MKI spiral beam screen measurements Agnieszka Chmieliska cole
MKI spiral beam screen measurements Agnieszka Chmielińska École Polytechnique Fédérale de Lausanne, CERN TE – ABT – PPE IWG Meeting 02. 08. 2019 Acknowledgements: M. Barnes, G. Bellotto, O. Bjorkqvist , S. Bouleghlimat, F. Caspers, W. Weterings, C. Zannini Agnieszka Chmielińska IWG Meeting, 02. 08. 2019
Outline 1. Spiral beam screen • Wrap-up 2. Longitudinal beam coupling impedance measurements • Experimental setup • Classical wire method • Resonant wire method 3. Conclusions and future R&D Agnieszka Chmielińska IWG Meeting, 02. 08. 2019 3
Spiral beam screen ü ü ü improved high voltage performance low longitudinal impedance flexibility in terms of different design options possibility to tune transverse resonant modes good field homogeneity (Opera simulations) fast field rise and fall times (Pspice simulations) Agnieszka Chmielińska For details see Ref. [1]. IWG Meeting, 02. 08. 2019 4
Spiral beam screen Design fully optimized for FCC-hh injection kicker magnet: (heating aspects, beam stability, field rise time, etc. ) Baseline FCC-hh design: ü ü ü For details see Ref. [1]. 24 screen conductors Integer number of turns in the aperture Symmetric vacuum gap (2 mm width) Ungraded lengths of screen conductors Reduced overlap length to 44 mm (compared to 56 mm for upgraded LHC MKI) Possibility to connect all spirals together in the overlap region Agnieszka Chmielińska IWG Meeting, 02. 08. 2019 5
Longitudinal beam coupling impedance measurements Agnieszka Chmielińska IWG Meeting, 02. 08. 2019
Remarks 1. The existing spiral tube is not optimized either for FCC or for MKI in terms of beam coupling impedance and HV operation. 2. The aim is to benchmark the impedance of the spiral beam screen with the available prototype on the LHC MKI for the proposed features: • symmetric vacuum gap (2 mm width) • Loverlap=44 mm 3. Special components had to be manufactured to install the spiral tube in the spare LHC MKI and a new CST model had to be developed. 4. The prototype beam screen has defects (slide 16). Agnieszka Chmielińska IWG Meeting, 02. 08. 2019 7
Prototype alumina tube with 21 spiral screen conductors applied using silver paint (~ 4. 5 turns). Agnieszka Chmielińska IWG Meeting, 02. 08. 2019 8
Prototype ~140 mm ~15 mm Capacitively coupled end Agnieszka Chmielińska Grounded end IWG Meeting, 02. 08. 2019 9
Prototype correct configuration 10. 5 44 removed Agnieszka Chmielińska 21. 5 removed IWG Meeting, 02. 08. 2019 10
Prototype Spiral tube in MKI Agnieszka Chmielińska IWG Meeting, 02. 08. 2019 11
Prototype installed in the spare LHC MKI Agnieszka Chmielińska IWG Meeting, 02. 08. 2019 12
Experimental setup LHC MKI with spiral beam screen and single wire stretched inside Sucoboxes with 220 Ohm resistors or capacitive coupling Network analyzer Calibration kit HF cables Agnieszka Chmielińska IWG Meeting, 02. 08. 2019 13
Measurement methods Classical single wire method (see Ref. [2]) Matching resistors: Resonant wire method (see Ref. [2]) Capacitive coupling: § Connection of the wire to the inner conductor of the measurement lines replaced by capacitive coupling § Resonant condition: L=nl/2 § The Q factor of each resonance is used to calculate line attenuation and thus real part of the longitudinal impedance § Higher sensitivity for low loss structures (i. e. LHC MKI) § Limited frequency resolution Agnieszka Chmielińska IWG Meeting, 02. 08. 2019 14
Results • Reasonably good agreement between simulations and two measurement methods (impedance peak due to the capacitive coupling pronounced around 1200 -1350 MHz). Agnieszka Chmielińska IWG Meeting, 02. 08. 2019 15
Results Eventual increase of the impedance in the low frequency range (except up to 10 MHz) can be attributed to the defects of the spiral, i. e. small thickness and broken connections. Nb. Stripe current Stripe voltage Stripe Resistance Average thickness* Cend-end [A] [V] [Ohms] [m] [p. F] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 0 0. 409 0. 417 0. 408 0. 395 0 0. 383 0. 363 0 0 0. 376 0. 387 0. 894 0. 895 0. 896 2. 19 2. 14 2. 19 2. 27 2. 34 2. 47 2. 38 2. 32 Agnieszka Chmielińska 7. 27 E-06 7. 42 E-06 7. 26 E-06 7. 02 E-06 6. 80 E-06 6. 44 E-06 6. 67 E-06 6. 87 E-06 151 143 98 73 63 57 55 54 56 62 67 105 - - 12 spirals broken average thickness: 7 µm (less than one skin-depth at 40 MHz ) - significant differences in measured capacitance IWG Meeting, 02. 08. 2019 16
Conclusions • The spiral beam screen has been developed and optimized for the FCC-hh injection kicker magnet. • Benchmark measurements of the longitudinal impedance has been performed using existing spiral tube installed in the spare LHC MKI. • The measurements show reasonably good agreement with the expected performance. • Defects of the existing spiral tube can influence the results. Agnieszka Chmielińska IWG Meeting, 02. 08. 2019 17
Future R&D • Improved coating quality and larger coating thickness (i. e. 30 µm) • Sensitivity of influence of small errors in induced voltage upon field rise time (for conductors coupled in overlap region) • High-voltage tests • Impedance measurements: • Concept of "switch" to connect all screen conductors to the beam pipe, once injection is complete (long term goal? ) verify low beam coupling impedance • Concept of "tuning" of frequency of transverse impedance resonances • Possibly in a test facility, e. g. CLEAR ? Agnieszka Chmielińska IWG Meeting, 02. 08. 2019 18
References 1. A. Chmielińska, “Spiral beam screen for the FCC-hh injection kicker magnet: longitudinal and transverse impedance", CERN, IWG meeting, 14 May 2019, https: //indico. cern. ch/event/820005/contributions/3427090/attachments/1844046/3024752/ IWG 201905 agav 2. pptx 2. T. Kroyer, F. Caspers, "Longitudinal and transverse wire measurements for the evaluation of impedance reduction measures on the MKE extraction kickers", CERN-AB-Note-2007 -028, July 2007. Agnieszka Chmielińska IWG Meeting, 02. 08. 2019 19
Thank you for your attention. Agnieszka Chmielińska IWG Meeting, 02. 08. 2019
Spare slides Agnieszka Chmielińska IWG Meeting, 02. 08. 2019
Conventional beam screen General features: Conventional beam screen (Catia model): • 24 (Ni. Cr) straight screen conductors inserted into grooved slots of alumina tube to shield ferrite yoke from the EM fields induced by the circulating beam: → reduced beam coupling impedance • Capacitive coupling to ground at the upstream end allows to preserve fast magnetic field rise time: → no major Eddy current loops • Overlap length determines resonances in the longitudinal impedance spectrum → possibility to shift resonant modes to higher frequencies [1]: i. e. : post LS 1 MKI: Loverlap=130 mm [2], upgraded MKI 8 D: Loverlap=56 mm [3]. • Power loss in the ferrite yoke is also reduced due to the damping provided by the ferrite rings positioned around each end of the beam screen [3]. Example: LHC MKI. Ferrite Rings Agnieszka Chmielińska IWG Meeting, 02. 08. 2019 23
Motivation High voltage issues: • • During magnetic field rise and fall, a significant voltage is induced on the screen conductors [2]: → possibility of HV breakdowns between adjacent screen conductors or/and between screen conductors and metallic cylinder. Non-symmetric vacuum gap is introduced to reduce the electric field. Induced voltage: Magnet voltage and voltage of screen conductor (#1): Maximum conductor and inter-conductor voltages (for 60 k. V PFN voltage): ferrite Max. induced voltage: Induced voltage: Agnieszka Chmielińska Max. difference between adjacent conductors: for IWG Meeting, 02. 08. 2019 24
Spiral beam screen Each conductor has a form of spiral: Spiral beam screen: The coordinates of the i-th spiral screen conductor: P-pitch, R-beam screen radius, α-spiral angle. Induced voltage: where and Nturn is the number of turns in the kicker magnet aperture. For the spiral beam screen, the distance of each screen conductor from the GND busbar changes along the z direction: For an integer number of turns along the aperture: Spiral beam screen is expected to provide significantly improved HV performance ü the induced voltage on each screen conductor is the same, ü this is approximately one-half of the worst case straight screen conductor. Agnieszka Chmielińska IWG Meeting, 02. 08. 2019 25
Spiral beam screen (optimized design): Longitudinal impedance spectrum: Power loss increase: Induced voltage: Conclusion: No heating issues are expected for the FCC-hh injection kickers with the spiral beam screen. Agnieszka Chmielińska IWG Meeting, 02. 08. 2019 26
Spiral tube (faultily installed) ~140 mm ~15 mm Capacitively coupled end Agnieszka Chmielińska Grounded end IWG Meeting, 02. 08. 2019 27
- Slides: 27