The Helical Hollow Cathode

 

Summary

The novel hollow cathode design possess many of the positive characteristics of the known hollow cathodes and in addition, it has some advantages, which allow to use the helical cathode successfully for building different hollow cathode metal vapor lasers. By changing the helical cathode transparency the discharge voltage and, hence, the excitation efficiency of different ion lines, changes too.

The most favorable features of the helical design of the hollow cathode for laser are:

  • possibility to change the discharge voltage by changing the helix transparency – high voltage HCD;

  • lack of current saturation with increasing discharge voltage;

  • excellent discharge stability at high current density and high discharge voltage;

  • high density of fast electrons – efficient excitation of high energy ion levels;

The helical cathode suggests some other new possibilities:

  • efficient cooling of the helix, wound of a metal tube through which water can flow which allows to obtain lasing of lines with a high threshold current or to get high laser output power at high input power, avoiding the danger of destroying the laser tube by overheating;

  • easy access of metal vapors in the cathode region;

  • an additional current through the helix can generate axial magnetic field to affect the plasma parameters;

The helical design enables to make a new electrode configuration - a double-helical hollow cathode discharge. The new design is symmetrical and the discharge can be ac excited, the two electrodes changing their polarity, so that each helix is successively either cathode or anode.
Similarly to the double-helical configuration, electrode configurations with three, four or more helical electrodes can be built and depending on the particular requirements can form different combinations of electrodes for three-phase or multi-phase excitation.
A novel helical cathode is developed for the case when the metal atoms are produced by heat evaporation,. The new helical cathode, like the sputtering cathode, acts as a source of metal atoms and a homogeneous distribution of metal vapors in the whole discharge zone of the cathode is achieved.
 
 
IMG 3942
The helical hollow cathode

 

Scientific results and their implementation

The first conference report on helical hollow cathode design initiated very quickly various investigations concerning the helical hollow cathode application in several different laboratories in Europe, Japan and Australia:

Metal vapour lasers 

Because of the possibility for efficient cooling of the helix, it appears to be very promising design for sputtering metal vapor lasers working at high current density. Soon after the first publication a group from the Technical University in Berlin reported oscillation on Cu ion lines using a helical hollow cathode discharge and demonstrated a 4 time higher output laser power compared to the conventional HCD. The advantages of the helical design are emphasized: a simple design, excellent discharge stability at high input power, good active volume homogeneity, efficient cooling, and stable cathode geometry for more than 1000 h operation. Later, the same group reported deep UV laser oscillation with record output power in the UV range.
Groups in Japan and France used the helical cathode for oscillation on Cd and Zn ion lines, realizing a “white-light” laser – simultaneous oscillation on blue, green and red lines. Due to the open geometry of the helical cathode, a higher electron density is achieved, considerably improving the laser excitation efficiency.

Inert gas ion lasers

In the first publication it was demonstrated that the excitation efficiency of Kr ion lines increases when increasing the helical cathode transparency. Later the helical design was used successfully for building a Penning laser, oscillating in He-H plasma at high laser power in the visible spectral range.

Other applications

The idea for a helical hollow cathode was also utilized in hollow cathode lamps for atomic absorption spectroscopy.