FileSystem problem - cordova Ionic Application

same problem :

Error: LocalFileSystem is not defined

code

app.controller('AddCtrl', function ($scope, $location, $ionicPlatform) {
  $ionicPlatform.ready(function() {

      $scope.addAss = function () {
            $scope.save();
      };

    $scope.save = function () {
            alert ('ok 1');
            window.requestFileSystem(LocalFileSystem.PERSISTENT, 0, fsReceived, errorHandler);
            $location.path('/');
    };

    var fsReceived = function (fileSystem) {
            alert ('ok 2');
    };

    var errorHandler = function (error) {
    };

  });
});