Electron number density in fusion plasma

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Discussion Overview

The discussion revolves around the calculation and prediction of electron number density in fusion plasma, particularly in the context of thermonuclear fusion and magnetically confined plasmas like those in ITER. Participants explore theoretical frameworks and practical considerations related to plasma density.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires about methods to calculate electron number density in plasma.
  • Another suggests the Saha equation as a potential tool for this calculation.
  • A participant notes that while the Saha equation provides an average electron density for ITER, it may not be suitable for fully ionized plasmas in thermonuclear fusion contexts.
  • It is mentioned that the Saha equation assumes thermodynamic equilibrium and uniform plasma, which may not apply to magnetically confined plasmas.
  • Discussion includes the complexity of calculating density profiles in fusion experiments, highlighting the role of transport theory and various transport processes such as turbulent and neoclassical transport.
  • The Greenwald limit is introduced as a stability boundary that affects maximum density in tokamak devices, with implications for reactor design.
  • A participant expresses the challenges of accounting for various factors in tokamak density calculations but finds the learning process enjoyable.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of the Saha equation and the complexity of density calculations in fusion plasma, indicating that multiple competing perspectives remain without a clear consensus.

Contextual Notes

The discussion highlights limitations in the applicability of the Saha equation to fully ionized plasmas and the challenges in accurately modeling density profiles due to various transport processes and stability considerations.

Dr. Octavious
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Hello everybody,

Is there a way to calculate/predict the electron number density of a plasma?

Thanks in advance
 
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Well, in thermonuclear fusion the plasma is fully ionized. I know in the ITER reactor they have an average electron density of 10^20 m-3. Is the Saha equation able to give me a fair approximation?
 
It depends on the context.

The Saha equation assumes that you have a uniform plasma in thermodynamic equilibrium. It really only takes into account the atomic processes. It's really not the right tool for understanding the density in the core of a magnetically confined plasma such as ITER. The Saha equation is more useful for colder partially ionized plasmas, such as those used in plasma processing. However, my understanding is that even here the Saha equation has limited utility.

The density is something that we can control in fusion experiments. We can fuel an experiment using gas puffing, neutral beams, and pellet injections.

The theoretical calculation of the density in magnetically confined plasmas is the subject of transport theory. This theory tries to calculate the density profile given the density sources and sinks. There are a number of different processes that give rise to different types of transport. For example there is turbulent transport, neoclassical transport, etc. In order to correctly calculate the density you have to correctly account for all the dominate types of transport. This is actually a very difficult problem with many open questions.

In current carrying devices there is also a hard stability boundary that limits the maximum density in a device. This is the Greenwald limit. The Greenwald limit is often used in design studies of tokamak reactors. Here is is assumed that the tokamak operates at some fraction of this limit.
 
Yes I understand. I know the Greenwald limit as a stability boundary and I thought of using this for my calculations. It is hard for an amateur to account for all the possible problems in a tokamak but at least it is fun learning new stuff. I've dealt with many of them in my research but they are so many and complicated in nature. Thank you all for your answers, they were really helpful! I appreciate it
 

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