An Electrodeless Moly-Oxide Discharge for Lighting Applications

J.L. Giuliani,¹ R.A. Meger,¹ R.E. Pechacek,² and G.M. Petrov³
¹Plasma Physics Division
²SFA, Inc.
³Berkeley Research Associates

Introduction: Lighting consumes 23% of the electrical energy in commercial, industrial, and military buildings. Thus there is a large potential for savings by improving lighting sources for general illumination. The efficiency of a light source for illumination, termed the efficacy, is measured in visible lumens per watt (lpw) of input electrical power. For example, the incandescent filament bulb of the Edison design has an efficacy of only ~15 lpw, the standard fluorescent tube ~70 lpw, and metal halide lamps ~100 lpw. In addition to efficiency, the ideal light source should also provide a broad emission spectrum throughout the visible region in order to produce a high quality of color rendition. A third criterion is the stability of the source output over a long lifetime to minimize operational and replacement costs. The fourth criteria of an ideal lamp, and the most challenging, is the future requirement of environmental safety. Both fluorescent tubes and metal halide lamps contain mercury (Hg), and on Navy vessels, Hg is already treated as a hazardous material. No existing commercial light source is optimal in all four criteria. In an effort to develop the ideal light source, NRL is investigating mercury-free, electrodeless, molybdenum- oxide (MoO₃) plasma discharges for use in lighting applications.¹

Operation of the Moly-oxide Lamp: As a lighting source, the moly-oxide discharge requires a multidisciplinary approach combining quartz fabrication, radio frequency (RF) electronics, plasma physics, oxide chemistry, and atomic excitation physics. The experiments are performed with specially designed quartz bulbs that contain a charge of MoO₃ as powder and an argon (Ar) buffer between 0.5 and 8 Torr. A plasma discharge is initiated in the Ar buffer via an external spiral coil driven by a 13.56 MHz RF generator. An electronic matching circuit, similar to the ballast in a fluorescent light, allows efficient transfer of the RF energy into the plasma. This RF coupling approach provides a long lifetime system because there are no internal electrodes to undergo plasma degradation. The resistivity of the partially ionized Ar leads to heating of the gas and walls of the bulb. As a pure metal, molybdenum (Mo) will not vaporize below the annealing temperature of quartz (1400 K); however, MoO₃ has a high vapor pressure of 1 Torr at 1007 K. Thus the moly-oxide undergoes a sudden evaporation from the quartz walls as they heat up to these temperatures. Once MoO₃ diffuses into the plasma ring, kinetic reactions dissociate it, and the Mo atom is subsequently excited by electron collisions to radiate in the near-UV region and throughout the visible domain. This process is similar to metal halide lamps except that oxygen takes the place of the halide in the metal recycling process and Hg is not used to produce a high-pressure, equilibrium plasma.

FIGURE 8
Calibrated spectrum from a moly-oxide electrodeless discharge with an Ar buffer. The photopic curve is the eye sensitivity.

Figure 8 presents an absolutely calibrated spectrum of the moly-oxide discharge. The photopic curve represents the relative sensitivity of the eye to various wavelengths. Some of the prominent atomic lines in the spectrum are denoted in the figure, and one can see the strong 550 nm emission from Mo at the peak of the photopic curve. The broadband continuum underlying the lines throughout the visible region comprises the white light emission and provides good color rendition. The efficacy of the present design is ~40 lpw. Improvements to the discharge as a general lighting source will require a reduction of the near-UV feature by shifting the energy into visible wavelengths.

FIGURE 9
Experimental configuration showing the moly-oxide bulb discharge in the center driven by an RF excitation coil with several diagnostics.

Fig10 Image

FIGURE 10
Ratio of line emission intensities from Mo and O with Ar as a function of the Mo partial pressure. Solid lines are from numerical simulations; the arrows indicate experimental data.

Mo partial pressure: The key to improvements for lighting applications is an understanding of the properties of the discharge, which is accomplished through a combination of diagnostics and modeling analysis.² Figure 9 shows the experimental setup. To highlight one result, the ratio of emission line intensities between Mo and Ar in conjunction with a Boltzmann model for the electron distribution function can be used to obtain the partial pressure of Mo atoms in the discharge. Figure 10 gives the results from spectroscopic analysis and simulations. The existing experiments indicate a Mo partial pressure of 2 to 4 Torr, but a significant enhancement of the 550-nm Mo emission, and correspondingly the efficacy, can be expected as the Mo pressure is doubled. This effect is due to the optical trapping of the UV resonance lines in Mo and the subsequent increase in visible emissions from untrapped levels.

Summary: The moly-oxide lamp is designed to combine the optimal properties of existing lighting systems, namely, the high efficacy, broadband white light emission found in high-pressure metal halide discharges and the long lifetime of the new low-pressure Hg fluorescent lamps driven by electrodeless RF coupling, such as the Philips Q-lamp. The objective of addressing these goals without the use of environmentally hazardous materials places the program at the leading edge of lighting research.

[Sponsored by ONR]

References
¹ V.A. Shamamian, D.J. Vestyck, Jr., J.L. Giuliani, and J.E. Butler, "Metal Oxide Discharge Lamp," U.S. Patent 6,157,133, issued December 5, 2000.
² J.L. Giuliani, R.E. Pechacek, G.M. Petrov, and R.A. Meger, "Plasma Conditions in a Moly-oxide Electrodeless Bulb Discharge,? in Proceedings" Pulsed Power Plasma Science Conference, June 2001, Digest of Technical Papers, Vol. 2, IEEE 01CH37251, pp. 1078-1081.