A method of measuring ion current between a plasma and an electrode in communication with said plasma, wherein a time-varying voltage is measured at said electrode and a time-varying current through said electrode is measured, the method comprising the steps of: (a) recording, for each of a plurality of voltage values, v', a plurality, n, of current values I(v'); and (b) obtaining from said current and voltage values a value of said ion current; wherein: said electrode is insulated from said plasma by an insulating layer, such that said current values lack a DC component; and said step of obtaining a value of said ion current comprises performing a mathematical transform effective to: (i) express said current and voltage values as a relationship between the real component of current through said electrode and the voltage by obtaining an average of the current values measured for each of a plurality of discrete voltage values, thereby eliminating a capacitive contribution to the current through the electrode; (ii) isolating from said real component of current through the electrode an isolated contribution attributable to an ion current and a resistive term, said contribution being free of any electron current contribution; and (iii) determining from said isolated contribution a value of ion current. (a) recording, for each of a plurality of voltage values, v', a plurality, n, of current values I(v'); and (b) obtaining from said current and voltage values a value of said ion current; wherein: said electrode is insulated from said plasma by an insulating layer, such that said current values lack a DC component; and said step of obtaining a value of said ion current comprises performing a mathematical transform effective to: (i) express said current and voltage values as a relationship between the real component of current through said electrode and the voltage by obtaining an average of the current values measured for each of a plurality of discrete voltage values, thereby eliminating a capacitive contribution to the current through the electrode; (ii) isolating from said real component of current through the electrode an isolated contribution attributable to an ion current and a resistive term, said contribution being free of any electron current contribution; and (iii) determining from said isolated contribution a value of ion current. said electrode is insulated from said plasma by an insulating layer, such that said current values lack a DC component; and said step of obtaining a value of said ion current comprises performing a mathematical transform effective to: (i) express said current and voltage values as a relationship between the real component of current through said electrode and the voltage by obtaining an average of the current values measured for each of a plurality of discrete voltage values, thereby eliminating a capacitive contribution to the current through the electrode; (ii) isolating from said real component of current through the electrode an isolated contribution attributable to an ion current and a resistive term, said contribution being free of any electron current contribution; and (iii) determining from said isolated contribution a value of ion current. (i) express said current and voltage values as a relationship between the real component of current through said electrode and the voltage by obtaining an average of the current values measured for each of a plurality of discrete voltage values, thereby eliminating a capacitive contribution to the current through the electrode; (ii) isolating from said real component of current through the electrode an isolated contribution attributable to an ion current and a resistive term, said contribution being free of any electron current contribution; and (iii) determining from said isolated contribution a value of ion current.