How did the Dementors guard Azkaban?

Question

This question is related to "Harry Potter - How can Dementors be trusted to guard Azkaban?" but the answers there don't cover it.

What, exactly, did the Dementors do while in Azkaban? We're told that they guard it — but how?

I assume that the prisoners were behind bars, and the Dementors basically just hung around waiting to see if someone managed to escape their bars, in which case they would feed on their souls.

This poses a few problems. If the bars (or other magical protections or whatever) held, the Dementors would effectively be starved, because there'd be no one to feed on. We also assume that they can't (or won't) pass through the bars. If it's a magical protection, this makes sense, but on the other hand, people must get in there somehow. At some point a wizard guarding Azkaban must bring in a new prisoner, and the Dementors would "agree" not to suck that person's soul.

According to The Harry Potter Wikia on Dementors:

They are also intelligent enough to be greedy: they obeyed the Ministry of Magic for years because, in guarding Azkaban, they were provided with sustenance of any remaining hope or happiness in the prisoners.

So, it would seem that they just sort of float around, surviving on the remaining hope or happiness of new prisoners (since we'd have to assume you'd run out of hope pretty quickly with a Dementor eating it). In addition, I'd think the greedy thing to do would be to suck the soul out of the prisoner as they're getting to their cell.

What, exactly, did the Dementors do in Azkaban? How did they guard it?

Answer 1

By continuously instilling despair

First, Dementors do not guard Azkaban in the hopes of eating the souls of escapees. Such escapes simply do not happen frequently enough for it to be a worthwhile incentive. After all, Azkaban is legendarily secure:

"Molly, they say Sirius Black's mad, and maybe he is, but he was clever enough to escape from Azkaban, and that's supposed to be impossible."

—Harry Potter and the Prisoner of Azkaban

It would seem that the Dementors continuously drain the emotions of their human captives. This makes sense: humans are continually producing emotions. The Dementors simply drain them away as quickly as they form. Every spark of hope, every beginning of happiness can feed the Dementors. Hagrid seems to indicate that the Dementors are constantly feeding on the happiness of prisoners.

Mind, the dementors weren't keen on lettin' me go."

"But you were innocent!" said Hermione.

Hagrid snorted. "Think that matters to them? They don' care. Long as they've got a couple o' hundred humans stuck there with 'em, so they can leech all the happiness out of 'em, they don' give a damn who's guilty an' who's not."

—Harry Potter and the Prisoner of Azkaban

Of course, after a while, a prisoner will be constantly depressed:

"Yeh can' really remember who yeh are after a while. An' yeh can' really see the point o' livin' at all.

—Harry Potter and the Prisoner of Azkaban

This makes sense. When new escapees come in, it takes time for all their excess emotions to be drained away to the level where the Dementors are taking their positive emotions as quickly as they can make them, so they offer a particularly rich food source for the Dementors.

But the Dementors are still draining emotions even once a prisoner reaches a permanently depressed state—indeed, that's precisely why the prisoners are always miserable!

The constant depression prevents even talented witches and wizards from mustering the will to escape. Further, who would try to break out prisoners when they are surrounding by hundreds or thousands of Dementors? (Except someone like Voldemort, who can bargain with them).

"Azkaban must be terrible," Harry muttered. Lupin nodded grimly. "The fortress is set on a tiny island, way out to sea, but they don't need walls and water to keep the prisoners in, not when they're all trapped inside their own heads, incapable of a single cheery thought. Most of them go mad within weeks."

—Harry Potter and the Prisoner of Azkaban

Answer 2

What, exactly, did the Dementors do while in Azkaban?

Based on what we've seen throughout the series, they seem to carry out all of the regular duties you'd expect from a prison guard - opening and closing cells, transporting prisoners, bringing food to prisoners (though they hopefully weren't responsible for preparing said food), etc.

'...So, one night when they opened my door to bring food, I slipped past them as a dog ...'
_{Harry Potter and the Prisoner of Azkaban, Chapter Nineteen - The Servant of Lord Voldemort}

That's in addition to their natural, passive effect of feeding off the positive emotions of those left in their "care", which would keep prisoners reasonably passive and result in very few escape attempts.