An electrode apparatus comprising: an electrode assembly comprising a bipolar arrangement of an even number of arrays (11,12), such as two arrays, wherein each one of said two arrays comprises one or more electrode elements (3,5); and wherein the electrode apparatus is configured such that said two arrays have opposite polarity, when in operation; and wherein said electrode elements of each one of said two arrays are at least three electrode elements, and wherein said at least three electrode elements of each one of said two arrays comprises a first electrode element, one or more central electrode elements and an ending electrode element, and wherein the electrode apparatus is configured such that said at least three electrode elements of each one of said two arrays produce an electric field having uniform intensity along an imaginary straight line between said first electrode element and said ending electrode element of said at least three electrode elements having the same polarity, said imaginary line connecting and crossing said first electrode element and said ending electrode element having the same polarity; and wherein said one or more central electrode elements are offset of said imaginary straight line, wherein the electrode apparatus is configured to produce an electric field region having a square or rectangular shape cross section avoiding cold spots along said imaginary line, when in operation; characterized in that said bipolar arrangement is a circular arrangement and wherein said first electrode element, one or more central electrode elements and ending electrode element of each one of said two or more arrays are arranged on opposite locations of said bipolar arrangement; wherein the electrode apparatus is further configured such that, when in operation, said at least three electrode elements of one of said two or more arrays have a positive polarity and said at least three electrode elements of one of said two or more arrays have a negative polarity and at least three elements having a positive polarity are arranged on the perimeter of a first half of said circular arrangement and said at least three electrode elements having a negative polarity are arranged on said perimeter of a second opposite half of said circular arrangement. an electrode assembly comprising a bipolar arrangement of an even number of arrays (11,12), such as two arrays, wherein each one of said two arrays comprises one or more electrode elements (3,5); and wherein the electrode apparatus is configured such that said two arrays have opposite polarity, when in operation; and wherein said electrode elements of each one of said two arrays are at least three electrode elements, and wherein said at least three electrode elements of each one of said two arrays comprises a first electrode element, one or more central electrode elements and an ending electrode element, and wherein the electrode apparatus is configured such that said at least three electrode elements of each one of said two arrays produce an electric field having uniform intensity along an imaginary straight line between said first electrode element and said ending electrode element of said at least three electrode elements having the same polarity, said imaginary line connecting and crossing said first electrode element and said ending electrode element having the same polarity; and wherein said one or more central electrode elements are offset of said imaginary straight line, wherein the electrode apparatus is configured to produce an electric field region having a square or rectangular shape cross section avoiding cold spots along said imaginary line, when in operation; characterized in that said bipolar arrangement is a circular arrangement and wherein said first electrode element, one or more central electrode elements and ending electrode element of each one of said two or more arrays are arranged on opposite locations of said bipolar arrangement; wherein the electrode apparatus is further configured such that, when in operation, said at least three electrode elements of one of said two or more arrays have a positive polarity and said at least three electrode elements of one of said two or more arrays have a negative polarity and at least three elements having a positive polarity are arranged on the perimeter of a first half of said circular arrangement and said at least three electrode elements having a negative polarity are arranged on said perimeter of a second opposite half of said circular arrangement.