Is it possible to know the SPEED of electron emission

[jeyurkon]

Originally Posted by old_coot88

Actually the beam is under constant acceleration from the time it leaves the gun till it hits the screen. The higher the HV, the higher the acceleration and the higher the final velocity. I dunno what the final velocity is, but it's really honkin' on.
Bill(oc)

That's partially true. If you look at the construction of CRT you'll notice that the aquadag, which is connected to the HV, extends back into the neck of the CRT. There are additional electrodes that we don't always think about. These are acceleration electrodes. The aquadag connects to the last electrode and the electrons reach their final velocity as they pass this electrode which happens before the beam passes the yoke.

While it may seem odd, the region past this last electrode inside the bell region is basically field free because it is all at the same potential.

John

[old_coot88]

Originally Posted by jeyurkon

That's partially true. If you look at the construction of CRT you'll notice that the aquadag, which is connected to the HV, extends back into the neck of the CRT. There are additional electrodes that we don't always think about. These are acceleration electrodes. The aquadag connects to the last electrode and the electrons reach their final velocity as they pass this electrode which happens before the beam passes the yoke.

While it may seem odd, the region past this last electrode inside the bell region is basically field free because it is all at the same potential.

John

Y'know it IS embarrassin' to have been in the trade for 30 years and now long retired, and never given thought to that accelerating electrode being connected to HV via the internal coating of the jug. So the beam is in fact not accelerating once it leaves that cylinder. Funny how such innocuous misconceptions can be carried for a lifetime unless someone corrects 'em. Thankya, John
Bill(oc)

[jeyurkon]

From "Electronic Fundamentals and Applications", John D. Ryder, 1964.

"Since the electron beam striking the screen is carrying negative charge and the screen is insulated by the glass, the removal of this charge is necessary; otherwise the potential of the screen would fall to such a negative value as to repel the beam. Fortunately, as the electrons strike the screen they not only cause the screen to give off light but also to emit other electrons. This effect, known as secondary emission (see Chapter 3), may result in an average of more than one secondary electron being emitted per electron in the beam. The secondary electrons are attracted to a graphite coating placed over the interior bulb walls and connected to the second anode. As a result, instead of the screen acquiring a negative potential, it may be a few more volts positive with respect to the second anode."

[jr_tech]

But this is true only below second crossover, the voltage at which the secondary emission ratio of the insulator again becomes unity. Aluminized crts overcome this problem and allow landing energies at the screen at almost full anode potential rather than the "sticking potential" determined by the secondary emission properties of the phosphor.

jr

[jeyurkon]

It's true that aluminization overcomes this problem.

Potassium silicate is normaly used in the deposition of the phosphor.

Strong silicate solutions, or evaporated magnesium oxide, allows them to adjust the second crossover point to closely follow the applied anode voltage.

A B McFarlane 1957 Br. J. Appl. Phys. 8 248-252

John

[jr_tech]

Ahhh Yes! nothing like a little MgO to fix up the old droopy secondary emission curve.

jr