Markus Löffler

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.

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.

Actively Controlling the Muzzle Velocity of a Railgun

The muzzle velocity variation is a parameter of many kinds of accelerators. Actively reducing this value is achieved by observing the motion of the accelerated object during launch and by adjusting the propelling force to achieve the desired velocity. Applying this method to classical gas guns requires a controllable energy source. Experiments with gas-driven accelerators achieve limited success due to severe energy transfer and timing problems. The railgun overcomes these limitations as it uses an electromagnetic field as a propellant that is able to transport power with almost the velocity of light. Using multiple independent capacitor modules, feeding the railgun enables a flexible release of energy. Results are presented from experimental investigations on the control of the acceleration in an augmented railgun. The observation of the accelerated objects motion is performed using a light barrier system. The output signal is read by a micro-controller that decides at which points in time the capacitor modules will be switched on based on a control algorithm. It is shown that the muzzle velocity variation can be reduced by shifting the switch-on points in time of the capacitor modules.

Augmented Electromagnetic Accelerators—Technical Solutions and New Ideas

Electromagnetic accelerators enable the acceleration of bodies in the gram to kilogram range to velocities above 2 km/s. The Lorentz force is responsible for the acceleration; it is proportional to the electric current in the bodys armature and the magnetic field produced by the electric current in the rails. Typically, a large electric current is necessary to create the accelerating force in accelerator configurations with one pair of rails. The large electric currents, which can reach several mega-amperes and the friction forces between the armature and the rails leading to wear the area of the contact. This causes a transition from solid contact to plasma contact increasing the wear of the material. To delay the transition, which limits the performance and the lifetime of the electromagnetic accelerator, the current through the armature has to be decreased. This can be realized by applying an external magnetic field, which augments the magnetic field between the rails and compensates for the reduced current in the armature. In the past, different techniques in the field of augmented electromagnetic accelerators were investigated. This paper gives an overview of augmentation techniques investigated so far. Furthermore, the concept of a modular augmented staged electromagnetic launcher (MASEL) is introduced. This MASEL is being implemented in a new accelerator for the experimental investigations.

Actively controlling the muzzle velocity of a railgun

The muzzle velocity variation is a parameter of many kinds of accelerators. Actively reducing this value is achieved by observing the motion of the accelerated object during launch and by adjusting the propelling force to achieve the desired velocity. Applying this method to classical gas guns requires a controllable energy source. Experiments with Busy Lizzie type gas-driven accelerators resulted in limited success due to severe energy transfer and timing problems. The railgun overcomes these limitations as it uses an electromagnetic field as propellant being able to transport power with almost the velocity of light. Using multiple independent capacitor modules feeding the railgun enables a flexible release of energy. Results are presented from experimental investigations on the control of the acceleration in an augmented railgun. The observation of the accelerated objects motion is performed using a light barrier system. The output signal is read by a micro-controller that decides at which points in times the capacitor modules will be switched on based on a control algorithm. It is shown that the muzzle velocity variation can be reduced by shifting the switch-on points in time of the capacitor modules.

First experiments with the modular augmented staged electromagentic launcher (MASEL)

Summary form only given. Augmented electromagnetic accelerators are advanced mass accelerators which are using an additional magnetic field to support the launching process. In conventional electromagnetic accelerators the electrical current through the rails and the armature has to be very large in order to achieve a sufficiently high electromagnetic force which accelerates the projectile. Due to magnetic flux diffusion effects, the electrical current is concentrated on the back of the armature, resulting in high current concentration and therefore heat generation. Together with the sliding contact between the armature and the rails, abrasive wear of the materials can occur and cause a transition from the solid contact to a plasma contact, which further increases the temperature and the wear of the rails/armature interface.

Electromechanical Modeling of Components of a Linear Electromagnetic Accelerator

Linear electromagnetic accelerators of the railgun-type are characterized by their capability to reach very high end velocities of more than 2 km/s for masses in the range of several hundred grams. The acceleration technology is characterized by the use of transient (ms-range) current pulses with amplitudes up to the mega-ampere range. Therefore, the layout of components has to consider the large magnetic forces acting on current-carrying conductors. Particular attention has to be paid to the contact regions between different conductors, as at these locations small mechanical deformations can lead to the formation of electric arcs. While in the past, the layout of electromechanical components had to base primarily on empirical knowledge, todays finite element software packages allow to model their behavior. In this paper, the 3-D-FEM software package COMSOL is used to investigate the electromechanical behavior of components of a railgun. The analysis starts with simple geometries assuming only the electric behavior and is extended to electromechanical calculations. The current distribution, the distribution of the magnetic forces as well as mechanical stresses and strains are calculated and discussed for several examples. Among the topics is one of particular interest for linear accelerators of the railgun-type: recoil.

The modular augmented staged electromagnetic launcher operated in the energy storage mode

Railguns use electrical energy to accelerate projectiles to large velocities. The electrical energy for the launching process is provided by a pulsed power unit. A pulsed power unit usually consists of a capacitor, a switching device, a crowbar diode and a pulse forming inductance. The pulse forming inductance is necessary to generate an appropriate current pulse length and amplitude. The idea of the energy storage augmented electromagnetic launcher is to use the magnetic field produced by the pulse forming inductance to augment the magnetic field between the rails of the launcher. To maximize the augmentation of the magnetic field, it is necessary to place the pulse forming inductance as close as possible to the rails of the railgun. The additional magnetic field causes not only an increased acceleration of the projectile, but also a larger mechanical load of the railgun structure. This requires a reinforced housing compared to conventional railgun housings. In this paper the design and the experimental setup of a railgun using an energy storage augmentation scheme are presented as well as the results of experiments. The results are compared with previous results of a non-augmented railgun configuration. The comparison shows an improvement of the energy efficiency.

First Experiment With the Modular Augmented Staged Electromagnetic Launcher

Augmented electromagnetic accelerators are advanced mass accelerators, which are using an additional magnetic field to support the launching process. In conventional electromagnetic accelerators, the electrical current through the rails and the armature has to be very large to achieve a sufficiently high electromagnetic force that accelerates the projectile. Due to magnetic flux diffusion effects, the electrical current is concentrated on the back of the armature, resulting in high-current concentration and therefore heat generation. Together with the sliding contact between the armature and the rails, abrasive wear of the materials can occur and cause a transition from the solid contact to a plasma contact, which further increases the temperature and the wear of the rails/armature interface. To decrease the electrical current through the armature without decreasing the accelerating force on the projectile, an additional energy input mechanism has to be implemented, realized by an additional magnetic field, augmenting the magnetic field generated by the current through the main rails. In this paper, the results from experiments with the first of the planned three stages of the modular augmented staged electromagnetic launcher are presented. Included are the measurements of the characteristic values for the rail module and the augmenting modules as well as the launching performance with and without the magnetic field augmentation. These results are compared with the previously calculated values.

Temperature Versus Magnetic Pressure at the Surface of a Semi-Infinite Plate

Transient high magnetic pressures are characteristic of pulsed power applications such as electromagnetic forming, electromagnetic acceleration, and other applications involving high electric currents. Typically, the current pulses are switched on rapidly (microsecond time scale) leading to very high current densities at the surface of the current-carrying conductors due to current-field interactions (skin effect). Electromagnetic diffusion is too slow to enable a spatially homogeneous current distribution inside metal conductors on this time scale. The very high current densities locally generate high ohmic power leading to Joule heating losses and increasing the conductors surface temperature rapidly. Moreover, high local thermal stresses are induced in the region close to the surface. The combination of magnetically and thermally induced stresses and Joule heating can lead to severe damage of the conductor, including phase transitions and deformations. This paper presents a formula that allows estimating the surface temperature of a semi-infinite conductor being exposed to a transient magnetic pressure at its surface. While this is a textbook problem, if adiabatic conditions are assumed, the approach taken here considers thermal diffusion inside the conducting material. The presented formula is valid if the conductors physical constants do not depend on temperature and magnetic flux density.