Cold Plasma Reactor: Principles & Applications

• Cold plasma reactors are devices that generate nonthermal, partially ionized gases at near-ambient conditions using electron-driven activation.
• They employ diverse configurations—such as DBD, plasma jets, and gliding arcs—optimized through tailored electrical excitation and reactor geometry.
• They enable energy-efficient surface modification, sterilization, and chemical conversion by leveraging low thermal loads and high reactive species output.

A cold plasma reactor is a device that generates and sustains a nonthermal, partially ionized gas (plasma) at low gas temperatures—typically close to ambient—under atmospheric or low-pressure conditions, in which electron temperatures are substantially higher than those of the neutral species. Distinct from thermal or arc plasmas, cold plasma reactors enable applications ranging from surface engineering and sterilization to selective chemical conversion, exploiting the unique property that electron-driven activation and radical chemistries proceed efficiently without substantial bulk heating. Technological implementations span a diverse set of configurations, including dielectric barrier discharges (DBD), plasma jets, gliding arcs, and hybrid plasma-catalytic systems, each defined by precise reactor geometries, electrical excitation schemes, and target operating parameters optimized for given reaction environments and processing objectives.

1. Reactor Architectures and Geometries

Cold plasma reactors manifest in multiple canonical geometries driven by application requirements, gas environments, and constraints on substrate compatibility:

• Radial Multi-Jet DBD Reactors: Exemplified by the flexible plasma multi-jet device described in (Corbella et al., 2021), this reactor consists of a 12 cm-long, 5.4 cm outer diameter PE foam cylinder with a dielectric barrier separating coaxial electrodes and multiple 1 mm-diameter gas outlets drilled through the wall. Brush mode (four jets, azimuthally separated) and comb mode (three axially aligned jets) architectures enable uniform internal surface coverage for hollow or intricate components.
• Microwave Anapole Line-Jet Reactors: The anapole-enabled reactor integrates a 14 mm-radius, 3.81 mm-thick high-permittivity dielectric disk with precision-etched slots and an embedded PCB microstrip for microwave excitation, producing a continuous 2 cm line plasma jet with uniform field distribution (Akram et al., 25 Mar 2025).
• In-liquid Plasma Catalytic Cells: Nitrogen reduction reactors as in (Grosse et al., 26 Jun 2025) use a hybrid cell comprising a Pt wire cathode within a ceramic tube, submerged in 0.1 M KOH, where a cold plasma is generated in a N₂ vapor sheath adjacent to the Pt surface.
• Transferred-Arc and Gliding-Arc Reactors: For waste conversion, pin–disc (transarc) and multi-rod (glidarc) reactors with adjustable electrode spacing process polymer substrates (e.g., LDPE) at atmospheric pressure by means of a rotating or transferred AC arc in N₂ (Tabu et al., 2022).
• Planar and Flexible DBD Arrays: Surface decontamination utilizes patterned copper or ENIG-coated electrodes separated by thin Kapton® dielectrics in flex-PCB construction schemes (Gershman et al., 2020).
• Remote CAP Jet Delivery: DBD chambers coupled to flexible plastic tubes with floating internal conductors facilitate plasma jet extension for nonlocal treatment, achieving skin-safe plasma at several meters from the generator (Kostov et al., 2014).

Representative geometrical parameters, active areas, and construction materials are summarized in the following table:

Reactor Type	Core Geometry/Active Area	Key Materials/Dielectric
Radial multi-jet DBD	Cylinder, L = 12 cm, D_out = 5.4 cm	PE foam, Kapton®, Al, Cu
Anapole line-jet	Disk: r = 14 mm, slot = 2 cm × 0.1 mm	TMM13i ceramic, PCB
Pin–disc arc	Pin above disc, H = 5–10 mm	W, Al, quartz
Gliding arc	3 rods, spread 135°, 30 mm gap	W, quartz
Flex DBD	16 mm × 26 mm rectangle	Cu, Kapton®, ENIG PCB
Remote jet + tube	24 mm ID × 4 cm DBD, tube L ≤ 4 m	Glass, quartz, PE tube

2. Electrical Excitation and Plasma Sustaining Modes

Cold plasma operation exploits a rich set of electrical strategies to maintain nonthermal ionization in neutral bulk gas environments:

• AC Dielectric Barrier Discharge (DBD): Sinusoidal voltage, typically 15 kHz, is applied (e.g., V_pp = 7.5 kV in (Corbella et al., 2021)), generating microdischarges across a dielectric-separated electrode pair. In the radial-jet geometry, the average power is P̄ = (1/T)∫₀ᵀ v(t) i(t) dt, with currents i_dis(t) deduced from capacitive subtraction.
• Microwave Resonant Drive (Anapole): At 960 MHz with ±1 MHz frequency agility (Akram et al., 25 Mar 2025), the field amplitude E(z, P) ≈ 1.0 × 10⁶·√(P/1 W) V/m enables plasma formation along the entirety of the slot at sub-10 W input powers due to near-perfect modal coupling.
• DC/AC Thermal Plasma Arcs: Gliding arc reactors are fed by high-voltage AC up to 40 kV_pp and 70 kHz; cold glow plasmas in in-liquid reactors ignite at DC voltages above ~80 V.
• Pulse-Resonant Excitation: Portable DBD arrays can be effectively driven by pulsed AC at resonance frequencies (e.g., 42 kHz), with duty cycles tuned to maintain low surface temperature (< 50 °C) and desired discharge activity.
• Remote Plasma Jet Launch: Floating inner wires in dielectric tubes provide capacitive field enhancement at the downstream tip, initiating a cold jet only at the distant outlet, leaving the transfer medium itself field-free (Kostov et al., 2014).

The electrical design directly determines discharge regime (corona, glow, arc), microdischarge statistics, and thermal load, and thus selectivity for application-specific chemistry.

3. Gas Flow, Discharge Environment, and Species Generation

Gas composition, flow regime, and volumetric residence time are critical in both sustaining the discharge and tailoring reactive species output:

• Feed gas selection: He, Ar, N₂, or ambient air are variously employed; He and Ar provide low breakdown thresholds and electron energy distributions favoring stable cold jets (Corbella et al., 2021, Akram et al., 25 Mar 2025, Kostov et al., 2014). N₂ is used in plastic waste conversion and plasma-driven reduction due to its relevance to the reaction stoichiometry (Tabu et al., 2022, Grosse et al., 26 Jun 2025). Ambient air suffices for surface sterilization DBDs (Gershman et al., 2020).
• Flow rates and regimes: For radial-jet reactors, each outlet is supplied with ~2 slm He, resulting in laminar flow (Re < 2300). Microwave line-jet devices operate from 1–40 slpm He, maximizing reactive species output in the 10–30 slpm regime (Akram et al., 25 Mar 2025).
• Residence time and discharge coupling: Lower flow in in-liquid reactors ensures a stable plasma sheath and sufficiently high N₂ activation (Grosse et al., 26 Jun 2025); intermediate flow rates optimize arc stability and polymer conversion in glidarc reactors (Tabu et al., 2022).
• Plasma-generated species: OES on DBD jets reveals OH, N₂ (second positive), N₂⁺ (first negative), He I, O(777 nm), and Hα emission (Corbella et al., 2021); significant RONS (O₃, NO·), UV, and radical flux underpin efficacy in disinfection and surface activation (Gershman et al., 2020, Akram et al., 25 Mar 2025). Plasma–catalytic NRR exploits excitable/ionized N₂, atomic N, and abundant H* radicals (Grosse et al., 26 Jun 2025).

4. Diagnostics, Modeling, and Performance Metrics

Evaluating cold plasma reactors involves rigorous experimental and computational methods:

• Plasma diagnostics: OES with high-resolution spectrometers (0.5 nm or better) quantifies radical species and rotational/vibrational temperatures (Boltzmann plots), while H-α Stark broadening yields electron density estimates (e.g., n_e ~ 10¹⁶ cm⁻³ for microwave jets (Akram et al., 25 Mar 2025)). Power is determined via Lissajous figures and current-voltage characterization.
• Thermal modeling: For low-pressure systems, the steady-state 2D Laplace equation for temperature (u(x,y)) is solved subject to measured boundary temperatures (e.g., cathode at 400 °C, walls at 115 °C) using parallel Jacobi methods on structured grids (Molleja et al., 2014).
• Performance metrics: Key metrics vary by application:
  • Plasma power density (W/cm² or W/cm³)
  • Electron density (n_e, cm⁻³)
  • Surface temperature (< 40–50 °C for nonthermal operation)
  • Disinfection log₁₀-reduction per treatment time (Gershman et al., 2020)
  • Hydrogen or ammonia yield rate (mmol/h or mmol h⁻¹ cm⁻²) (Tabu et al., 2022, Grosse et al., 26 Jun 2025)
  • Energy efficiency (η = yield per kWh or % of thermodynamic minimum)

A table of representative performance values from selected reactors is provided:

Reactor	Electron Density (cm⁻³)	Power (W)	Surface/Jet Temp (°C)	Application
Radial DBD Multi-jet	Not quoted	0.1–1.0	<40	Hollow internal surfaces
Anapole Line-jet	1×10¹⁶	4–27	<40	Large-area surface treat.
Flex DBD (air)	1×10¹²–10¹⁴ (est.)	≤1.1	<50	Disinfection
Gliding Arc	Not measured	32–52	~300	H₂ from waste plastics
In-liquid NRR (Pt)	10¹⁴–10¹⁶	10–50 (per cm²)	Electrode up to 1,600	NH₃ electrosynthesis

5. Application Domains and Operational Advantages

Cold plasma reactors are adapted to a broad spectrum of advanced processing and biomedical tasks:

• Materials modifications: Uniform, low-thermal-load activation of internal tube or cavity surfaces (radial multi-jet), polymer surface functionalization, maskless micro-etching of semiconductors (Corbella et al., 2021, Akram et al., 25 Mar 2025).
• Sterilization and disinfection: High log₁₀ bacterial reductions (>6 log₁₀ in 90 s for plasma + H₂O₂) with rapid, low-power protocols, well suited to portable and flexible devices (Gershman et al., 2020).
• Chemical conversion: Nonthermal valorization of plastic waste to H₂ under atmospheric conditions (0.33–0.42 mmol/g LDPE, η ~ 0.15–0.16 mol/kWh) and plasma-catalytic in-liquid ammonia synthesis exceeding conventional 2e⁻-only electroreduction (Tabu et al., 2022, Grosse et al., 26 Jun 2025).
• Biomedical interventions: Remote or conformal plasma jets for wound care, endoscopic decontamination, and cell stimulation without thermal injury or electrical hazard (Kostov et al., 2014).

Key operational advantages include:

• Intrinsic low thermal load, enabling direct application on delicate substrates.
• Rapid, scalable treatment architecture via arrayed/plural jet, slot, or multi-electrode arrangements.
• High electron density and reactive species yields per unit input power (notably in anapole-jet geometry with ~1 µW per V/m ratio).
• PCB-compatibility and flexible device construction.
• Broad environmental compatibility, requiring from pure process gas to ambient air, with tailored power conditioning for each environment.

6. Design Constraints, Safety, and Scaling Principles

Designing cold plasma reactors requires attention to boundary condition control, electrical and gas handling safety, thermomechanical stability, and process scalability:

• Dielectric and electrode configuration: Precision in dielectric barrier thickness and spatial electrode arrangement (e.g., flush sample embedding, distributed electrode geometry) is critical for discharge uniformity and field management (Molleja et al., 2014, Corbella et al., 2021).
• Cooling strategies: Effective external water-cooling and chamber thermal modeling are essential to prevent premature seal or insulator failure during sustained operation (Molleja et al., 2014).
• Electrical safety: Remote plasma launchers with floating conductors isolate user-facing surfaces from high voltage, enabling operator contact and in situ medical application (Kostov et al., 2014).
• Scaling laws: In pin–disc hydrogen reactors, performance is strongly correlated with V_rms²/H (transarc) and P_rms³/Q (glidarc), providing predictive scaling for yield optimization (Tabu et al., 2022). In anapole line-jets, field amplitude follows square root of input power, and dielectric geometry must be scaled commensurately for longer slots (Akram et al., 25 Mar 2025).
• Material compatibility: Dielectric and conductor selection impact long-term reliability, electrical breakdown thresholds, and maintenance cycles, particularly under continuous high-duty cycles in PCB-based devices (Gershman et al., 2020).
• Gas handling and environmental controls: Flow rate optimization (e.g., keeping Re < 2000) is vital for maintaining discharge stability and uniform radial jet coverage (Corbella et al., 2021, Akram et al., 25 Mar 2025).

A plausible implication is that integration of cold-plasma modules into industrial and clinical tools requires modular scalability, process-gas flexibility, and minimally invasive energy delivery architectures, parameters now being demonstrated in pilot systems.

7. Limitations, Open Problems, and Development Trends

Despite technical maturity in many configurations, cold plasma reactors present open challenges:

• Energy efficiency and scale: Current energy requirements for chemical conversions (e.g., ~3300 kWh/kg H₂ in plastic cracking reactors) remain above those of thermochemical benchmarks but offer process integration with renewable electrical sources for otherwise recalcitrant waste valorization (Tabu et al., 2022).
• Uniformity and process control: Active feedback on plasma plume topology, electron density, and surface temperature at scale is required for further industrial adoption, particularly in slot-jet and multi-modal designs (Akram et al., 25 Mar 2025).
• Catalyst/process matching: Synergistic plasma–catalytic systems show major metal-specific yield sensitivity, as with Pt-enabled ammonia synthesis; process reliability hinges on lifetime and poisoning resistance of catalytic electrodes under combined plasma/thermal load (Grosse et al., 26 Jun 2025).
• Fluid–thermal modeling: Coupling 3D thermal–flow simulation (conductive/convective/radiative) with OES-based diagnostics will enhance predictive reactor design for extreme geometries (e.g., deep cavities, conformal surfaces) (Molleja et al., 2014).
• Device miniaturization and deployment: Flexible, portable, and battery-powered designs open new avenues for decentralized decontamination and on-demand medical plasma therapy, but as batch size and area increase, drive electronics and dielectric lifetime become limiting (Gershman et al., 2020).

Overall, cold plasma reactors have transitioned from proof-of-concept to robust, scalable platforms across diverse domains. Optimization of field geometries, nonthermal energy delivery, and process-specific radical chemistry remain the active frontier for both fundamental studies and engineering realization.