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Hot and Cold Plasma

Overview of difference between hot plasma and cold plasma.

The desiccation or carbonisation layer is the layer of the skin which experiences the the local highest temperatures. Carbonisation also occurs when using “cold plasma”.

The definition of hot plasma or cold plasma is dependant on the temperature at the carbonisiation point. In aesthetic treatments and medicine, the carbonisation point is where the electrical arc meets the surface of the skin or the soft tissue dehydrating it quickly into carbon.

Cold plasma example, plasma in Argon. The arc is far more stable and looks almost like a flame. The temperature at the point of contact of the arc with the skin is not as high as it is in case of arcing in air because the energy associated with the arc is lower.

When the electrical arc strikes, the temperature at the carbonisation point  is proportional to the energy associated with the arc. In turn, the energy associated with the electrical electrical arc is dependant on the dielectric strength of the medium used (normally ambient air). Generally the higher the dielectric strength the higher the energy required to create the electrical arc. In case of most plasma devices for aesthetic applications, the medium used is ambient air. As we know the air normally behaves as an insulator  due to its inherent high dielectric strength. Air is generally behaving as an insulator and it takes a relatively high amount of energy to generate an electrical arc in air.  This is because the energy required for the arc to take place in a high electric strength material is relatively high. This is why the energy associated to the electrical arc  in air has generally high power.

Conversely the lower the dielectric strength the lower the energy associated to the arc.  Therefore the energy required to strike the electrical arc is relatively low and hence the low energy allows lower temperatures at the carbonisation point. For this reason one of the most common ways to produce low energy arcs and consequently cold plasma is by using “carrier gasses” with relatively low dielectric strength.

Generally hot plasma is defined as such when electrical arc takes place in air. This is because the temperature of electrical arc in air (the energy associated to the arc) at the carbonisation point is  relatively high. So what is generated by over 90% of devices in the market is hot plasma, because the arc  in air and the temperature of the arc is relatively high. When the temperature at he carbonisation point is reduced to a certain factor we refer to it as “cold plasma”. As we will see the reduction of the temperature at the carbonisation point is possible in various ways.

 

 

 

What is “hot plasma”?

“Hot plasma” is that electrical arc generated in atmospheric air. In other words “hot plasma” is electrical arcing which takes place in ambient air without the use of any carrier gasses. It is the same electrical spark you see generated by most devices in the market place. Also because air is made up mostly by nitrogen, some marketers refer to this type of electrical arc as “nitrogen plasma”.

The temperatures of electrical arcing in air are extremely high. In case of lightening the temperature at the striking point is extremely high. It can set a tree on fire.

The temperature at the carbonisation point is very high (over 1000 degrees Centigrade). The high temperature is localised at the carbonisation point and this high temperature occur because the dielectric strength of air is relatively high, hence the air it usually behaves as an insulator. In order to generate an electrical arc in air a relatively high amount of energy is required. In other words, in  to convert the air from an insulator to a conductor, which is the phenomenon of plasma or electrical arcing a relatively high amount of energy is required. Hence the electrical plasma in air carries a relatively high amount of energy relative to other types of plasma (like plasma in argon). Hence the energy associated to plasma in air is relatively high (high energy = high temperatures = high heat transfer).This is the type of plasma  we find in most common plasma or electrical arcing devices for medical and aesthetic purposes.

Lightening is an example of “hot plasma”. The energy and temperature at the striking point is so high that it can cause serious damage.

The high temperature at the carbonisation point makes the arc painful during aesthetic procedures (therefore the requirement for numbing products for the treatment). The heat propagated into the skin is relatively high. At low power intensity the arc is not very easy to sustain.

The amount of heat transferred into the dermal layer is relatively high compared to cold plasma.

To summarise “hot plasma” in aesthetic treatment is:

  1. More painful than “cold plasma”, this is because the temperature at the carbonisation point is relatively high.
  2. The heat transfer at the dermal layer is relatively high.
  3. The cross section of the carbonisation point is relatively small. 

 

What are carrier gasses in Plasma?

 

Carrier gasses are those gasses which have a dielectric strength lower than air. A lower dielectric strength means that the arc can take place more easily than in air. These gasses require less energy than air to become ionised (become a conductor). This means that the energy associated to the arc is lower, the temperature at the point of contact between the arc and the skin is also lower. These types of gasses are also inert, non flammable and non poisonous.

Argon has a far lower breakdown voltage than air see this link for more information.

The most common carrier gasses used for this type of application is Argon. This is because:

For the above reasons Argon has been used for several years in aesthetics and medicine. The advantage of using a carrier gas is that the plasma generated is much more stable, delivers less energy  and so less pain than normal hot plasma. The arc in Argon is easier to sustain and the diameter of the surface contact between the arc and the skin is larger.

 

 

What is “cold plasma”?

As we know “hot plasma” is the electrical arcing in air. Air is normally an insulator (relatively high dielectric strength) therefore the energy required to generate the arc is relatively high. This causes high temperatures at the point of contact between the skin and the electric arc.
 


 

In order to stabilise the arc and facilitate the ionisation process (which is another way to refer to as “plasma”, electrical sparking etc), Argon or other inert gasses with lower dielectric strength than air are used. This is because lower dielectric strength means lower breakdown voltage and in turn this means that the electrical arc can be generated more easily. The term “cold” is used because, when an inert gas with far lower breakdown voltage than air is used, the temperature at the carbonisation point is much lower. Additionally the arc is much more stable and the diameter of the surface of contact between the arc and the skin is much larger than electrical arc in air (“hot plasma”). This makes the aesthetic treatment less painful and and also allows for some effects not possible with “hot plasma”.

Cold plasma hand-piece design. Argon is used as carrier gas,

In other words when we use certain carrier gasses with dielectric strength lower than air we have “cold plasma”, this is referred to as such because of the lower temperature at the carbonation point. This is because the energy associated with the electrical arc in Argon is much lower than of plasma in air. The temperature is lower because the carrier gasses (or external gasses) used have a lower “breakdown voltage” than air. Because of this, the energy required in order to generate the arc is much lower in this type of electrical arc. In turn since the energy is much lower also the temperature is lower than plasma in air. Hence plasma in Argon is usually referred to as “cold plasma”. The lower temperature and lower energy associated to the arc cause less heat transfer into the skin and therefore less pain associated to this type of electrical arc. Also the higher stability of the arc makes this type of electrical arc one of the preferred ways to carry out skin resurfacing using plasma.

The main application of “cold plasma” in aesthetics is skin resurfacing. This is because the stabilisation allows the user to control the arcing more easily making this type of aesthetic treatment: