Voitech Stankevic

Highly Local Measurements of Strong Transient Magnetic Fields During Railgun Experiments Using CMR-Based Sensors

The accurate measurement of the magnetic field distribution during electromagnetic launch experiments is an ambitious task. Loop sensors are widely used for detecting the change in magnetic flux. However, this technique is mostly used only for qualitative purposes, e.g., for triggering various devices. This paper deals with the use of another type of high magnetic field sensor based on thin (<1 mum) manganite films, which exhibit the colossal magnetoresistance (CMR) effect. This sensor measures the magnitude of the magnetic induction and can have very small sensitive areas (e.g., 0.5 mmtimes50 mum). Some basics about CMR and the design of the sensor are given. Several sensors were used in experiments performed with the ISL-launcher EMA3 (Eprim =0.6 MJ, l=3 m, cal=15 mmtimes30 mm). Transient magnetic field profiles with rise times of approximately 50 mus and amplitudes up to 4 T were recorded. The results obtained with the CMR sensors are compared with those of conventional loop sensors. Also, some metrological peculiarities due to high-frequency coupling to the detector circuit are mentioned. The highly local measurements of these CMR sensors were validated by results obtained from 3-D finite element (FE) calculations of the magnetic field distributions

Nanostructured thin manganite films in megagauss magnetic field

We report on the use of the colossal magnetoresistance (CMR) effect in manganites for the measurement of pulsed magnetic fields up to the megagauss limit. To increase the application range in a magnetic field, we fabricated nanostructured La-Sr-Mn-O films consisting of nanocrystallites cummulated into clusters separated by highly amorphous inter-cluster boundaries. We demonstrate that the CMR effect does not saturate in these films at 77 K up to 91.4 T. Moreover, the magnetoresistance behavior at 290 K shows that nanostructured manganite films are promising candidates for the development of magnetic field scalar sensors operating in wide field and temperature ranges.

B-Scalar Measurements by CMR-Based Sensors of Highly Inhomogeneous Transient Magnetic Fields

We present local measurements of absolute values of pulsed magnetic fields in a typical electromagnetic launching system-a small coilgun. For this purpose, we designed magnetic field sensors based on thin polycrystalline La0.83Sr0.17MnO3 films exhibiting a colossal magnetoresistance (CMR) effect. We measured the magnetic field distribution inside the bore of a coilgun consisting of a multilayer coil and inserted a copper projectile in the shape of a hollow cylinder using an array of CMR-based sensors. In order to identify places with highly inhomogeneous magnetic field changes in direction and value, we simulated a pulsed magnetic field inside the coilgun. The measurements of magnetic induction compared well with simulations. CMR-based sensors are able to measure highly inhomogeneous magnetic fields in very small areas independently of the magnetic field direction with respect to the orientation of the sensor.

Pulsed magnetic field measurement system based on colossal magnetoresistance-B-scalar sensors for railgun investigation

A high pulsed magnetic field measurement system based on the use of CMR-B-scalar sensors was developed for the investigations of the electrodynamic processes in electromagnetic launchers. The system consists of four independent modules (channels) which are controlled by a personal computer. Each channel is equipped with a CMR-B-scalar sensor connected to the measurement device-B-scalar meter. The system is able to measure the magnitude of pulsed magnetic fields from 0.3 T to 20 T in the range from DC up to 20 kHz independently of the magnetic field direction. The measurement equipment circuit is electrically separated from the ground and shielded against low and high frequency electromagnetic noise. The B-scalar meters can be operated in the presence of ambient pulsed magnetic fields with amplitudes up to 0.2 T and frequencies higher than 1 kHz. The recorded signals can be transmitted to a personal computer in a distance of 25 m by means of a fiber optic link. The system was tested using the electromagnetic railgun RAFIRA installed at the French-German Research Institute of Saint-Louis, France.

Electric field-induced effects on yeast cell wall permeabilization.

The permeability of the yeast cells (Saccharomyces cerevisiae) to lipophilic tetraphenylphosphonium cations (TPP(+) ) after their treatment with single square-shaped strong electric field pulses was analyzed. Pulsed electric fields (PEF) with durations from 5 to 150 µs and strengths from 0 to 10 kV/cm were applied to a standard electroporation cuvette filled with the appropriate buffer. The TPP(+) absorption process was analyzed using an ion selective microelectrode (ISE) and the plasma membrane permeability was determined by measurements obtained using a calcein blue dye release assay. The viability of the yeast and the inactivation of the cells were determined using the optical absorbance method. The experimental data taken after yeasts were treated with PEF and incubated for 3 min showed an increased uptake of TPP(+) by the yeast. This process can be controlled by setting the amplitude and pulse duration of the applied PEF. The kinetics of the TPP(+) absorption process is described using the second order absolute rate equation. It was concluded that the changes of the charge on the yeast cell wall, which is the main barrier for TPP(+) , is due to the poration of the plasma membrane. The applicability of the TPP(+) absorption measurements for the analysis of yeast cells electroporation process is also discussed.

Manganite Sensor for Measurements of Magnetic Field Disturbances of Pulsed Actuators

Magnetic field sensors based on polycrystalline La0.83Sr0.17MnO3 films were used to measure the magnetic field distribution and disturbances during the operation of an electromagnetic launcher. Hollow cylinders made from dural aluminum and iron were used as propelled objects inside the solenoidal coil. The obtained results revealed the ability of manganite sensors to rapidly measure changing high magnetic fields of arbitrary waveforms.

CMR-B-Scalar Sensor Application for High Magnetic Field Measurement in Nondestructive Pulsed Magnets

In this paper, we present the investigation of the axial and radial magnetic-field distribution inside and outside of the bore of a nondestructive pulsed-field coil presently installed at the Dresden High Magnetic Field Laboratory (HLD). The array used for these magnetic-field measurements was made up of three CMR-B-scalar sensors based on nanostructured La-Sr-Mn-O films. The investigations were performed at a temperature of 270 K and at a peak field of 46 T. The experimental results fit well with calculations obtained using ANSYS code based on the finite element method (FEM). It is concluded that the CMR-B-scalar sensors can be successfully used for the investigation of the magnetic-field distribution in pulsed high magnetic field coils.

Theoretical Analysis and Experimental Determination of the Relationships Between the Parameters of the Electric Field Pulse Required to Electroporate the Cells

Here, theoretical relationships between the parameters of the electric pulse, which is necessary to porate the cell by electric pulse of various shapes, have been obtained. The theoretical curves were compared with the experimental relationships. Experiments were carried out with human erythrocytes, Chinese hamster ovary and mouse hepatoma MH-22A cells. The fraction of electroporated MH-22A cells was determined from the extent of the release of intracellular potassium ions and erythrocytes-from the extent of their hemolysis after long (20-24 h) incubation in 0.63% NaCl solution at 4°C. The dependence of the fraction of electroporated cells on the amplitude of the electric field pulse was determined for pulses with the duration from 95 ns to 2 ms. The shapes of theoretical dependencies are in agreement with experimental ones. The cell poration time depended on the intensity of the pulse: the shorter the pulse duration, the higher the electric field strength has to be. This dependence is much more pronounced for pulses . For example, if the pulse amplitude required to electroporate 50% of human erythrocytes increased from 1.0 to 1.76 kV/cm, when the duration of a square-wave pulse was reduced from 2 ms to 20 μs, it increased from 3 to 12 kV/cm, when the pulse duration was reduced from 950 to 95 ns. The relationships between the electric field strength required for electroporation and the frequency of the applied ac field were calculated for different pulselengths. It has been obtained that although the electric field strength is constant for frequencies but its value depends on the pulselength decreasing with increasing pulse duration. At higher frequencies, electric field strength is dependent on the frequency of the ac field.

Magnetoresistance and Resistance Relaxation of Nanostructured La-Ca-MnO Films in Pulsed Magnetic Fields

The results of magnetoresistance (MR) and resistance relaxation of nanostructured La1-xCaxMnO3 films, with different composition x grown by metal-organic chemical vapor deposition technique, are presented and compared with the La0.83Sr0.17MnO3 films. The MR was investigated in pulsed magnetic fields up to 60T in the temperature range 1.5-294K while the relaxation processes were studied in pulsed fields up to 10T and temperatures in the range of 80-300K. It was demonstrated that at low temperatures the MR has higher values in the LCMO films in comparison with the LSMO ones, while at room temperatures, the highest MR values are obtained for the LSMO films. The fast (~100 μs) and slow (~ms) resistance relaxation processes were observed after the magnetic field pulse was switched off. It was shown that the fast process could be analyzed using the Kolmogorov-Avrami- Fatuzzo model, considering the reorientation of magnetic domains into their equilibrium state, while the slow process-by the Kohlrausch-Williams-Watts model considering the interaction of the magnetic moments in disordered grain boundaries having spin-glass properties. It was concluded that La1-xCaxMnO3 films having a higher sensitivity and lower memory effects and should be favored for the development of fast pulsed magnetic field sensors operating at low temperatures.

Permeabilization of yeast Saccharomyces cerevisiae cell walls using nanosecond high power electrical pulses

The electrical field-induced changes of the yeast Saccharomyces cerevisiae cells permeabilization to tetraphenylphosphonium (TPP+) ions were studied using square-shaped, nanosecond duration high power electrical pulses. It was obtained that pulses having durations ranging from 10 ns to 60 ns, and generating electric field strengths up to 190 kV/cm significantly (up to 65 times) increase the absorption rate of TPP+ ions without any detectible influence on the yeast cell viability. The modelling of the TPP+ absorption process using a second order rate equation demonstrates that depending on the duration of the pulses, yeast cell clusters of different sizes are homogeniously permeabilized. It was concluded, that nanosecond pulse-induced permeabilization can be applied to increase the operational speed of whole cell biosensors.