Question: what is the purpose of a van de graaff generator
[{'answer': 'physics research', 'provenance': [{'wikipedia_id': '52642', 'title': 'Van de Graaff generator', 'start_paragraph_id': 3, 'start_character': 72, 'end_paragraph_id': 3, 'end_character': 88, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '52642', 'title': 'Van de Graaff generator', 'start_paragraph_id': 3, 'start_character': 0, 'end_paragraph_id': 3, 'end_character': 426, 'bleu_score': 0.9338603241255111, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 1617192204262268300}, 'section': 'Section::::Abstract.'}]}]