Enhancing the Luminosity Quality of Sulfur Plasma Lamps by the HF2PLL Phase-Locked Loop

Introduction

The choice of technology in lighting industry requires a trade-off between color rendering and luminous efficacy because the spectral sensitivity of the human eye varies with wavelength. As sustainable development has become a priority, the manufacturing industry of lighting sources is expected to put high efforts on the luminous efficacy. In this context, sulfur plasma lamps have received a great amount of attention because they provide an outstanding luminous efficacy and at the same time generate a continuous spectrum of high correlation with the sunlight. Ideal for high power spots, this light source suites to cases where a lack of daylight needs to be compensated by periodic exposures to high illumination with a high spectral quality. This is a concern for workplaces where people have to stay indoors during the daytime, far from any window, reducing greatly their sunshine yearly dose, like for instance in the depth of buildings, in open-plan offices, shopping malls or undergrounds.

Similar to discharge and incandescent bulbs, the luminous efficacy of plasma lamps depends on the temperature; the higher the temperature, the better the efficiency. The plasma is heated by a microwave signal produced by a magnetron. For sulfur lamps, the plasma is so hot that the most refractory glass melts in case of an extended contact. Therefore, the bulb is maintained in rotation to avoid melting the glass. This rotation is the main drawback of plasma lamps which causes technical and reliability issues. An alternative approach to avoid bulb rotation is to force the plasma to resonate so that it forms a ball staying at the center of the light bulb [1]. Fig. 1 shows the plasma out of resonance on the left and in spherical resonance on the right, clearly demonstrating the unshaped plasma and the ball-shaped plasma, respectively.

 
Fig. 1. Plasma out of resonance (left) and in spherical resonance (right).

The Institute of Micro- and Nano-Techniques (MNT) at the University of Applied Sciences and Arts Western Switzerland (HES-SO) has a research project supported by the Swiss Foundation Gebert Rüf Stiftung in which scientists aim to develop an energy-saving and high-color rendering lamp. In this framework, an HF2LI Lock-in Amplifier from Zurich Instruments equipped with the HF2LI-PLL Phased-locked Loop option plays a critical role in realizing the goal and implementing the corresponding experiment.

Experimental Setup

The purpose of this experiment is to achieve and maintain an ultrasonic resonance of the plasma inside the bulb in order to keep the plasma in a spherical shape in the middle of the bulb [2]. The experimental setup constitutes of the main parts shown in Figure 2. A DSP pulse generator produces a 4.5-kV pulse train to drive the magnetron which generates a microwave signal at 2.45 GHz with a pulse duration of less than 8 μs and a repetition rate of 30 kHz [3]. The bulb is essentially filled with sulfur and enclosed inside a metal mesh which acts as a Faraday cage preventing any harmful leak of microwaves. The plasma response is measured by a fast photodiode which gives a voltage called photo-signal proportional to the light intensity.

Fig. 2. Experimental setup.

One of the major challenges of this experiment is the photo-signal instability. The modulation depth is sometimes very small and the DC component can vary substantially. The photo-signal is synchronized with the pulse train when the plasma is out of spherical resonance. However, as soon as this resonance occurs the synchronization gets lost, unless the phase is locked by a closed control loop. In this respect, the HF2LI Lock-in Amplifier is used to generate a signal synchronized with the plasma vibration. Without phase-locking, the frequency of the plasma oscillation is no more equal to the pulse repetition rate when the spherical resonance happens. As a result, the pulse repetition rate cannot be used as a reference input to the HF2LI. This difficulty can be easily overcome by using the “auto-reference mode” of the HF2LI lock-in amplifier.

Another challenge is due to the fact that the modulation falls at the very moment of the resonance onset. Therefore, the photo-signal which is intermittently aperiodic is passed through a band-pass filter upstream the HF2LI in order to ease its locking. Furthermore, taking the advantage of PLL option, one can modify the PID controller parameters to stabilize the PLL. The plasma moves sometimes in a chaotic way as long as the resonance is not set on. We have been able to stabilize the HF2LI locking by tuning the PI parameters thanks to the LabOne User Interface, especially its PLL adviser shown in Figure 3. The phase set-point in the PLL is essential for timing so that the pulses occur at the right moments during the rising of the light output.

Fig. 3. Tuning the PI parameters in the LabOne PLL adviser.

Measurement Results and Analysis

Before discussing about the results, it is worth mentioning the labels used in the plots of the figures to come.

  • Cmd/3V: Command signal, expressed in a unit of 3 V.
  • PSFiltre/V: AC component of the photo-signal, expressed in a unit of 1 V.
  • 10*PSPLLFiltre/V: Output of the HF2LI, expressed in a unit of 0.1 V.
  • realReg/3V: Operation indicator of the regulation, expressed in a unit of 3 V.

 

Fig. 4. Measured signals out of resonance.

Figure 4 shows the results when the plasma is out of resonance. In this condition the AC component of the photo-signal has a non-sinusoidal shape; while as the plasma moves to its spherical resonance, the signal changes to a sinusoidal shape as it is evident from Figure 5.

Fig. 5. Detection of the resonance onset.

Figure 5 clearly shows a sinusoidal shape for the photo-signal meaning that the resonance has onset. But the “realReg” signal is not active, because the phase regulation of the pulse generator is not yet on. This will be the case at the next scan of the oscilloscope.

Fig. 6. Signals when the resonance occurs and the PLL is locked.

Figure 6 depicts the measured signals when the resonance is occurring and the PLL is locked. Since the photo-signal is sinusoidal and the “realReg” signal is in its upper level, the plasma is in its spherical resonance and the PLL is in proper operation. One can see that, as a result, the pulse phase is locked on the photo-signal at a set-point of 40° before the rising zero crossing.

Conclusion

This blog shows what has been achieved with an HF2LI Lock-in Amplifier in a research project on a pulsed-plasma lamp and the unique results obtained with this piece of equipment. The HF2LI allows us to produce a signal synchronized with the plasma vibration in order to lock the microwave pulses. This operation is essential to control the resonance that produces the plasma ball formation.

Acknowledgments

Zurich Instruments would like to thank Dr. Gilles Courret, Ms. Fanny Lorant and Mr. Jonas Faillétaz from HES-SO, Yverdon-les-Bains, Switzerland for performing the experiment and providing support in preparing this blogpost as well as the Swiss Foundation Gebert Rüf Stiftung for funding this project. 

References

  1. G. Courret, P. Nikkola, S. Wasterlain, O. Gudozhnik, M. Girardin, J. Braun, S. Gavin, M. Croci and P. W. Egolf, “On the plasma confinement by acoustic resonance,” Eur. Phys. J. D 71: 214, 2017. DIO: 10.1140/epjd/e2017-70490-6
  2. Andreas Meyer, Gilles Courret, and Mirko Croci, “Plasma lamp with means to generate in its bulb a resonant ultrasound wave,” European Patent Office, EP 1 876 633 B1, Sept. 2010.
  3. Gavin, M. Carpita and G. Courret, “Power electronics for a sulfur plasma lamp working by acoustic resonance: Full scale prototype experimental results,” 16th European Conference on Power Electronics and Applications, Lappeenranta, pp. 1-7, 2014.
    DIO: 10.1109/EPE.2014.6911061