Harold Mirels

The following is an expanded version of an interview of Dr. Harold Mirels regarding the invention of the continuous wave HF/DF chemical laser edited in 2013. The interview was conducted by Joan Bromberg, of the Niels Bohr Library and Archives, on January 31, 1985.

I came to The Aerospace Corporation from NASA as an aeronautical engineer. This was in 1961. I headed the Aerodynamics and Heat Transfer Department in Laboratory Operations. The department conducted research related to the launch and re-entry of space and missile systems. One of our missions was to develop advanced test facilities.

In the mid-1960s, a major problem was the creation of a satisfactory ground facility to test nose tip materials under conditions corresponding to the severe re-entry conditions encountered by slender re-entry vehicles. No such facility then existed for simulating both the heating and the mechanical shear and pressure conditions. The failure mechanism of nose tips was not understood, so a facility was much wanted. In response to this need, we developed a concept to use a wind tunnel to achieve both the mechanical shear and high pressures encountered, and to simultaneously use a radiation source to irradiate the tip and duplicate the heat load. A re-entry nose tip experiences about 100 atmospheres and 10,000 degrees Kelvin, causing material to burn off. Our original plan was to use an electric arc, to be built by Electro-optical of Pasadena, as the radiation source. It may have been a xenon electrical arc. NASA had already used radiation loads for the simulation of thermal loading. In this situation, the wavelength is not important but the heat is. The pressure was to be created by a shock associated with supersonic flow past the nose tip. The entire facility was to be the Radiation and Cold Flow Facility (termed RASTA). An Air Force contract was let to AVCO-Wilmington to build the RASTA facility.

We then became aware that AVCO-Everett had demonstrated the CO₂ gas dynamic laser. In this device, CO₂ and N₂ are heated by combustion in a plenum and are expanded to a supersonic stream where vibrational energy is transferred from the N₂ to the CO₂ which then lases. We therefore started an in house program to study the CO₂ gas dynamic laser as a potential radiation source for RASTA. An existing electric arc, rather than combustion, was used for the plenum heating of the CO₂ and N₂ and the supersonic stream was in the form of a free jet. Donald J. Spencer conducted the experiments. We went to AVCO-Wilmington to discuss these concepts. Sometime after that trip, George Sutton came from AVCO-Everett to make a presentation to the Air Force, and to convince it to change the RASTA contract so as to use a CO₂ gas dynamic laser as the radiation source. The meeting was held in San Bernardino, and we were present in the role of advisors. Since we had already been studying the feasibility of using the CO₂ laser, we were able to agree with Sutton. The Electro-optical contract was terminated, so that we could have AVCO-Everett build a high power CO₂ gas dynamic laser as the radiation source for RASTA.

AVCO-Everett had shortly before demonstrated the CO₂ laser at a power of 10Kw. They had wanted the Avionics Laboratory to fund a project to reach 100 Kw. The Avionics laboratory had felt that this was premature and thought that AVCO-Everett should do more work on the science. Since we needed an order of magnitude higher power, we in effect gave AVCO-Everett the rationale they sought for building a 100 Kw laser. The AVCO-Everett CO₂ gas dynamic laser program was classified because of the laser's power. However, the RASTA program itself was not classified.

With regard to our in house CO₂ gas dynamic laser development efforts: We had a common background in fluid dynamics with the AVCO-Everett people. Like them, we knew about nonequilibrium flow because of its association with re-entry phenomena. And we learned the necessary optics and laser technology. We did not have appreciable interaction with The Aerospace Corporation’s Electronics Research Laboratory. There was too large a power level difference between our work and theirs, and the lasing materials were different. We were also not in touch with Pratt-Whitney, in Florida and Hartford, who were independently working on the CO₂ gas dynamic laser. Abraham Hertzberg was a consultant for us, and may have been the source for us as to what was happening elsewhere.

At this time, the Aerodynamics and Propulsion Laboratory, which was within Aerospace’s Laboratory Operations, was directed by Joseph Logan. As a manager, he encouraged research in new fields. My group, the Aerodynamics and Heat Transfer Department, of which Spencer was a member, and Theodore A. Jacobs’ group, the Chemical Kinetics Department, with Rolf W. F. Gross as a member, were both under Logan. Jacobs, Robert R. Giedt and Gross were then conducting shock tube experiments in which an HF laser pulse was observed in a thin reaction zone behind a strong shock propagating in a dilute mixture of H₂ and F₂O in argon. A continuous wave (cw) HF laser had not yet been demonstrated. In a discussion between Jacobs, Gross and myself, it was decided that my department and Jacobs’ department collaborate on an attempt to demonstrate a cw HF laser by replicating the shock tube conditions in the arc tunnel CO₂ laser facility. Nominally, this could be achieved by establishing a stationary strong shock wave in a low temperature supersonic flow containing a dilute mixture of H₂ and F₂O. However, this concept proved to be too difficult to implement. Instead, a concept evolved in which a porous blunt nosed body, effusing H₂ at its nose, would be injected into an arc heated supersonic free jet containing F atoms. It was hoped that vibrationally exited HF would be created at the interface between the H₂ and F streams, in the nose tip region, leading to cw HF lasing. Tests using this configuration were conducted by Spencer. No lasing was initially observed. However, unexpectedly, and fortuitously, HF lasing was observed by Spencer when the porous nose emitted H₂ external to the free jet. It was thus apparent that a cw HF laser can be constructed by having H₂ diffuse latterly into a supersonic jet containing F atoms. A cw HF laser configuration was rapidly implemented by affixing perforated copper tubing, emitting H₂, at the down steam end of the two dimensional nozzle used to generate the supersonic jet. The F atoms were generated in the electric arc plenum chamber by the dissociation of SF₆ (i.e. the use of flammable F₂ was avoided). The first observation of cw HF lasing occurred on May 9, 1969. A U.S. patent was later issued with inventors listed as Donald J. Spencer, Harold Mirels, Theodore A. Jacobs and Rolf W. F. Gross. The original arc tunnel nozzle, with perforated copper tubing attached, has been donated to the Smithsonian Institute.

After the initial demonstration, the subsequent development of the cw HF chemical laser device, at The Aerospace Corporation, was conducted by my department. Donald A. Durran designed efficient multiple nozzle array configurations, Richard A. Chodzko designed novel resonator configurations, Robert Hofland, William S. King and I provided analytic studies of laser performance, Robert L. Varwig conducted flow diagnostic studies and Spencer conducted laser performance studies in the arc facility. Efficient kilowatt level cw HF lasers were rapidly developed. The primary contributions of the Chemical Kinetics Department, to cw HF laser device technology, were the evaluation of rate coefficients for the HF reaction by Norman Cohn and spectral measurements in the arc facility by Munson A. Kwok. The Aerospace cw HF laser development effort is well documented in the 1976 book “Handbook of Chemical Lasers” by Gross and Bott. After the initial demonstration of the diffusion type cw HF laser, Jacobs and Gross continued to promote the concept of a cw HF laser via the flow of hydrogen and fluorine through a stationary strong shock or detonation wave. Gross was unsuccessful in a later experimental attempt to do so.

Following the initial demonstration, the cw HF chemical laser development effort at Aerospace was largely funded by the AF Weapons Lab with Major R. Oglukian as program manager. During 1970/1971, in collaboration with Hofland and King, I developed a simplified analytic flame sheet model to describe cw HF laser performance. In this model, a flame sheet was assumed to exist in the fluorine stream, the flame sheet shape depending on the H₂ lateral diffusion rate. The reactant H₂ was assumed to be added to the fluorine stream at the flame sheet location. The subsequent chemical reaction along each fluorine stream tube was calculated, using a simplified two HF vibrational energy level chemical model and assuming no further lateral diffusion. Closed form analytic expressions for cw HF laser performance were derived. The flame sheet model indicated that, for a given device, and given chemical composition, laser power would first increase with mass flow (i.e. plenum pressure) but, because of diffusion effects, would reach a condition where the laser power remains constant with further plenum pressure/mass flow increase. At the same time, George Emanuel, a member of Jacobs’ department, had developed a complex computer code to estimate cw HF laser performance. His code assumed premixing of all of the reactants, thereby neglecting diffusion effects, and included multiple HF vibrational energy levels and multiple chemical reactions. Due to his neglect of diffusion effects, Emanuel’s code predicted unlimited increase of power with plenum pressure/mass flow. The power versus mass flow issue was important because of its implications regarding the high power potential of cw HF chemical lasers. In view of the complexity of Emanuel’s code, and the simplicity of the flame sheet model, Oglukian chose to believe the results from the Emanuel code. He termed the flame sheet mass flow-power relationship a “disaster curve” and forbade me from presenting the flame sheet model at a laser conference to be held at the Weapons Lab. At that conference I discussed preliminary computer calculations by King which included the full set of fluid, chemical and radiation equations describing cw HF laser performance, and which also indicated the power limitation. As a result of having mentioned the power limitation, Oglukian termed me “persona non grata” and terminated my Weapons Lab funding. The performance predictions of the analytic flame sheet model proved to be correct and it provided important scaling laws. A paper on the subject was published in the AIAA Journal. In the chapter in the 1976 Gross/Bott book which deals with the analysis of cw HF laser performance, the authors G. Grohs and G. Emanuel, both then at TRW, used the simplified Aerospace flame sheet and chemical model to derive analytic expressions for cw HF laser performance. Their derivation and results duplicate the derivation and results that we, at Aerospace, had previously published. The flame sheet concept was also used by others to generate complex cw HF laser computer codes. Nevertheless, Oglukian never acknowledged the validity of the flame sheet model and continued to demand that I not receive financial support from the Weapons Lab laser program at Aerospace. I never did receive Weapons Lab funding thereafter.

Substitution of D₂ for H₂ results in a cw DF chemical laser which has better atmospheric transmission than does HF. High power cw HF/DF lasers, essentially, scaled up versions of the original Aerospace device, have been investigated by various contractors for missile defense applications. A megawatt level cw DF chemical laser, MIRACL, was built by TRW with US Navy funds. This laser was first demonstrated in 1980 and is believed to be the highest power cw laser, of any type, constructed to date.

It is interesting to note that all the components used in the original cw HF laser experiment were already available from previous re-entry related studies. The porous blunt nosed body had been used to study the use of boundary layer blowing to reduce re-entry heating. The arc tunnel, in addition to ablation studies, had been used to dissociate SF6 to provide a supersonic stream of F atoms in studies with the objective of using F atom electron attachment to reduce re-entry wake radar cross-sections. Similarly, the resonator optics were in place as part of the CO₂ gas dynamic laser study conducted in support of the RASTA program. It is also noted that the variety of skills that were involved — chemical kinetics, fluid dynamics, nozzle design, analytic modeling — are all involved in rocket launch and re-entry science; it's the same technology.

Several years prior to our CO₂ laser studies, Logan had read an article by Pimentel in Scientific American that discussed pulsed chemical lasers which emitted radiation in the infrared. Logan suggested to our chemical kinetics group (at that time led by Richard Hartunian) that they investigate the feasibility of a pulsed chemical laser which provided radiation in the visible spectrum. This, then, was the origin of pulse chemical laser studies by the chemical kinetics group and is indicative of Logan’s stressing of new research efforts. Sometime later, Hartunian briefly replaced Logan as laboratory director. Then Walter R. Warren came in. Most of the cw HF laser development occurred under Warren.

In the early 1960s, The Aerospace Corporation Laboratories had an academic atmosphere. 20% of our effort was in direct support of existing AF programs while the rest of our mission was to be a recognized science center, capable of attracting good people. We were to be knowledgeable about advanced technology in order to help in both current AF program development and future technology. Over the years, our discretionary research funds have been reduced, and we have shifted more and more to direct support of existing AF programs.

The RASTA facility itself proved not to be useful. Part of the problem was that, in flight, ablation was altering the nose tip shape. The resulting fluid dynamics — heat transfer interaction was not properly simulated.