Ions and ion traps

[dtvmcdonald]

I was just thinking about ion traps.

Does anybody know exactly what ions? What's off is that if they hit the screen,
they have to be NEGATIVE ions!

The only common negative ions that could be in a CRT are H-. O-, and
OH-, of which is first is by far the easiest to make.

Does anybody know?

Doug McDonald

[electroking]

There is some air (O_2 and N_2) in the CRT. If an air molecule captures an electron,
it does become a negative ion. The trick with ion traps was to install the electron
gun slanted and straighten the path of the electrons with the magnet. The same
magnetic field acting on the heavier and slower negative ion shas a negligible effect,
and the ion just hits the first anode and is neutralized.

[old_coot88]

Originally Posted by electroking

The trick with ion traps was to install the electron
gun slanted and straighten the path of the electrons with the magnet. The same
magnetic field acting on the heavier and slower negative ion shas a negligible effect,
and the ion just hits the first anode and is neutralized.

Just like winnowing grain in the wind.

[Electronic M]

The reason for ion traps is that before aluminized screens(a coating of vaporized aluminum over the phosphor which reflects light towards the viewer and shields the phosphor from ions) the unprotected phosphor screen was vulnerable to damage from being hit by ions. This effect known as ion burn is common in pre-ion trap era CRTs and manifests itself as a browning of phosphors in the center region of the screen(this brown region also does not get as bright as the surrounding area).

[bandersen]

Some early post war sets also didn't have ion traps. Here's an example of ion burn in a Stromberg Carlson TV-12.

[compucat]

I read the instruction manual for one of the early pre-war RCA TVs. It was the console model with the 5 inch screen and shortwave radio. I forget the model number. It was one of the TRK sets. The manual said that the screen would naturally discolor as the kinescope ages. This obviously referred to ion burn which was unable to be prevented at the time. Aluminized tubes were a real improvement in CRT performance.

[compucat]

I have never seen an ion burn in an electrostatic set yet they do not use an ion trap. Anyone know why a 7JP4 doesn't need an ion trap? Is it because the high voltage is relatively low or are the ions absorbed by the deflection plates?

[bandersen]

I've read that electrostatic fields deflect the ions along with the electrons. So the ions are spread evenly over the screen rather than bombarding a circle in the middle. Eventually the screens do get dimmer but it takes a while.

[compucat]

Originally Posted by bandersen

I've read that electrostatic fields deflect the ions along with the electrons. So the ions are spread evenly over the screen rather than bombarding a circle in the middle. Eventually the screens do get dimmer but it takes a while.

Maybe that explains why older electrostatic tubes are not as bright as ones that are new old stock. I have a NOS 8BP4 for my Motorola 9VT1 that I have never tested as I am saving it until I really need it. I wonder how much brighter it would be compared to the one in my daily use set that was installed in 1954. Apparently the original CRT only lasted five years.

[old_coot88]

Originally Posted by bandersen

I've read that electrostatic fields deflect the ions along with the electrons. So the ions are spread evenly over the screen rather than bombarding a circle in the middle. Eventually the screens do get dimmer but it takes a while.

Ions being vastly heavier than electrons would make them (ions) harder to deflect, causing them to hit nearer center of the screen. Seems like, anyhow.

The principle of the ion trap itself is the easier deflectability of electrons vs. ions ("winnowing").

[bandersen]

It's my understanding that ions are more easily deflected by an electric field vs a magnetic field used in ion traps.

[jr_tech]

IIRC, the force on a charged particle in an electrostatic field depends upon the amount of charge and the strength of the field. In a magnetic field the force depends on the amount of charge, the magnetic field strength as well as the *velocity* of the charged particle.

jr

[egrand]

Here's a couple of links about it and different ion traps:

http://books.google.com/books?id=Pno...20tube&f=false

http://www.r-type.org/articles/art-004e.htm

[old_tv_nut]

It just struck me that the magnet gizmo we call the ion trap is really the electron un-trapper.

[old_coot88]

Originally Posted by old_tv_nut

It just struck me that the magnet gizmo we call the ion trap is really the electron un-trapper.

Eggzackly. It's the electron beam re-directer.
Sorta like the definition of a shock absorber is really the rebound damper.. while the spring is what does the shock absorbing.