Re: [Phys-l] Sparks in vacuum

[John Denker <jsd@av8n.com>]

On 03/27/2009 07:16 AM, CARABAJAL PEREZ, MARCIAL ROBERTO wrote:

> Students made me an interest question. In a perfect vacuum, Which is
> the potential (voltage) required to obtain a spark between two
> parallel plates at distance d ?.  SInce ionization is not possible in
> perfect vacuum, I find it difficult to answer. 

You can get an electrical current, but it won't look like a
spark and it won't properly be called a spark.

The phenomenon is called _field emission_.

The onset occurs when the field is on the order of 300 MV/m for
typical metallic materials.  The Wikipedia article quotes 1000 MV/m 
but it's the same idea.
  http://en.wikipedia.org/wiki/Field_emission

The underlying physics revolves around an escape by tunneling
through a barrier, which sounds like an undergrad-level QM
problem ... but there are lots of devilish details, and a
complete treatment of the problem is a real tour de force
at the grad-school level or above.

Field emission has tremendous practical applications.  Conversely
preventing it also has tremendous real-world significance.