# Effect of Operating Parameters and Anode Configuration on Plasma Generated by a Modular Cascaded Torch

Myriam Sleiman, Geoffrey Darut, Ralph Seulin, Marie Pierre Planche, Jean-Jacques Gonzalez, Pierre Freton, Francis Sambou, Armando Salito, Manfred Rösli

► To cite this version:

Myriam Sleiman, Geoffrey Darut, Ralph Seulin, Marie Pierre Planche, Jean-Jacques Gonzalez, et al. Effect of Operating Parameters and Anode Configuration on Plasma Generated by a Modular Cascaded Torch. Journal of Thermal Spray Technology, 2024, 34 (9), pp.875 - 884. 10.1007/s11666-024-01867-4. hal-05356850

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Submitted on 13 Nov 2025

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# Effect of operating parameters and anode configuration on plasma generated by a modular cascaded torch

**Myriam SLEIMAN, Geoffrey DARUT, Ralph SEULIN, Marie Pierre PLANCHE**

UTBM, CNRS, ICB, F-90010 Belfort cedex, France

corresponding author, @: [myriam.sleiman@utbm.fr](mailto:myriam.sleiman@utbm.fr)

**Jean-Jacques GONZALEZ, Pierre FRETON, Francis SAMBOU**

LAPLACE, UMR 5213 CNRS-INP-UPS, Toulouse, France

**Armando SALITO, Manfred RÖSLI**

Guhlfi AG, IBLEO, Wohlen, Switzerland

## Abstract

New generations of plasma torches have emerged with a cascaded or segmented configuration. These torches consist of a series of electrodes named stages before the final anode at the torch output. Thanks to this new configuration, the movement of the arc is limited to this last stage, reducing arc voltage fluctuations. The study of these torches is not yet sufficiently developed. This work consists of an experimental study of the operation of a modular plasma cascaded/segmented torch. This torch allows the modification of the anode length (39.1 to 61.8mm) and internal diameter (8 and 10mm). Several operating parameters are studied (argon and hydrogen flow rates, arc current intensity). Also, to characterize the operation of torch configurations, several diagnostics are performed, including the measurement of arc fluctuations. The results show that depending on the plasma torch configuration, the variation trends of the plasma as a function of operating parameters are not the same. With a larger anode diameter, the arc voltage trends to decrease of about 10%. In addition, the arc voltage increases with the arc current intensity with a ratio of 10V / 50A or with the plasma gas flow rate (5V / 10L/min for argon gas, and 20V / 4L/min for hydrogen gas). It appears that the torch geometry is a key factor influencing plasma properties.
**Keywords**

Cascaded plasma torches; voltage; plasma properties; operating parameters

**Introduction**

Thermal spray coatings are versatile materials including metals, ceramics, polymers, alloys, and composites (Ref 1). They can be used for a variety of applications including wear and corrosion protection, thermal and electrical insulation (Ref 2–6). Concerning thermal spray processes, plasma spraying involves generating a high-energy plasma that can melt and spray material onto a targeted surface. Injected particles are treated into an argon-based plasma to which other gases such as hydrogen, helium or nitrogen are added to increase its thermal conductivity or momentum properties. Particles are sprayed onto prepared substrates in a molten or semi-molten state according to desired coating microstructure (Ref 4). The heat source is a plasma jet which is typically generated by a direct current (DC) arc (about 99% of plasma spraying applications). In the most common type of plasma process applications as Atmospheric Plasma Spraying (APS), the plasma jet is generated by a DC arc in air at atmospheric pressure.

The plasma is created within a torch formed by a cathode and a concentric anode, into which plasma gases are injected. The cold gas traveling through this chamber is ionized by an electric arc, forming plasma. The hottest regions of the plasma at the exit of the anode can reach around 15,000°K and travel at velocities of approximately 2,000m/s (Ref 7). A plasma jet consists of two main components: a main arc column, attached to the end of the cathode tip serving as the source of electrons, and a connecting column that terminates at the arc root on the inner surface of the anode. The main tool for generating the plasma jet is the plasma torch. The conventional plasma torch design comprises a cathode and an anode (one piece), but it is often plagued by random and unstable phenomena (Ref 8). A particularly troublesome instability comes from the movement of the arc root, which constantly oscillates back and forth across the anode surface. This arc root movement is associated with fluctuations in the arc voltage. The complexity of APS makes real-time process monitoring and control a cognitive challenge for human operators (Ref 9).

To address this issue and precisely control the arc voltage variations, new generations of plasma torches have emerged in recent years (Ref 10). These so-called cascaded/segmented torches feature a reduced arc displacement due to the presence of insulated rings positioned in anode configuration. These torches use a cascaded anode composed of stacked
copper rings (tunnel) that are isolated from each other (the "neutrodes") and enclosed by an outer ring (the final part of the torch) upon which the arc resides, Fig 1.

<figure><img src="image_2.png" /><figcaption>**Fig. 1** Larmor Debye segmented plasma torch scheme. Reproduced with permission from Gulhfi AG</figcaption></figure>

This design effectively stabilizes the arc by confining its movement to the final anode ring (Ref 11,12). Another approach that was explored is the possibility of controlling the arc movement through the application of an external magnetic field to the torch (Ref 13,14). Furthermore, this torch design enables the plasma to extend closer to the nozzle outlet (anode), enhancing torch efficiency by reducing heat loss during cooling and increasing the amount of energy available for particle treatment. This improved efficiency leads to enhanced coating reproducibility (Ref 15). Additionally, lower power and secondary gas flow rates are required to melt materials and produce coatings (Ref 16,17). For example, typical arc voltages in plasma torches, such as F4 of Sulzer Metco or SG100 of Praxair ST, range from 30 to 70V and electric power is up to 80kW (Ref 18). For modular torches studied, voltages range 100 and 150V can be obtained with electrical power between 20 and 40kW. Achieving the desired characteristics of coatings requires the successful completion of multiple steps or sub-tasks.

This experimental work presents the study of different torch configurations of a modular cascaded plasma torch in terms of assembled neutrode number (tunnel length) and final anode diameter. The results concern the effect of operating parameters and geometry of the torch, on the evolution of arc voltage, fluctuations and net powers.

## Materials and processes

### Spray Process and Related Operating Parameters
The Larmor plasma torch used in this study is manufactured and commercialized by Gulhfi AG company (Wohlen, Switzerland). Two types of torch configuration were tested: Larmor 9 and 15 corresponding to a different number of stacks. This parameter could be correlated to the length from the cathode tip to the final anode in such a way that 9 stacks correspond to 39.1 mm and 15 stacks to 61.8 mm respectively. Furthermore, two anode internal diameters for neutrodes/nozzle assembly (tunnel), 8 and 10mm, were investigated regarding the number of stacks.

<table><caption>**Table 1** Configurations of Larmor cascaded plasma torch and operating parameters investigated</caption><thead><tr><th>Plasma torch</th><th colspan="2">Larmor</th></tr></thead><tbody><tr><td>Stack number or Cathode/Anode distance, mm</td><td>9 39.1</td><td>15<br>61.8</td></tr><tr><td>Anode diameter, mm</td><td>8</td><td>10</td></tr><tr><td>Arc current intensity, A</td><td colspan="2">From 350 to 450</td></tr><tr><td>Argon flow rate, L/min</td><td colspan="2">From 35 to 55</td></tr><tr><td>Hydrogen flow rate, L/min</td><td colspan="2">From 0 to 4</td></tr></tbody></table>

Table 1 summarizes the data variables. Please note that when torches are referenced subsequently, such as 'Larmor 9 tunnel 8' or 'Larmor 15 tunnel 10', the number following 'Larmor' denotes the number of stack, while 'tunnel' (8 or 10) refers to the neutrode/nozzle internal diameter.

<figure><img src="image_3.png" /><figcaption>**Fig. 2** Zoom on the operating window (a) arc voltage, (b) electrical power, (c) net power of the torch</figcaption></figure>
For the four studied configurations, the following plasma operating parameters were studied: arc current intensity, argon and hydrogen flow rates. The range of operating parameters is also provided in Table 1. The operating window presented in the Figure 2 dictates the limits of the voltage range and displays the different net and total powers measured.

For coating manufacturing an alumina Amdry 6062 powder (Oerlikon Metco, Switzerland) was chosen. This powder features a particle size distribution ranging from 22 to 45µm, corresponding to a mean diameter of 33µm. A powder mass flow rate of 30 g/min was employed.

The electrical power, which is also shown on the control panel, is calculated by multiplying the torch voltage (U) by the arc current intensity (I) (Eq. 1).

$$P_{\text{electrical}} (\text{W}) = U(\text{V}) \times I(\text{A}) \quad (1)$$

For the net power, the following formula is used (Eq.2 and 3), where $\dot{m}$ is the water mass flow rate (kg/s). $C_p$ is the heat capacity at constant pressure of water (J/Kg. °C), $T_{\text{outlet}}$ is the water temperature at the torch outlet (°C), $T_{\text{inlet}}$ is the water temperature at the torch inlet (°C).

$$P_{\text{net}} = P_{\text{electrical}} - P_{\text{losses}} \quad (2)$$

$$P_{\text{losses}} = \dot{m}C_p(T_{\text{outlet}} - T_{\text{inlet}}) \quad (3)$$

Electrical arc fluctuations were monitored via an oscilloscope. The data were collected with a 50 MHz Rigol DS1052E oscilloscope (Rigol, Germany). Each signal contains 512 points with a sampling rate of 1 MHz and is real time processed.

The microhardness was measured on polished cross-sections using a Vickers indenter (Leitz, RZD-DO, Germany). A load of 300 g was applied for 15 seconds, and ten indentations were randomly selected from various regions of each sample. And the porosity was assessed using ImageJ software by analyzing five randomly chosen SEM images (200X magnification) from the cross-section, excluding the edges.

# Results

## Influence of the arc current intensity on arc voltage

The increase in arc current intensity is presented in Figure 3 for a fixed flow rate of argon and hydrogen gases (45/4 L/min). The voltage increases linearly with the arc current whatever the torch configuration. One of the most essential
electrical aspects of the plasma arc is the quasi-linear dependence of voltage with the plasma torch configuration and the arc current intensity.

<figure><img src="image_4.png" /><figcaption>**Fig. 3** Evolution of arc voltage as a function of arc current intensity (Ar/H₂ = 45/4 L/min) for the 4 configurations: Larmor torch (9 and 15) with inner tunnel diameters (8 and 10 mm)</figcaption></figure>

In conventional torches, a decrease in voltage is observed with increasing arc current intensity, which is explained by a widening of the arc column. The electrical radius is proportional to the square root of the arc current intensity value (√I) (Ref 19). The electrical resistance of the arc then decreases due to the increase in its cross-sectional area. This decrease in arc resistance is accompanied by a decrease in the electric field and therefore in the average arc voltage. In the case of conventional torches, the increase in arc current intensity is also accompanied by a shortening of the arc (Ref 20).

This study of the different Larmor torch configurations exposes, the opposite trend to that observed with conventional torches: voltage increases with arc current intensity. An explanation is that increasing arc current intensity essentially forces the arc inwards, acting as a constraint. This requires a corresponding increase in voltage. The increase in arc current intensity also leads to an increase in temperature (Ref 15) and therefore in radiation losses, which in turn leads to an increase in voltage.

In addition, it should be noted Larmor 15 tunnel 8 has the highest voltage, while Larmor 9 tunnel 10 has the lowest voltage for comparable operating conditions. An increase ratio of around 10V / 50A is observed whatever the torch configuration, leading to quasi-parallel trends. Finally, increasing the number of stacks while maintaining the same internal diameter leads to an increase in voltage. This subject will be discussed in another paragraph. This increasing trend of arc current intensity / voltage characteristics of the plasma arc, with a cascaded plasma torch, was also reported by Osaki et al (Ref 21) and Tu et al (Ref 22). An increase from 36 to 42V was observed when increasing intensity from 80 to 160A. Ruelle et al. (Ref 23) also studied this relationship. Their research documented a voltage rise from 0 to 75V due to the increase in arc current intensity from 0 to 500A.
Influence of the plasma gas flow rate on arc voltage

The argon flow rate is first increased while the arc current intensity and hydrogen flow rate are kept constant at predetermined values (400A, 4L/min). Voltage evolution is presented in Figure 4.

<figure><img src="image_5.png" /><figcaption>**Fig. 4** Evolution of arc voltage as a function of Argon flow rate ($I$ = 400 A, $H_2$ = 4 L/min) for the 4 configurations: Larmor torch (9 and 15) with inner tunnel diameters (8 and 10 mm)</figcaption></figure>

Regardless of the Larmor torch used, increasing the argon flow rate from 35 to 55L/min systematically increases voltage by around 6.5%. This rising flow rate increases the total plasma resistance, resulting in a rise in voltage. That corresponds to an average voltage rise of around 5V / 10L/min. Tu et al. (Ref 22) have obtained the same behavior. They explained their results by the fact that increasing argon gas flow rate requires higher energy to heat the plasma gas, which in turn requires a higher electric field, resulting in a higher arc voltage. Increase gas flow rate results in a greater drag force, due to the presence of the gas flow acting on the anode attachment and pushes the arc downstream. At the same time, the arc column becomes more constricted, and the cold gas boundary layer thicker, thus lengthening the arc. These two effects contribute to an increase in arc voltage as gas flow rate increases (Ref 22). This tendency is observed in conventional torches, but for segmented torches, the jet is more constricted, so the arc can be attached a little further on the last stage of the final anode.

<figure><img src="image_6.png" /><figcaption>**Fig. 5** Evolution of arc voltage as a function of Hydrogen flow rate ($I$ = 450 A, $Ar$ = 35 L/min) for the 4 configurations: Larmor torch (9 and 15) with inner tunnel diameters (8 and 10 mm)</figcaption></figure>
Figure 5 illustrates the effect of increasing hydrogen flow while keeping the arc current intensity and argon flow constant at predetermined values (450A, 35L/min). This general trend applies regardless of the specific combination of settings used. For all four configurations used in this study, the voltage increases by around 20V as the hydrogen flow rate increases from 0 to 4L/min.

This trend is observed because, on the one hand, increasing the percentage of hydrogen constricts the plasma jet (Ref 2). This allows it to attach itself a little further to the last stage of the anode, as observed with the increase in argon flow. On the other hand, increasing the hydrogen percentage also affects the electrical conductivity which induce the decrease of its electrical radius. The jet becomes more resistant, which also leads to a higher voltage. This increase is observed in the study of Qing et al (Ref 24).

In this study, the variation of plasma gases flow rates (argon and hydrogen) was done for two different arc current intensities.

- 400A for the variation of argon flow rate for a constant hydrogen value 4L/min.

- 450A for the variation of hydrogen flow rate for a constant argon value 35L/min.

These values were chosen to have a broader understanding of voltage variations and to demonstrate the general evolution trends regardless the arc current intensity. It is important to note that, while different current intensities are used (400 or 450A), similar trends are expected for both graphs of Argon or Hydrogen.

## Influence of the number of stacks and the tunnel internal diameter on arc voltage

According to the previous figures, an increasing of the arc current intensity from 350 to 450A, or of the argon flow rate from 35 to 55L/min or the hydrogen flow rate from 0 to 4L/min, increases the voltage, around 0.1V per 1A, 0.5V per 1L/min, and 5V per 1L/min, respectively. It is obvious that increasing hydrogen flow rate affects arc voltage more than increasing arc current intensity or argon flow rate. Figures 3, 4, and 5 offer valuable insights into the relationship between voltage and torch configuration in a cascaded plasma torch. Based on the observations of all these graphs, the possible voltage intervals for each configuration are represented in Table 2.
<table><caption>**Table 2 Evolution of voltage intervals**</caption><thead><tr><th>Torch</th><th>Diameter, mm</th><th>Voltage intervals, V</th></tr></thead><tbody><tr><td rowspan="2">Larmor 9 (39.1 mm)</td><td>8</td><td>120-140</td></tr><tr><td>10</td><td>110-125</td></tr><tr><td rowspan="2">Larmor 15 (61.8 mm)</td><td>8</td><td>150-170</td></tr><tr><td>10</td><td>130-150</td></tr></tbody></table>

Higher voltages are obtained using the Larmor 15 rather than the Larmor 9, regardless of torch diameter. Depending on the operating parameters studied and the torch configuration, different voltage ranges are noticed:

* the Larmor 15 tunnel 8 presents the highest voltage range, from 150 to 170V, whereas for tunnel 10 the voltage is lower and ranged from 130 to 150V.

* the Larmor 9 tunnel 8 has a smaller range from 120 to 140V, whereas the tunnel 10 has the smallest range from 110 to 125V.

Increasing the number of stacks while maintaining the same internal diameter leads to an increased voltage.

This phenomenon arises from two factors:

* Increased contact points: Each additional stack introduces a new obstacle to the arc root attachment increasing the arc column length and consequently raising the voltage.

* Increased plasma volume: With more stacks, the plasma occupies a larger volume between the electrodes. This translates into a higher inherent electrical resistance of the arc within the plasma itself, and therefore a higher voltage.

By contrast, expanding the plasma diameter while keeping the number of stacks constant has the opposite effect, causing a decrease in voltage. This phenomenon can be understood with an analogy of the widening of a riverbed. As the cross-section expands, the current density within the plasma reduces, effectively lowering its overall resistance (Ref 25). An analogy can be made with water flow: water flow is more freely across a wider channel, encountering less friction per unit area. As also demonstrated by Planche et al. (Ref 26), decreasing the nozzle diameter leads to a rise in average voltage for a consistent arc current intensity level, attributable to significant arc column narrowing and
a shorter average arc length. This characteristic can be explained by the fact that at a small diameter, the arc column is strongly constricted, the weaker electric radius leads to higher voltages.

## Evolution of the electrical power and net power

As arc current intensity and argon / hydrogen flow rates increase, the electrical and net power output of the Larmor torch increases in a predictable way. Changes in plasma power regarding gas flow rate are minimal for low current intensity levels (4 to 8 kW from 35 to 55L/min of argon according to the torch configuration). The change is more significant for higher arc current intensity levels (400 – 450 A), particularly with the highest hydrogen flow rate of 4L/min (6 to 12 kW). The same prediction was noticed by Bora et al. (Ref 27) and Shicong et al. (Ref 28). The evolutions of electrical power and net power versus the 4 torch configurations and operating parameters are summarized in Table 3 and Table 4, respectively.

<table><caption>**Table 3** Evolution of Electrical Power for the 4 torch configurations versus the operating parameter</caption><thead><tr><th rowspan="2"></th><th colspan="3">Electrical power, kW</th></tr><tr><th>I = 350-450 A Ar/H<sub>2</sub> = 45/4 L/min</th><th>Ar = 35-55 L/min I = 400 A, H<sub>2</sub> = 4 L/min</th><th>H<sub>2</sub> = 0-2 L/min I = 450 A, Ar = 35 L/min</th></tr></thead><tbody><tr><td>Larmor 9 tunnel 8</td><td>✓ (44-63) +19</td><td>✓ (52-56) +4</td><td>✓ (54-63) +9</td></tr><tr><td>Larmor 9 tunnel 10</td><td>✓ (41-56) +15</td><td>✓ (47-50) +3</td><td>✓ (50-59) +9</td></tr><tr><td>Larmor 15 tunnel 8</td><td>✓ (56-78) +22</td><td>✓ (64-68) +4</td><td>✓ (60-72) +12</td></tr><tr><td>Larmor 15 tunnel 10</td><td>✓ (50-69) +19</td><td>✓ (57-60) +3</td><td>✓ (60-64) +4</td></tr></tbody></table>

<table><caption>**Table 4** Evolution of Net Power for the 4 torch configurations versus the operating parameter</caption><thead><tr><th rowspan="2"></th><th colspan="4">Net power</th></tr><tr><th>I = 350-450 A Ar/H<sub>2</sub> = 45/4 L/min</th><th>Ar = 35-55 L/min I = 400 A, H<sub>2</sub> = 4 L/min</th><th>H<sub>2</sub> = 0-2 L/min I = 450 A, Ar = 35 L/min</th><th>H<sub>2</sub> = 2-4 L/min I = 450 A, Ar = 35 L/min</th></tr></thead><tbody><tr><td>Larmor 9 tunnel 8</td><td>✓ (28-36) +8</td><td>✓ (28-33) +5</td><td>30</td><td>✓ (30-33) +3</td></tr><tr><td>Larmor 9 tunnel 10</td><td>✓ (22-31) +9</td><td>✓ (24-29) +5</td><td>26</td><td>✓ (26-31) +5</td></tr><tr><td>Larmor 15 tunnel 8</td><td>✓ (28-38) +10</td><td>✓ (28-36) +8</td><td>32</td><td>✓ (30-38) +8</td></tr><tr><td>Larmor 15 tunnel 10</td><td>✓ (22-32) +10</td><td>✓ (23-31) +8</td><td>26</td><td>✓ (25-34) +9</td></tr></tbody></table>

In order to simplify the graph and because the evolution follows the same trend for all 4 torch configurations, Figures 6, 7 and 8 present the evolution of electrical power (a) and net power (b) for only the Larmor 15 tunnel 8 as a function of arc current intensity, argon flow rate and hydrogen flow rate respectively.

Obviously, there is an increase in electrical power due to the increase of the voltage versus the arc current intensity, argon flow rate or hydrogen flow rate. But is interesting to note the evolution of net power which exhibits a smoother
slope with the increasing of arc current intensity. Consequently, it can be deduced from the equations 1, 2 and 3 that the power losses increase with the arc current intensity. This is understandable since the arc column is wider in such a case and more thermal exchanges occur. It is important to clarify that the losses calculated in this study refer to the losses lost in water during the working period.

<figure><img src="image_7.png" /><figcaption>**Fig. 6** Evolution of electrical power (a) and net power (b) as a function of arc current intensity (Ar/H₂ = 45/4 L/min) for Larmor torch (15) with inner tunnel diameter (8 mm)</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 7** Evolution of electrical power (a) and net power (b) as a function of Argon flow rate (I = 400 A, H₂ = 4 L/min) for Larmor torch (15) with inner tunnel diameter (8 mm)</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 8** Evolution of electrical power (a) and net power (b) as a function of Hydrogen flow rate (I = 450 A, Ar = 35 L/min) for Larmor torch (15) with inner tunnel diameter (8 mm)</figcaption></figure>

As previously calculated, power losses were deduced and in accordance with the hypothesis of jet constriction with argon flow rate, losses decrease meaning that exchanges with water are reduced. As hydrogen flow rate increases, losses increase due to the convection (hydrogen increases thermal conductivity, thermal exchange by conduction is more efficient) and radiation (hydrogen has a higher radiative emissivity than argon).

Qing et al. (Ref 24) observed a similar trend in thermal efficiency: they found that the efficiency increased by approximately 37% as the argon flow rate rose from 1 to 10 L/min. Conversely, they observed a decrease in efficiency with increasing both arc current intensity and hydrogen flow rate.
However, another interesting observation in the study emerges when examining the diameter and number of stacks.

An increase in the number of neutrodes leads to an increase in losses due to the larger volume of plasma jet present in the torch, Fig 9.

<figure><img src="image_10.png" /><figcaption>**Fig. 9** Evolution of power losses as a function of current intensity (Ar/H₂ = 45/4 L/min) for Larmor (15/9) with inner tunnel diameter (8 mm)</figcaption></figure>

Whatever the number of stacks, for a same tunnel internal diameter (either 8 or 10mm) the net power remains relatively similar, regardless of gas flow rate or arc current intensity values (less than 1kW). This suggests that the torch has already achieved optimal heat exchange efficiency. Additionally, increasing the number of neutrodes creates a larger volume of plasma in the torch, which in turn increases the exchange with water and therefore the losses. Since the electrical power with Larmor 15 is greater than with Larmor 9, a similar net power for a different number of neutrodes is obtained (higher electrical power but also higher losses).

Finally, increasing the diameter from 8 to 10mm leads to a decrease in net power (from 4 to 6kW). This can be attributed to the larger surface in contact with water; the electrical radius of the plasma column is larger thus more exchange with water will occur.

## Torch configurations and voltage fluctuations

As developed previously and illustrated in Figure 3, the voltage increases as the internal diameter of the torch decreases and, in reverse, as the length of the torch increases. Figure 10 shows the voltage fluctuations for the Larmor configurations as a function of diameter and length. Compared to conventional plasma torch, fluctuations are considerably reduced for Larmor torch (24V for F4 with I = 650A, Ar = 40L/min, H₂=12L/min compared to 2V for Larmor 9 tunnel 10 with I = 450A, Ar = 35L/min and H₂ = 2L/min). Fluctuations can be caused by a variety of factors, including magnetic fields, turbulence, and collisions between particles where present. In a cascaded torch, neutrodes
force the arc to maintain a longer distance, anchoring the arc root on the terminal anode. This arrangement of a longer arc and a smaller anode surface effectively limits arc oscillations, leading to enhanced arc stability. This stability prevents voltage drops that often occur at higher input current levels in conventional torches.

In this study, the Larmor torch exhibited stability in arc fluctuations regardless of variations in arc current intensity, argon flow rate, or hydrogen flow rate. For example, the voltage fluctuations are typically within 1-2%, as evidenced by Figure 10 and values resumed in Table 5. Further analysis reveals that stack number as well as internal diameter does not influence arc fluctuation.

<figure><img src="image_11.png" /><figcaption>**Fig. 10** Fluctuations of the arc voltage for the 4 Larmor torch configurations—*I* = 350 A, Ar/H₂ = 45/4 L/min</figcaption></figure>

<table><caption>**Table 5** Variation of fluctuations, *I* = 350 A, Ar/H₂ = 45/2 L/min and *I* = 350 A, Ar/H₂ = 45/4 L/min</caption><thead><tr><th rowspan="2">Torch</th><th colspan="2">I = 350 A, Ar/H₂ = 45/2 L/min</th><th colspan="2">I = 350 A, Ar/H₂ = 45/4 L/min</th></tr><tr><th>Voltage, V</th><th>Fluctuations, V</th><th>Voltage, V</th><th>Fluctuations, V</th></tr></thead><tbody><tr><td>Larmor 9 tunnel 8</td><td>118</td><td>± 1.75</td><td>125</td><td>± 1.75</td></tr><tr><td>Larmor 9 tunnel 10</td><td>109</td><td>± 1.25</td><td>115</td><td>± 1.25</td></tr><tr><td>Larmor 15 tunnel 8</td><td>152</td><td>± 2.25</td><td>159</td><td>± 2.25</td></tr><tr><td>Larmor 15 tunnel 10</td><td>134</td><td>± 2</td><td>140</td><td>± 2</td></tr></tbody></table>

An amplitude of voltage variations within 90-96 V in the case of SinplexPro from Oerlikon was measured (Ref 15).

Voltage fluctuations are significantly smaller than voltage fluctuations of F4MB type plasma torch (± 35 V) (Ref 18).

This suggests that, compared to other conventional plasma torches, cascaded design allows superior arc stability.
Within the range of parameters studied, the thermal efficiency varied between 40% and 50% for Larmor 15 and 50, and 60% for Larmor 9.

## Coatings comparison

This paragraph provides a comparison of alumina coatings manufactured with Larmor torches, and the conventional F4 torch with the same net power of 27 kW. The same net power was chosen since this power is the effective power of the plasma jet. The resulting parameters were:

* F4 torch: $I= 650\text{A}$, $Ar/H_2 = 40/12\text{L/min}$, generating an arc voltage of 73V.

* Larmor 9 and 15 (same tunnel 10): $I= 450\text{A}$, $Ar/H_2 = 35/2\text{L/min}$ generating an arc voltage of 115 and 140V respectively.

Comparing these values, the Larmor torch could be more efficient than the F4 torch, because of less arc current intensity or gas flow rate are used to generate higher arc voltage. In these conditions by achieving the same net power as F4 torch (27 kW), it seems to be more interesting to use a cascaded torch.

<figure><img src="image_12.png" /><figcaption>Fig. 11 Coating image for different torches: (a) F4, (b) Larmor 9 tunnel 10 and (c) Larmor 15 tunnel 10</figcaption></figure>

Figure 11 compares the coating properties obtained with the three different torches, focusing on porosity and hardness. The porosity with Larmor torches is reduced by 3 leading to a higher hardness.
Deposition efficiency measurements are given in the Figure 11. An increase of 36% is obtained with the cascaded torch: deposition efficiency of 63% for Larmor torches vs 40% for the F4 torch.

Another article will be dedicated to the coating obtained by the cascade torch.

## Conclusions

The study reveals a clear interaction between operating parameters and plasma properties (voltage, power) for the Larmor modular cascaded plasma torch. Increasing arc current intensity, argon and hydrogen flow rates leads to a predictable rise in both voltage and power output. This observation underscores the torch's responsiveness to adjustments in these primary operating parameters. In addition, increasing the tunnel diameter, leads to a decrease in arc voltage and net power. The opposite is observed on arc voltage, when increasing the stacks number. In this case the net power is not affected.

Furthermore, the Larmor torch offer an effective arc stabilization like SinplexPro. As evidenced by the negligible fluctuations throughout the parameter variations, its design demonstrably minimizes turbulent perturbations within the plasma. This stability offers a significant advantage compared to other conventional torches on the market.

Moving forward, the investigation will aim to investigate the impact of operating parameters on both in-flight particle properties and the sprayed coating's microstructure. By understanding how these crucial aspects are affected, further optimization of the Larmor torch's efficiency and performance for specific applications will be done.

## Acknowledgments

This project was made possible by the financial support of the Bourgogne-Franche-Comté region.

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