{
     "model": {
        "_all": {
             "index_analyzer": "nGram_analyzer",
             "search_analyzer": "whitespace_analyzer"
        }

       ...
  }

var result = this._client.Map<ElasticsearchProject>(m => m
    .AllField(a=>a
        .Enabled()
        .IndexAnalyzer("nGram_analyzer")
        .SearchAnalyzer("whitespace_analyzer")
        .TermVector(TermVectorOption.with_positions_offsets)
    )
    ...
    ...