Calibration of a second generation pulsed electron gun for hypersonic flow diagnostics

A new diagnostic technique to measure test gas density, rotational temperature and vibrational distributions is being developed for use in high enthalpy, chemically reacting flows. A multielectrode Pulsed Electron Gun (PEG) having a FWHM pulse of tens of nanoseconds and typical current of 1000A is used to excite room temperature nitrogen in static conditions. A spectral survey extending between 350-550nm was performed with a N2 pressure of SOmTorr. Fluorescence from the First Negative System (N; ,B E-»X E) and the Second Positive System (N2,Cn-> Bn) was analyzed and rotational and vibrational population distributions were measured and compared to theory. An intensified CCD array was used with lOnsec. and 200nsec. gate windows to capture the spectral signatures. In addition, beam penetration tests were performed on a 1.5|iin thick Mylar diaphragm to be used in the future for proper isolation between the pressure sensitive PEG and the flow field being studied. Significant improvements in beam collimation and higher pulse currents have been achieved over the first generation single electrode PEG. Although preliminary results are encouraging, further development is necessary to apply this technique for hypersonic flowfield diagnostics. 1.0 Introduction Sophisticated computational fluid dynamics (CFD) techniques are used to design future generations of high speed vehicles operating in the hypersonic flight regime (Mach number ID20). A large portion of the flight envelope of these aircraft/probes involve hypervelocity speeds with associated nonequilibrium flowfield chemistry. * Student Member AJAA, Research Assistant, Aerospace Engineering ** Fellow AIAA, Professor, Aerospace Engineering There is a critical scarcity of detailed experimental flowfield measurements that can be used to assess the accuracy and validity of flow simulations. A sketch of a test model to be used in CUBRC's Large Energy National Shock Tunnel is shown in Fig.l illustrating the installation of a PEG for flowfield studies. Figure 1: PEG mounted within test model. Electron Beam Fluorescence (EBF) is a well established and mature diagnostic technique for obtaining density, temperature, internal state population distributions and velocity measurements at points in low enthalpy gas flows.' The fluorescent intensity observed from a volume of gas excited by an electron beam can have a linear relationship to test gas density below approximately 10 cm" (SOOmTorr at 300K)' At greater densities, collisional quenching causes a nonlinearity between intensity and density, with measurement accuracies becoming increasingly uncertain as the density rises. Since nonequilibrium reacting flow studies must be conducted to quite high densities (*= 3xl0 cm for an equivalent altitude of 20 km) in the presence of uncertain quantities of quenching species, an alternative to EBF has been proposed, Pulsed Electron Beam Fluorescence (PEBF)''. PEBF retains the advantage of EBF, which is the simultaneous broad band excitation sampling of vibrational and rotational internal energy states of most molecular test species, while making it possible to ameliorate quenching up to number densities of about 10 cm"3.7 In addition, typical PEG beam currents are 6 orders of magnitude greater than cw electron guns used in EBF. Therefore, background light levels that are the bane of high enthalpy diagnostics are not as problematic since the signal integration time is small (i.e. 10-25nsec.). This paper outlines preliminary results of a second generation multi-electrode PEG to generate PEBF and measure rotational and vibrational population distributions of N2 at SOmTorr in a static cell. Penetration tests were performed on a l.Spn thick Mylar diaphragm to be used in future applications for proper isolation between the pressure sensitive PEG and the flow field of interest 2.0 Theory This discussion will briefly highlight the PEG and its quenching resilience and a detailed treatment of vibrational population measurements. The main advantage of the second generation multi-electrode PEG is its ability to operate at potentials of order 40kV and currents between roughly 500 2000A in extremely short pulses (20 100ns), resulting in two main advantages. First, the short pulse times delay the serious complications of quenching to densities of around 10 cm". Second, the elevated current produces a high density of excited states, thereby allowing observation of a strong signal relative to the high background light levels commonly found in high enthalpy flow facilities. The development of pseudospark discharge switches has provided a simple and compact means of generating electron beams with the aforementioned energies. The PEG operates on the left side of the Paschen curve, thereby suppressing any short path or surface discharges. This forces electrons during commutation, which is initiated by a surface discharge trigger electrode, to follow field lines that take the longer path between electrodes. By placing small orifices (5mm diameter) in the centers of the PEG'S 9 electrodes, an axial discharge generating energetic electrons is produced. As the electrons are accelerated along the field lines, they acquire enough ballistic energy to detach from the field at the exit plane of the gun (Fig. 2). Factors such as distance between electrodes, triggering system, materials, gun chamber initiator gas and operating potential all impact the specific operating conditions of pulsed electron guns. These devices produce high current, self focusing beams when propagating through higher gas densities, thus maximizing surface discharge trigger machinable ceramic cathode