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The concept dates to the 1950 s , and was briefly advocated by Hans Bethe during the 1970 s , but largely remained unexplored until a revival of interest in 2009 , due to delays in the realization of pure fusion .
 Hans Bethe
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 Hans Bethe