You'll know afterwards, when I get mad. [Analysis of 'when']

Donna: Well, you'll know afterwards, when I get mad.

Is when a relative word taking afterwards as antecedent?

When, adverb = the time at which

This is spoken “Well, you'll know afterwards, [slight pause] when I get mad.” = “Well, you'll know afterwards, which is when I will get mad.” or “Well, you'll know afterwards because afterwards is the time at which I will get mad.”

“when I get mad” is in apposition to “afterwards” – you can omit either “when I get mad” or “afterwards” (but not both.)

Afterwards is an adverb, and thus “when I get mad” is a descriptive adverbial phrase, explaining the meaning of “afterwards” – it is thus ‘relative’.

Consider:

“John, the man who owns the shop, is very ill.” You can omit "John" or "the man who owns the shop"

Yes, the "afterwards" is an antecedent to the "when" in this case.

One of the examples of an antecedent that is followed by a corresponding proform, given in this wikipedia page is

d. He arrived in the afternoon, when nobody was home. - Prepositional phrase as antecedent

where "in the afternoon" is a prepositional phrase that is the antecedent to "when". The difference between this example and the one in "Well, you'll know afterwards, when I get mad.", is that this example is in the past, hence, "when nobody was home". In "Well, you'll know afterwards, when I get mad.", the "afterwards" is in the future, so the present simple is used in "when I get mad".

As we can see in the Cambridge Dictionary's grammar section

The present simple is used to refer to events in the future which are certain because they are facts, or because there is a clear or fixed schedule or timetable.