Railgun Diagnostics Program Accomplishments

What follows is a paper presented at the 13th Electromagnetic Launcher Symposium this past May. The Army and the Navy are both pursuing the development of railgun technology for various defense scenarios. This past year ESTD personnel conducted two successful feasibility tests aimed at demonstrating on-the-fly temperature measurement of railgun armatures.

Railguns utilize a magnetic field to accelerate an armature placed between two conducting rails. High currents flow through the armature as it slides in contact with the rails.

Railgun technology has the primary goal of developing ultra-high-velocity projectile launchers for weapons systems, with potential for high accuracy, very high lethality, and significant increases in range. A number of advances in this technology are needed to reach its full potential and make it available for battlefield use. Critical to achieving these necessary advances will be the gathering of diagnostic data on the dynamic electromagnetic launch process, including temperatures, vibrations, surface damage, and other physical measurements. One of the major problems associated with the projectile launch concerns the phenomenon known as “transition,” a plasma-inducing electric breakdown across the rail pair as the armature slides along the bore. This breakdown is more likely to occur as the launch velocities increase. Transition degrades energy efficiency of the launch and increases surface damage to the rails that severely limits their operating lifetime. An important goal is to eliminate or minimize transition, while continuing the effort to increase the launch velocities. It is well understood that more and better diagnostic tools are needed to characterize railgun/armature interaction and overall railgun functioning. ORNL has developed the capability to measure temperature of railgun armatures (projectiles) as they accelerate within and after they exit the rails. The dynamic thermal behavior of the armatures provides real-time information on the efficiency, consistency, and energy profile of the launch. In addition ORNL has capability to measure dynamic strain on moving armatures, the rails, rail-support assemblies, and the pulsed high-current power supply structures. Strain profiles help characterize vibration, material expansion, the dynamic progress of the armature along the rails, structural damage mechanisms, or other phenomena related to the mechanical response of the system. Other ORNL capabilities might also be considered, including use of expertise in material science for improvement in the durability of the rail surfaces or the structural assemblies.

In-Flight Armature Diagnostics
S. W. Allison, M. R. Cates, S. M. Goedeke, M. A. Akerman (retired)
Oak Ridge National Laboratory

M. T. Crawford, S. B. Ferraro, J. Stewart, D. Surls
Institute for Advanced Technology, The University of Texas at Austin

I. Introduction
The purpose of this effort is to develop temperature measurement capability for railgun armatures while in motion, both within the bore and outside in free flight. The method utilizes pulsed laser illumination of a phosphor coating applied to the armature and the fluorescence lifetime indicates temperature. A review article [1] and subsequent publications by the Oak Ridge National Laboratory (ORNL) authors further document the method. Additional experimental details and results are contained in two ORNL reports [2] and [3].

II. Test Setup
A nitrogen laser (Laser Science 337ND) with an output at 337 nm (3 ns duration) is optimum for the selected phosphors. One fiber (800 µm diameter) within a metal-sheathed fiber probe delivers the laser light to the armature to produce the fluorescence. Another fiber of the same size and situated next to it captures the fluorescence and conveys it to a photomultiplier tube (PMT) for detection. References [4] and [5] further discuss the use of this type of optical fiber probe, sometimes called a dual-fiber probe. Fig. 1 depicts the muzzle of the portable railgun with this dual-fiber probe inserted underneath the channel for viewing the bottom surface of the armature. The black-jacketed fiber also shown in this figure inserted from the top performed the time-of-arrival and velocity measurement function. It is a single fiber connected to a 2 × 1 fiber splitter. Light from a red diode laser was injected into the input end. Light emerging from the output end illuminates the channel. When the armature moves into the beam, an increased amount of light is reflected back into the fiber, and that signal is conveyed to a PMT (not shown). This signal provided a timing mark from which the laser trigger pulse was generated. The dual fiber was slightly down stream of the time-of-arrival fiber. So, an approximate 100-microsecond delay between the timing mark and the laser trigger coincided with the armature being directly above the fluorescence-sensing dual fiber. Similarly, Fig. 2 depicts the benchtop railgun with fiber probes situated near the end of the muzzle. Fluorescence and timing probes both view the armature from above. An additional measurement station is also seen. A dual-fiber probe views from above, but the timing probe views the side of the armature as it passes. Fig. 3 reveals a closer look at the exterior probe station and a stationary armature illustrates the fluorescence.

Figure 1. Muzzle of portable railgun and fiber probes.

The signals from the PMTs are acquired by digital oscilloscope, which subsequently transfers the data to an instrumentation computer. A specialized program coded in National Instruments Labview software captures the signals and can perform some analysis.

Figure 2. Muzzle of benchtop railgun and fiber arrangements.

Figure 3. Illumination for armature at external location.

If there are interfering effects from bright backgrounds, motion effects, or other concerns, the signal is post-processed using a spreadsheet program (Sigma Plot or Excel, in practice) and then the corrected signal may be returned to the Labview program in order to ascertain temperature.

Two phosphors, La2O2S:Eu and Gd2O2S:Eu, have high-temperature sensitivity in the expected range for this application and were used. The phosphor is mixed with Sperex, a clear-paint base, as described in previous work [1].

One oscilloscope was dedicated to the timing signals and the other to capturing the fluorescence signal. The most time consuming activity related to triggering and timing is to gain understanding of the signals returned from the armature, and then to focus on illuminating a specific spot on the armature regardless of velocity variation. For the portable railgun, the velocity was determined using optical methods. For the benchtop railgun, B-dot probes and a high speed camera gave instantaneous velocities at various locations in the armature flight probes and a high-speed camera gave instantaneous velocities at various locations in the armature flight.

III. Results—Portable Railgun
The portable railgun is a very small system used for demonstration in an indoor setting. The power supply is capable of delivering about 20 kA and the launcher can accelerate 2 g armatures to a velocity of around 100 m/s. Fig. 4 shows the processed fluorescence signal for railgun shots 42, 45, 47 and 48. Each of these signals was uploaded into the Labview program, following background subtraction and other post-processing described below, to obtain the temperature determination. It was seen that as the fluorescence duration, or lifetime, decreased, the temperature increased. Table I shows the calculated temperature for these and other shots, as well as the velocity data. Temperatures ranged from 22–92°C. There are two different decay time algorithms that were used, and they differed at most by 2°C. That figure, therefore, is taken as the uncertainty in temperature measurement. This level of uniformity is quite adequate considering that it is attained from single-shot (unaveraged) data.

Figure 4. Fluorescence signals for portable armature.

A major concern is that the armature does move appreciably during the measurement period. For example, an armature moving at 1 km/s moves 20 mm during a 20 µs observation period. The effect on the signal was slight for the portable test but more pronounced for the benchtop test. The method that is used to correct for motion, when needed, is discussed in [1] and [2].

While developing the process to accurately trigger the source laser while the armature was properly positioned, a method of measuring armature velocity was demonstrated. The reflection of a CW laser diode beam was used to detect the passage of the armature. By placing two grooves in the armature face, the reflected signal gave good temporal resolution of the passage of the grooves by the fiber optic face. Data from shots with and without the grooves along with a picture of the grooved armature from the portable railgun are shown in Figure 5.

IV. Results—Benchtop Railgun
The series of tests using the benchtop railgun allows for some comparison between the electrical parameters of the launch and the temperature measurements. This comparison was not possible on the portable railgun, because current measurements were not made during the tests.

Figure 5. Reflected light waveforms for grooved and ungrooved armatures.

The benchtop railgun consists of a low-voltage, electrolytic, capacitor-based pulsed-power source and has a nominal bore cross-section of 12.7 × 25.4 mm and an overall bore length of 1 m. The armature is a standard C-shaped design constructed from aluminum with a mass of 14 g. The throat cross-section is 12.7 × 6.35 mm for a conduction area of 80.6 mm². Typical waveforms for the benchtop railgun tests are shown in Fig. 6. From these waveforms, the total electrical action through the armature can be determined. An estimate of the local temperature rise in the throat is made using the room-temperature properties of the armature material.

Figure 6. Representative waveforms for the benchtop railgun (shot 31).

Figure 7. High-speed image of railgun armature flight for shot 10.

Fig. 7 shows four successive images of a railgun firing and flight of an armature for shot 10. The first frame was taken prior to arriving at the measurement station. The third and fourth frames clearly reveal the fluorescing spot.

The measurements and calculations for the benchtop railgun series are given in Table II. The top three results are for outside the muzzle and were obtained using a different pair of rails than used for later in-bore results. The fluorescence signal for shot 11 from the outside position is shown in Fig. 8. An exponential signal with a decay time constant of 4 µs is plotted through the data for comparison. Based on an uncertainty of +/- ½ µs, the temperature is 92+/-4°C is determined. The difference between the measured temperature and the calculation based strictly on the electrical action through the throat can probably be attributed to two primary reasons.

First, the calculation method did not account for the increase in resistivity with temperature, nor for the magnetic diffusion processes that will tend to concentrate the current in the surfaces of the armature. Both of these simplifications will lead toward under-prediction of the armature temperature. Second, there are potentially several other mechanisms that could heat the armature besides ohmic losses. The most obvious additional thermal sources are friction and plasma due to either transition or muzzle arcing.

The effects of motion could be readily ignored for the free-flight cases but were more significant for the in-bore shots. In addition, owing to the nearby surfaces, laser scattering into the receiver fiber was more of a problem. An analysis in [3] concludes that motion does not affect the first 4 µs. The in-bore measurement is illustrated for shot 23 in Fig. 9. It shows the temperature-dependent signal with a representative 2.6 µs signal exponential curve through the data. An uncertainty of ±0.4 µs implies a temperature of 112±6°C.

Figure 8. Fluorescence waveform and curve firt for free-flight shot 11.

Figure 9. Fluorescence waveform and curve fit for in-bore shot 23.

V. Conclusions
Although the absolute value of the measured temperature appears reasonable when compared to the calculated temperature rise from ohmic heating (within 50%), the shot-to-shot variation of measured-to-calculated temperature does not correlate well. This would seem to indicate that a large part of armature heating is due to a mechanism that does not change readily with velocity or electrical action. During this short series of experiments, many variables were not controlled—such as armature transition, muzzle arcing, surface condition of the rails, and condition of the armature (armatures were used multiple times). There may be significant variations in armature heating as these conditions change. In addition, the small scale of the railguns utilized for these tests make repeatable operation difficult.

These tests on both the portable and benchtop railguns have demonstrated that non-contact temperature measurements of an armature in motion can be made. Several proof-of-concept measurements have been made. For both railguns, a laser signal was successfully synchronized to illuminate the phosphor on the armature. This allowed absolute temperature measurements of the armature within the bore of the railgun. Additionally, the temperature was also measured in free flight for the benchtop railgun, and high-speed images showed the fluorescence on the armature in flight. The measurements were made with a viewing time as little as 5–20 µs. It was also noted that the signal fluorescence level was strong, while the normalizing fluorescence levels were weak. It may be possible to improve these signals by using larger-diameter pick-up fiber or direct viewing with a detector. These changes could also improve viewing time as well. Finally, it was possible to optically measure the velocity of armature on the portable railgun system. It should be possible to scale this measurement up with railgun size. For future work, larger-diameter fiber bundles, faster amplifiers, and a more intense trigger laser will likely improve the results. The next step is to use the techniques developed on the portable and benchtop railguns to demonstrate that it is possible to measure the temperature of an armature moving at a velocity of over 2 km/s.

An important goal of this effort is to eventually measure temperature distributions on an armature. Possibly, the best approach would be to use imaging. An imaging system would acquire a snapshot of the moving armature and provide a temperature profile. If the imaging system is based on infrared emissions, a phosphor-based point measurement could be used to provide absolute temperature calibration for the image.

Based on the success of these trials, it is possible that similar optical diagnostic techniques have the potential to make other measurements on railgun systems:

1. Temperature and possibly velocity of the armature at several positions along the rails during acceleration.
2. Electromagnetic interference-immune (EMI-immune), transient temperature measurements of railgun bore components during the shot.
3. Spectral analysis of the transition or muzzle arc plasma, including time-dependent information.
4. Strain profiles at selected points along the rails and on armatures.
5. EMI-immune, transient temperature measurements of the pulsed-power supply during discharge.

Acknowledgment
The research reported in this document was performed in connection with Contract number DAAD17-01-D-0001 with the U.S. Army Research Laboratory. The views and conclusions contained in this document are those of the authors and should not be interpreted as presenting the official policies or position, either expressed or implied, of the U.S. Army Research Laboratory or the U.S. Government unless so designated by other authorized documents. Citation of manufacturers or trade names does not constitute an official endorsement or approval of the use thereof. The U.S. Government is authorized to reproduce and distribute reprints for government purposes notwithstanding any copyright notation hereon. Oak Ridge National Laboratory is operated for the U.S. Department of Energy by Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725.

References

Submitted by: Steve Allison, Advanced Lasers, Optics, and Diagnostics Technology Group

[an error occurred while processing this directive]