred Daltons and would be the most suitable for low energy charged particle-molecule interaction studies. Performance of the ToFMS has been tested by measuring the partial ionization cross sections for electron impact on CF(4).

Antibinding of atomic electrons in strong inhomogeneous magnetic fields

The ground-state energy of heavy one-electron ions in an inhomogeneous locally bounded magnetic field is estimated by the variational principle. The ions are described by means of the pseudorelativistic Herbst-Chandrasekhar operator. Two classes of magnetic fields are considered which model a field-free region around the central charge. It is shown that for a certain size of this region the ground-state energy becomes positive and increases strongly with the magnetic field strength. This behavior is in contrast to the two-dimensional case where electrons can be bound by such a field-free region.

Molecular Ion Dissociation Processes in a Partially Cooled Electrostatic Trap

An experimental capability being developed at the Oak Ridge National Laboratory Multi-Charged Ion Research Facility (ORNL MIRF) is described that enables stored cooling and state characterization of atomic and molecular ions of essentially any mass. Ions selected from a variety of available (hot and cold) sources are injected into a 1.5 meter long electrostatic mirror trap, in which excited internal states decay by radiative cooling. Ion lifetime studies or direct measurements of 'in-trap' interactions of molecular ions with electrons or atomic targets are accomplished with either a crossed electron beam or atomic beam (with recoil ion detection) located in the field-free region of the ion-trap, coupled with product velocity imaging. Similarly, extraction of trap-cooled ions can be used 'out-of-trap' for near state-specific electron capture (EC) and dissociative charge exchange (DCE) studies employing higher resolution COLTRIMS techniques coupled with fragment imaging.

Ion optical design of a collinear laser-negative ion beam apparatus

An apparatus for photodetachment studies on atomic and molecular negative ions of medium up to heavy mass (M ≃ 500) has been designed and constructed. Laser and ion beams are merged in the apparatus in a collinear geometry and atoms, neutral molecules and negative ions are detected in the forward direction. The ion optical design and the components used to optimize the mass resolution and the transmission through the extended field-free interaction region are described. A 90° sector field magnet with 50 cm bending radius in combination with two slits is used for mass dispersion providing a resolution of M∕ΔM≅800 for molecular ions and M∕ΔM≅400 for atomic ions. The difference in mass resolution for atomic and molecular ions is attributed to different energy distributions of the sputtered ions. With 1 mm slits, transmission from the source through the interaction region to the final ion detector was determined to be about 0.14%.

Parallel ion strings in linear multipole traps

Additional radio-frequency (rf) potentials applied to linear multipole traps create extra field nodes in the radial plane which allow one to confine single ions, or strings of ions, in totally rf field-free regions. The number of nodes depends on the order of the applied multipole potentials and their relative distance can be easily tuned by the amplitude variation of the applied voltages. Simulations using molecular dynamics show that strings of ions can be laser cooled down to the Doppler limit in all directions of space. Once cooled, organized systems can be moved with very limited heating, even if the cooling process is turned off.

Electrostatic beams from tailored plasmas in a Penning–Malmberg trap

In recent work, a technique was developed to extract high quality beams from single-component plasmas confined in a Penning–Malmberg trap in a 4.8 T magnetic field. In this paper, a procedure is developed to extract these beams from the confining magnetic field and then focus them to create especially tailored electrostatic beams. Electron beams are extracted from the field in two stages: they are first transported to a region of reduced field (1 mT), and then taken to zero field with a nonadiabatic, fast extraction. Once in the field-free region, the beams are focused using an Einzel lens. Experimental results and numerical simulations are presented to illustrate the extraction and focusing process. Theoretical expressions are developed to describe the modifications to the relevant beam energy and spatial distributions. Where possible, analytic expressions are presented for the case relevant here of beams with Gaussian radial profiles. Beam emittance considerations are discussed as well as prospects for further development of these techniques. Application of these techniques to provide high-quality positron beams is also discussed.