Speaker
Description
I. Introduction
Ablation experiments have traditionally been conducted using plasma wind tunnels. Plasma wind tunnels can generate high-temperature flows for relatively long durations and are suitable for measuring recession rates and internal temperature histories of ablative materials. However, because the test gas is generated as a plasma flow, its degree of dissociation can be higher than that in actual re-entry environments, which limits the reproduction of chemical species. To reproduce flow conditions closer to actual atmospheric re-entry environments, heating experiments in hypersonic impulse facilities have attracted increasing attention. Zander et al. [1] conducted experiments at a surface temperature of 2400 K by electrically heating graphite using a DC power supply. Their results showed that CN formation near the surface increased with increasing surface temperature. Chang et al. [2] proposed another approach in which a test model was preheated in an arcjet facility and subsequently exposed to an expansion-tube flow. This method can reproduce a realistic internal temperature gradient by arcjet heating and has the advantage of being applicable to various model materials and geometries. Murray [3] performed electrically heated carbon–carbon composite ablation experiments in the Hypersonic Shock Tunnel at Sandia National Laboratories. Spallation from the C/C surface was observed, and the measured particle velocities ranged from 50 to 2000 m/s, with a mode of approximately 300–340 m/s. TDLAS measurements of CO indicated that the CO number density increased as the surface temperature was raised from 1900 K to 2200 K.
In this study, a graphite test model was preheated using a 4 kW continuous-wave laser before being exposed to an expansion-tube flow. Laser heating has been widely used in ablation studies, and its effectiveness has already been demonstrated. After laser preheating, the graphite model was tested in an expansion tube, and optical emission spectroscopy focused on the CN violet band was conducted to investigate changes in chemical species emission caused by an increase in surface temperature.
II. Experimental Setup
A Test Model
IG-11 grade isotropic graphite manufactured by Toyo Tanso Co., Ltd. was used as the test material. This graphite is manufactured by cold isostatic pressing (CIP) and has a purity of 99.96%, a density of 1770 kg/m³, and an emissivity of 0.80. The test model had a cylindrical shape with a diameter of 20 mm.
B Expansion Tube, CW laser and Rotating Sting
The experiments were conducted using the MX-6 free-piston expansion tube at Tokyo Metropolitan University. The reservoir pressure was set to 1 MPa. The driver gas was helium at 50 kPa, the driven gas was air at 120 Pa, and the acceleration tube was filled with air at 20 Pa. Under these conditions, the shock velocity was approximately 7.4 km/s, the Pitot pressure was approximately 200 kPa, and the test time was approximately 50 μs. JPT 4 kW continuous-wave fiber laser was used for preheating. The central wavelength of the laser was 1080 nm. The laser beam was collimated to a diameter of 20 mm using optics attached to the laser head. Because the beam profile was centrally peaked, the effective beam diameter was approximately 9.5 mm.
In laser preheating, uniform irradiation of the model surface is important for obtaining a nearly uniform surface temperature distribution. To achieve this, a specially designed rotating sting was used to rotate the test model about the centerline of the expansion tube. Although the laser irradiated the model from above, rotation enabled heating of the lower part of the model as well, resulting in a more uniform circumferential temperature distribution.
C Optical Emission Spectroscopy and Surface Temperature Measurement
Optical emission spectroscopy was performed with a focus on the CN violet band as an indicator of changes in surface-temperature-dependent species emission. An Andor Kymera 193i Czerny–Turner spectrograph and an Andor DH334T-18H-13-8SW ICCD camera were used. The surface temperature was measured using a FLIR T860 infrared camera. Although the upper temperature limit of the camera is 2100℃, the measurable temperature range was extended to above 3000℃ by using an infrared neutral-density filter. The infrared camera and neutral-density filter system was calibrated using a CHINO-S14 blackbody furnace at JAXA over the temperature range of 800℃ to 1400℃.
III. Results
The graphite test model was heated using a laser output power of 4 kW. The surface temperature reached 1500 K after 4.3 s, 2000 K after 8.5 s, and 2400 K after 15 s. The temperature distribution on the model surface was approximately within ±5%. Optical emission spectroscopy showed an increase in CN emission intensity as the surface temperature increased.
IV. Conclusion
A new ablation experiment combining laser preheating with an expansion tube was conducted. The graphite test model was successfully heated up to a maximum surface temperature of 2400 K. The increase in surface temperature enhanced the CN emission intensity, showing qualitative agreement with previous studies. These results indicate that laser preheating combined with an expansion tube is a promising approach for reproducing ablation phenomena under conditions closer to actual atmospheric re-entry environments.
References
1. F. Zander, R. G. Morgan, U. Sheikh, D. R. Buttsworth, P. R. Teakle, “Hot-Wall Reentry Testing in Hypersonic Impulse Facilities”, AIAA JOURNAL, Vol. 51, No. 2, February 2013, pp. 476-484.
2. Eric Won Keun Chang, Omar Valeinis, Joseph Steer, Matthew McGilvray, Tobias Hermann, Thomas E. Schwartzentruber, Berk Gur, Rui Fu, Alexandre Martin, “Operational Prediction of Arcjet Model Preheating within Expansion Tube Facility”, AIAA SciTech 2026 Forum, 12-16 January 2026, 2026-0459.
3. John S. Murray, Chris Murzyn, Erin Mussoni, Justin L. Wagner, “Carbon-Carbon Ablation Experiments in the Sandia Hypersonic Shock Tunnel”, AIAA SciTech 2026 Forum, 12-16 January 2026, 2026-461.
Summary
Laser-preheated graphite ablation experiments were conducted in an expansion tube. The graphite surface was heated to 2400 K, and the CN emission intensity increased with increasing surface temperature. These results indicate that laser preheating is a promising technique for reproducing ablation phenomena in expansion-tube testing.