Hypersonic Plasmas Laboratory
ESTHER - European Shock-Tube for High Enthalpy Research
ESTHER Institutional Description
The European Shock-Tube for High Enthalpy Research (ESTHER) is a twostage high-speed shock tube designed to reach shock speeds in excess of 12km/s. This facility is hosted in the Hypersonic Plasmas Laboratory, on the premises of Instituto de Plasmas e Fusão Nuclear (IPFN), in the Instituto Superior Técnico (IST) Nuclear and Technical Campus of Bobadela, Loures and inaugurated on the 24th July 2019. This infrastructure was developed in the scope of a joint funding by ESA and IST, for an overall amount of about 3M€.
Figure 1: Outside view of the Hypersonic Plasmas Laboratory
Facility Description
The facility is comprised of a 47 L volume, 2m long combustion driver where high-pressures and temperatures are reached (in excess of 600 bar), linked to a 5m-long compression tube filled with helium at an intermediary pressure of 0.1–0.5 bar, where a shock wave is allowed to accelerate. This tube is in turn linked to a 5.4m-long test section at typical pressures around 0.1–10 mbar, with gas compositions simulating the upper layers of an arbitrary planetary atmosphere. The shock wave crosses this tube with a speed in the range of 3–18km/s (Mach = 10−40). Finally, a 1000 L dump tank collects the flow prior to its evacuation through a piping system. The 20m long facility is free-standing on a bench, with a removable braking load transfer structure behind the combustion chamber which absorbs the facility recoil during testing. This bench is in turn mounted in a 40 Ton slab placed over anti-seismic pads, which insulate the laboratory frame (where the shock tube instrumentation is located) from vibrations resulting from the shock tube operation.
The facility is developed with the specific aim of investigating the physicalchemical processes of shock-induced plasmas, through the observation of the radiative properties of the plasma created downstream of the shockwave, as well as the measurement of the time-dependent electronic densities. This allows furthering the understanding of the physics for such kind of plasmas, and more specifically improving the understanding of meteoroid and spacecraft entry physics, allowing for better predictive capabilities of existing high-temperature hydrodynamic codes. This has in turn a direct impact into the design of entry spacecrafts, as it allows for a better tailoring of its thermal protection systems.
Shock tubes are conceptually-speaking very simple facilities comprised of several sections at different pressures, separated by diaphragms designed to burst at specific pressures. Once the driver reaches a critical pressure, a shockwave propagates from the higher pressure section towards the lower pressure section, which in turn may rupture a second diaphragm, triggering a secondary shockwave that travels at an even higher speed.
Despite the straightforward physical concept of a shock-tube, the extreme pressure ranges involved in the generation of such high speeds represent a significant challenge, since the facility needs to generate very high and very low pressures (ranging from 1 × 103 bar to 1 × 10−6 mbar with a ratio of twelve orders of magnitude), and further since all control and acquisition systems need to operate at the microsecond (μs) timescale, characteristic of shockwaves of this class. Safety concerns also mandate that the facility is operated inside a blast-resistant ATEX experimental hall fully segregated from a control room, with the facility being fully remotely handled.
The ESTHER shock-tube has been designed with the objective of enforcing high-performance (high-speeds), clean (low-contamination), repeatable shots with a low turnaround time, allowing for up to three experiments per day. This is achieved by an innovative laser-ignited combustion driver design (a worldwide first of its kind).
A gas filling system delivers an He/H2/O2 mixture with an 8:2:1.2 stoichiometry to pressures up to 100 bar into the driver 47 L vessel. Ignition is then triggered by Nd:Yag laser system, which advantageously replaces a previous hotwire ignition system, improving the cleanliness of the ignition process and streamlining operations, as the consequence of fully locating the ignition system outside of the chamber.
The test section is placed under the gas conditions of the upper layers of a planetary atmosphere (air for Earth, CO2–N2 for Venus and Mars, N2–CH4 for Titan, H2–He for Gas Giants, etc...). Eight windows are placed at the end of the test section, 4m from the diaphragm separating the compression tube and the test section. This distance is required to ensure the shockwave is fully formed into a disk shape once it crosses the windows.
Figure 2: ESTHER starboard view near the driver (top) and portside view near the dump tank (bottom)
Diagnostics
The optical diagnostics are typically tailored for measuring the timedependent radiation emitted/absorbed in a specific spectral region. A typical setup consists on a broadband spectrometer coupled to a streak camera which records a 2D signal with the time and spectral domain variables on a CCD detector. Alternatively, if only the radiative signal integrated over a specific spectral window is required, then a more cost-effective photomultiplier may replace the streak camera.
The optical setup of the ESTHER facility is currently composed of a near- UV/Visible-range (250–800nm) streak camera coupled to a spectrometer; An identical VUV-capable setup (in the 120–350nm range); and finally an IR-capable setup in the FIR-MWIR range (1–5 μm) is under development. The optical setup is complemented by a 75–110GHz microwave interferometer which allows measurements of time-dependent electron concentrations behind the shockwave, allowing validating the ionization processes implemented in kinetic models.
Since the typical test times for one shot are about one to a few microseconds, fast acquisition systems with a sampling rate larger than 1MHz (and up to 250MHz) have been developed to trigger the optical and electron interferometry diagnostics through a calculator who samples signals from the pressure ports installed along the test section, calculates the shock velocity on the fly, and sends the proper trigger signals to the diagnostics.
In addition to the test-section diagnostics, the facility dump tank may also operate as a dedicated high-speed aerodynamic facility through the deployment of scaled models which may be observed through three large windows which have been placed near the forebody of the dump tank.
Figure 3: Vacuum Ultraviolet Spectrograph and Streak Camera
A New Generation Facility for Supporting European Planetary Exploration Endeavors
Overall, it is expected that ESTHER will improve our understanding of high-speed shockwave phenomena, through the commissioning of dedicated experimental campaigns and the reconstruction of the shots obtained in the facility, which are expected to have improved performance, less noise and pollution, and better repeatability than those obtained in previous generation facilities of its kind. ESTHER is one of the Ground-Base Facilities (GBF) participating in ESA’s “Science in Space Environment” (SciSpacE) programme, which allows research teams to apply with experiment proposals that use the capabilities of the facilities in this programme. More information about SciSpacE may be found here