A seal for a vacuum chamber, wherein the vacuum chamber comprises a first wall and a second wall, the seal comprising: a first outer insulation block for sealing between the first wall of the vacuum chamber and a first power transmission plate that extends from the interior of the vacuum chamber to the exterior of the vacuum chamber between the first and second walls of the vacuum chamber; a second outer insulation block for sealing between the second wall of the vacuum chamber and a second power transmission plate that extends from the interior of the vacuum chamber to the exterior of the vacuum chamber between the first and second walls of the vacuum chamber; wherein the seal is characterised by: an intermediate insulation block (34, 36) for sealing between the first and second power transmission plates, wherein the intermediate insulation block comprises one or more slots formed in a first side of the intermediate insulation block for facing the interior of the vacuum chamber and one or more slots formed in a second side of the intermediate insulation block for facing the exterior of the vacuum chamber; one or more interior insulation sheets (74, 76, 78) for extending between the first and second power transmission plates on the interior of the vacuum chamber, wherein the one or more interior insulation sheets are arranged in the one or more slots (62, 64, 66) in the first side of the intermediate insulation block; and one or more exterior insulation sheets (80, 82, 84) for extending between the first and second power transmission plates on the exterior of the vacuum chamber, wherein the one or more exterior insulation sheets are arranged in the one or more slots (68, 70, 72) in the second side of the intermediate insulation block. a first outer insulation block for sealing between the first wall of the vacuum chamber and a first power transmission plate that extends from the interior of the vacuum chamber to the exterior of the vacuum chamber between the first and second walls of the vacuum chamber; a second outer insulation block for sealing between the second wall of the vacuum chamber and a second power transmission plate that extends from the interior of the vacuum chamber to the exterior of the vacuum chamber between the first and second walls of the vacuum chamber; wherein the seal is characterised by: an intermediate insulation block (34, 36) for sealing between the first and second power transmission plates, wherein the intermediate insulation block comprises one or more slots formed in a first side of the intermediate insulation block for facing the interior of the vacuum chamber and one or more slots formed in a second side of the intermediate insulation block for facing the exterior of the vacuum chamber; one or more interior insulation sheets (74, 76, 78) for extending between the first and second power transmission plates on the interior of the vacuum chamber, wherein the one or more interior insulation sheets are arranged in the one or more slots (62, 64, 66) in the first side of the intermediate insulation block; and one or more exterior insulation sheets (80, 82, 84) for extending between the first and second power transmission plates on the exterior of the vacuum chamber, wherein the one or more exterior insulation sheets are arranged in the one or more slots (68, 70, 72) in the second side of the intermediate insulation block. an intermediate insulation block (34, 36) for sealing between the first and second power transmission plates, wherein the intermediate insulation block comprises one or more slots formed in a first side of the intermediate insulation block for facing the interior of the vacuum chamber and one or more slots formed in a second side of the intermediate insulation block for facing the exterior of the vacuum chamber; one or more interior insulation sheets (74, 76, 78) for extending between the first and second power transmission plates on the interior of the vacuum chamber, wherein the one or more interior insulation sheets are arranged in the one or more slots (62, 64, 66) in the first side of the intermediate insulation block; and one or more exterior insulation sheets (80, 82, 84) for extending between the first and second power transmission plates on the exterior of the vacuum chamber, wherein the one or more exterior insulation sheets are arranged in the one or more slots (68, 70, 72) in the second side of the intermediate insulation block.