Bender, M.

Körber, B.

Severin, D.

GSI Repository

Impedance Spectroscopy on High Performance PolymersB. Ko¨rber1,2, D. Severin1, and M. Bender∗11GSI, Darmstadt, Germany; 2HS RheinMain, Wiesbaden, GermanyDue to the outstanding properties, polyimide films suchas Kapton are widely used as electrical and thermal insula-tion in harsh environments. Compared to other organic in-sulators, Kapton exhibits enhanced radiation hardness andcan be applied in a wide temperature range from 4 up to670 K.For the new FAIR accelerators, Kapton will be used as in-sulating material in superconducting magnet coils. Here itis exposed to a continuous particle shower of primary andsecondary ions, e.g. from scattering events with residualgas. In the long run, this leads to degeneration of the mate-rial and might cause the insulator to fail. To follow the ag-ing process of the insulator under radiation exposure and topredict its life time, we have developed an in-situ monitorof the dielectric strength by means of an impedance mea-surement as possible low-cost device for the FAIR magnets[1]. Briefly, the dielectric value is measured by the res-onance frequency of an LC oscillator, where the foil rep-resents the dielectric medium of the capacitor. Here, wewill report on results obtained with different ion beams andenergies, corresponding to different dose loads. The radi-ation dose accumulated over time equals the total energydeposited per mass unit of the test sample and is thus in-dependent of the different beam parameters such as speciesand energy of ions. A direct comparison of the irradiationeffects is scaled by the dose D:D =Em=e ·ΔE · Φd · ρ , (1)where e is the elementary charge, ρ the mean target density,ΔE is the deposited energy as∫ x0δEδx Δx and Φ the total ir-radiation fluence in projectile ions per area.In our irradiation experiments, we used a flux of1.4 · 108cm−2s−1 for Xe and Au and 4 · 108cm−2s−1 forTi ions. The beam energy was varied between 1.4 and 4.8MeV/u. The Kapton foils had a thickness of 7.6 μm andwere coated with Al on both sides. For beam exposure, thefoils were mounted in a supporting frame to ensure reliableelectrical contact. The initial capacity of the pristine foilvaried within ±15% around the mean value of 4 nF for allinvestigated foils. For an overall comparison of the differ-ent experiments, we plot the change in capacity ΔC/C0against the applied dose. The change in capacity is a directtransformation of dielectric changes, since no geometricalchange of the target is observed such as swelling or shrink-ing.The irradiation experiments were performed at the highcharge state injector (HLI) of GSI with 1.4 MeV/u Xe ionsand at the M3 branch of the UNILAC with Au and Ti ions∗ corresponding author: m.bender@gsi.deof 4.8 MeV/u. The energy loss of the Ti, Xe, and Au ions inthe Kapton target was 4, 12, and 16 MeV/μm respectively.Figure 1 shows the capacity change of the various irradia-tion runs with Ti, Xe, and Au beams. The end of irradiationis marked in red; maximum dose of foils that broke due tobrittleness are marked in black.Figure 1: Capacity change of 7.6 μm thick aluminizedKapton foils from different irradiation experiments vs. re-spective dose.The foils irradiated with Ti do not show a significantchange in capacity up to a dose far above 10 MGy (5 · 10 12ions per cm2). This effect is ascribed to the low energy lossof the Ti ions being below the threshold for severe damageformation reported earlier [2].All other irradiations lead to a steep capacity change be-tween 10 and 20 MGy, which is in good agreement withearlier results from beam-induced structural changes [3].At low dose values, i.e. at the initial stage of the irradia-tion, the capacity change shows a slightly decreasing slope.This effect is ascribed to water desorption. Virgin Kap-ton is known to incorporate water with a dielectric constantof 80 compared to 3.4 for Kapton. Thus, small amountsof water are sufficient to change the foil capacity. For theplanned on-line monitoring, the water desorption is not rel-evant, because the desorption is finalized within the initialirradiation phase.References[1] B. Ko¨rber, et al., GSI annual report 2011-PNI-MR-21[2] P. Salvetat, et al., Phys. Rev. B 55 (1997) 6238[3] D. Severin, et al., Nucl. Inst. Meth. B 236 (2005) 456GSI SCIENTIFIC REPORT 2012 PNI-INHOUSE-EXP-34403

Impedance Spectroscopy on High Performance Polymers

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Impedance Spectroscopy on High Performance Polymers

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