Plasma at Your Fingertips: The Impact of Atmospheric Pressure Plasma Jet Technology 

Atmospheric Plasma Jet

Atmospheric Pressure Plasma Jets (APPJs) represent a fascinating area of plasma technology that operates at atmospheric pressure and near room temperature. When this plasma is generated at atmospheric pressure—meaning at the same pressure as the air we breathe—it can interact directly with its surroundings without the need for a vacuum. These jets are created using a variety of techniques, such as applying high voltage to a gas, which ionizes it and causes it to emit light and heat. The result is a jet of plasma that can be directed for various applications, offering a unique tool for applications ranging from surface modification to biomedical treatments. Unlike traditional plasmas that require vacuum systems and high temperatures, APPJs are compact, cost-effective, and versatile, making them suitable for both industrial and medical applications. 

What is an Atmospheric Pressure Plasma Jet? 

An Atmospheric Pressure Plasma Jet is a device that generates a plasma plume at atmospheric pressure. The setup typically involves a small nozzle or tube through which a working gas (like helium, argon, or a gas mixture) flows. When a high-voltage electric field is applied, the gas becomes ionized, creating a plasma plume that extends out from the jet. This plasma is not only hot but also highly reactive, which makes it useful for a range of applications. There exist a high number of different designs and generation mechanism which they can be classified based on:  

  • The discharge geometry, electrode arrangement and field configuration 
  • The type of plasma, which is generated (e.g. arc/torch, non-thermal and/or cold plasma) 
  • The excitation frequency (e.g. dc, ac-driven, RF-driven, MW-driven) and excitation pattern (e.g. continuous wave, pulsed) 
  • The type of gas or combination of gases injected to the discharge arrangement 
Key characteristics of Atmospheric Pressure Plasma Jet:
  1. Non-thermal plasma: Most APPJs operate as non-thermal (cold) plasmas, where the electrons are at a much higher temperature than the ions and neutral species (heavy particle temperature in the range of 300–1000 K). This allows the plasma to remain at room temperature, enabling its use on heat-sensitive materials, including biological tissues. Also, electron densities are typically 1019 m−3 in these plasma jets.
  2. Reactive species generation: APPJs produce a variety of reactive species, including reactive oxygen and nitrogen species (RONS), which are crucial for many of their applications, particularly in surface treatments and biomedical uses.
  3. Flexibility and control: APPJs offer precise control over the plasma parameters, such as plume length, power, and reactive species concentration, making them adaptable for different applications.
Applications of Atmospheric Pressure Plasma Jets 

APPJs are employed in many fields due to their unique ability to deliver reactive plasma species at atmospheric pressure and room temperature. Some key applications include: 

  1. Biomedical Applications: APPJs promote wound healing by enhancing blood flow, reducing bacterial load, and stimulating cell proliferation. The reactive species generated by the plasma play a crucial role in this process. The reactive species can also induce apoptosis (programmed cell death) in cancerous cells while sparing healthy tissue. APPJs are also being used in dental treatments for disinfecting cavities, enhancing the adhesion of dental materials, and promoting oral tissue healing.
  2. Surface Modification: APPJs can modify the surface chemistry of materials, improving adhesion, wettability, and bonding properties. This is particularly useful in industries like electronics, textiles, and coatings.
  3. Sterilization of medical devices: The reactive species generated by APPJs can effectively kill bacteria, viruses, and fungi, making them ideal for sterilizing medical instruments and surfaces.
  4. Semiconductor industry: APPJs find applications such as removal of photoresist, oxide films and organic residues from wafers for the electronics industry.
  5. Food Safety: APPJs can be used to decontaminate food products and packaging materials, enhancing food safety and shelf life.
  6. Environmental Applications: APPJs can degrade organic pollutants and pathogens in water and air, offering an environmentally friendly method for purification and decontamination.
Advantages of Atmospheric Pressure Plasma Jets 
  1. Non-Invasive and Gentle: The cold nature of APPJs allows them to treat sensitive surfaces, including biological tissues, without causing thermal damage.
  2. Localized Treatment: APPJs provide a focused plasma plume, allowing for targeted treatment of specific areas, which is particularly beneficial in medical and surface treatment applications.
  3. Cost-Effective and Portable: Operating at atmospheric pressure eliminates the need for vacuum systems, reducing costs and allowing for compact, portable devices.
  4. Versatility: APPJs can be used with various gases and can be easily tailored to generate specific reactive species for different applications.
Plasma Diagnostic Challenges with Atmospheric Pressure Plasma Jet

While the general mechanisms of plasma jet generation are understood, there is a lack of knowledge on the basic plasma parameters for most of the non-thermal plasma jets. This is due to the fact of small size and transient discharge behavior, which makes them very challenging objects for plasma diagnostics. As plasma jets are always operating in an open arrangement the ambient conditions interfere with the plasma properties, which needs to be understood and preferably controlled for any application. Some key challenge on diagnostic front is listed below 

  1. Spatial resolution: APPJs are typically small and exhibit sharp gradients in parameters like temperature, electron density, and species concentration. Achieving high spatial resolution in diagnostics is crucial for accurately characterizing these variations. 
  2. Temporal resolution: APPJs can operate in pulsed modes with rapid changes in plasma properties. High temporal resolution is needed to capture the dynamics of the plasma, which can vary on microsecond to nanosecond timescales.
  3. High electron density and collisionality: APPJs have high electron densities and frequent collisions due to atmospheric pressure. This affects the interpretation of diagnostic signals, as collisional broadening and quenching can complicate measurements particularly in spectroscopy. 
  4. Optical Emission Spectroscopy (OES) Challenges: OES is commonly used in plasma diagnostics, but in APPJs, the spectra can be complex due to the presence of multiple species and overlapping emission lines. Additionally, self-absorption and reabsorption effects can distort the measured spectra. 
  5. Reactive Species Detection: APPJs produce a variety of reactive species (e.g., radicals, ions, excited atoms) that are important for applications. Detecting and quantifying these species requires sensitive diagnostic techniques like laser-induced fluorescence (LIF) or mass spectrometry. 
  6. Environmental Interference: Being operated at atmospheric pressure, APPJs are susceptible to environmental influences such as humidity, temperature, and background air composition, which can affect plasma behavior and complicate diagnostics. 
  7. Probe Diagnostics: Using Langmuir probes or other intrusive probes in APPJs is difficult due to the small size of the jet and high collisionality, which can lead to perturbation of the plasma and inaccurate measurements. 
Impedans Technology for RF and plasma measurements in Atmospheric Pressure Plasma Jet:

Addressing the above said challenges often involves combining multiple diagnostic techniques and carefully designing experiments to minimize interference and maximize accuracy. Impedans offers solutions like the Octiv VI Probes for electrical measurements of RF and pulsed DC-powered APPJs. These probes provide unique insights that help optimize the design and operational regimes of APPJs for optimal performance. The data from these electrical measurements can also be used to validate models that predict plasma parameters generated by APPJs. 

For real-time plasma parameter measurements, Impedans offers Langmuir probes specifically designed for atmospheric pressure operation. Their double-tip configuration minimizes collisional perturbations on the measured I-V curves, ensuring accurate measurements of plasma density and temperature. These probes are available with tips made from materials such as tantalum and platinum, which are durable and can withstand the reactive exhaust of APPJs. 

Conclusion 

Atmospheric Pressure Plasma Jets (APPJs) are powerful and versatile tools used in a wide range of applications, from enhancing material surfaces to aiding in medical treatments. Their capability to generate reactive species at atmospheric pressure and near room temperature makes them especially valuable in situations where low-pressure plasma technologies are impractical. As research in this field progresses, APPJ technology is expected to have an increasingly significant impact on both industrial and medical innovations. 

Impedans is at the forefront of enhancing sensor capabilities to improve control and efficiency in plasma-based applications, thereby advancing APPJ technology. The Octiv VI Probe and Langmuir Probe offer cost-effective solutions for RF/DC and plasma monitoring in APPJs, helping to optimize performance and explore new applications. 

To know more about our plasma measurement solutions contact us at [email protected]