when one Citta perishes,from where does the next arise?

The Buddhist body-mind model is similar to the hardware-software model of how a computer or robot works.

Hardware and software (of a computer or robot) are both machines or automatons on their own right and also depend on each other to continue functioning. It is similar in the case of the mind and the body. Even if the exact mechanism is not the same, the general principle is the same.

Just as how a robot or computer (hardware-software) does not require a soul or Self to function, similarly the body-mind also does not require a soul or Self to function. The self is just an idea of the mind.

Please note from DN 2:

“When his mind is thus concentrated, pure and bright, unblemished, free from defects, malleable, wieldy, steady and attained to imperturbability, he directs and inclines it to knowledge and vision. He understands thus: ‘This is my body, having material form, composed of the four primary elements, originating from father and mother, built up out of rice and gruel, impermanent, subject to rubbing and pressing, to dissolution and dispersion. And this is my consciousness, supported by it and bound up with it.’

The last statement shows the dependence of mind on body to continue functioning ("supported by it") and also the inter-dependence of mind and body with each other ("bound up with it").

From here, I quote the Buddhist teacher Stephen Batchelor who describes this operation as a fluidic stream of events:

I would argue that there is really no present moment. The present moment is one of these things that Buddhists have become terribly attached to. If you think about it, you try to find the present moment, you will never find anything. The present moment is actually just a concept; it can be a very useful, strategic concept … I will often say when instructing in meditation, “Stay in the present moment.” But I don’t mean by that, try to find this elusive thing called the present moment and stay in it. It’s basically a way of saying, Don’t get caught up in the unknown future. Don’t get caught up in reminiscing about the past. But confront the situation at hand. And the situation at hand is always unfolding. It’s fluid. It’s like water, it’s like a stream. Things are constantly impacting your senses, constantly bubbling up in your thoughts, constantly emerging as emotions and feelings; it’s always in motion, it’s moving. And it’s such a mobile experience that notion of ‘The present moment’ really has no place there. There’s no point really. So, every situation that occurs, and you could call it ‘at the present moment’ if you wish, but it’s basically an unfolding of events that is calling forth an appropriate response. And in this sense, it’s always in time.

A river looks like it flows uninterruptedly, just like the mind. But the river is composed of moving water molecules - each time you look at a flowing stream, you are looking at a different set of water molecules.

Similarly, the movement of the mind involves arising and ceasing cittas - so fast, that they appear uninterrupted.

Now scientific studies are also validating this - please see this answer for details.

Citta arises from mentality-and-materiality (nama-rupa). As long as the physical body (materiality) is alive, cittas will continue to arise.

Note: citta is of the same substance as consciousness.

Mentality-and-materiality is the cause and condition for the manifestation of the consciousness aggregate. SN 22.82

With the origination of mentality-and-materiality there is the origination of citta. With the cessation of mentality-and-materiality there is the passing away of citta. SN 47.42

By avija (not knowing) and tanha (craving) do saṅkhāras (formation) arise, good householder, does world come to arise. Here a possibility for gatī in another sphere.

I think the problem is that you're mixing two school of thought, the concept of momentary flux,momentary consciousness, etc has no place in Nagarjuna's teaching or that of Mahayana in general.

That which is called mind, intellect or consciousness arises as one thing and ceases as another. The conditioned element changes as it persists. The past is one end, the future is the other end, the present is the middle and craving is the seamstress.

The premise of your question is flawed as nowhere in the Sutta is mind explained to be arising and ceasing as one and the same thing before the arising of another. Such interpretations are wrong and easy to disprove by contradiction.

As I explained in my answer about dharmas, the dharmas (including cittas) are not discrete. There's no distinct "previous" and "next" dharmas. Instead, things morph and stir continuously like clouds - and this includes the thoughts too.

[Nagasena:] ‘Just so, O king, does the continuity of dharmas connect. One emerges, another dissolves, connecting as it were without [a clear boundary between] the previous and the next, thus the former-consciousness and the next-consciousness cannot be categorized as either the same nor as different.’

This invalidates all the points made in your question.

You might as easily ask where the next wave comes from once a wave has crashed on the beach. The answer is that the wave is not the water. The wave is the result of movement within the water, of various internal flows within the water pushing against each other (including the flow of water out from the last wave as it recedes). In philosophy jargon, a wave is epiphenomenal: it is the outwardly observable expression of some phenomenon we cannot easily observe.

Dharmas are like waves. Cittas are like waves. Thoughts are like waves. Selves are like waves. They are upwellings caused by tangled movements within in the vast ocean of karma. The practice is to untangle and smooth over those deep movements so that the upwellings abate.

OP: Anatman/Anatta says that the 'self'is a momentary flux of Cittas-Vijnanas.

There's a stream of consciousness but it's not self. There's no self at all. That's why the blessed one said, "Sabbe Dhamma Anatta".

OP: There can be no stable permanent Citta.

Yes, it's true. Everything unless Nibbana is impermanent. So is Citta.

OP: In kshanabhangavada a dharma perishes before its successor arises.

Here, if you are asking about the stream of consciousness, it is true according to Teravada Abhidhamma. One Citta perishes before the next arises. How different Dhammas be the causes in different causalities to make an effect is very complex. No single Dhamma arise with a single cause. There are many causes and one Dhamma may be a cause for another Dhamma in different causalities (paccaya). For example two Dhamma can be co-exist being the causes for each other. This is called, Annamanna-paccaya.

OP: One citta arises then perishes and another takes its place all going on until Nirvana is established.

This process permanently ends with Anupadisesa Nibbana. But it can be temporarily stopped (paused) with Nirodha Samapatti.

OP: However,when one citta perishes,from where and what does the next Citta arise?

This is not a suitable question to ask since "Where" is not applicable for Citta. Citta doesn't acquire space. It's a Nama Dhamma. You can ask "How, When, What, Which" questions about Citta, but not "Where" questions. May I ask you a question, "If you plant a seed in a fertile soil and water it, it will become a plant. From where that plant came from? Was it inside that seed or was it somewhere else?". You may answer, "Don't ask such a question. The plant doesn't come from anywhere. The plant is produced when the conditions are met." Same applies for Citta. If certain conditions satisfied Citta will arise. You can't say where that Citta arise from.

OP: It cannot be that the Citta has the same substance as the former Citta.

Even if the next (or another) Citta has the same mental formations as the present Citta, those two Citta won't be identical. For the ease of learning about Citta, we may classify all the Citta with same mental formations into a same group and give it a common name. But two Citta won't be identical. For example, if we take two Citta in the same stream of consciousness which take the same thing as their object won't be identical. Then how about taking different things as Citta's objects? They are definitely different.

Note: This is how I understood. I may be wrong but not Dhamma.