Is the "wa-particle" in katakana ワ or ハ？
(Just a quick question - I think I have seen both used, but if ワ is correct then I have just seen the results of a computer o/p automatically using ハ)
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ハ for the topic particle. There's no difference from hiragana.
The basic answer is that は is written ハ in katakana.
However, I think it depends on why it's written in katakana. One reason you might write something in katakana is to communicate pronunciation, and in this case the particle は would be written ワ:
spelling pronunciation おはよう オハヨー こんにちは コンニチワ
You can see this sort of use of katakana in some official documents, for example.
On the other hand, if you're just writing regular Japanese using katakana, you'd usually spell は as ハ. Another reason you might use katakana is for the use-mention distinction; some books or papers about Japanese grammar, for instance, write particles in katakana when they're mentioned rather than used, and in this context は would be ハ (and を would be ヲ, and so on).
You can see it's ハ as what the old MS office assistant F1 said
You can also see the usage of ヲ in the above sentence
As a side note the text above isn't a translation of the English version
Katakana is just used for foreign words, the grammar elements are not written in Katakana.