Evaluation of HOM Coupler Probe Heating Problemtips by HFSSÔ Ssimulation
G. Wu, H. Wang, R. A. Rimmer, C. E. Reece
Abstract:
Three different tip geometiesgeometriesr iesin a HOM coupler on a CEBAF Upgrade Low Loss cavity have been evaluated by HFSS simulation to understand the tip surface heating problem under various situations. The surface heat loss was calculated for modified tip designs and for these different tips and for the standard tip design under different notch tuning conditions. The result shows that by sacrificing some of the HOM damping by detuning the notch frequency reduces surface heating a little, but insignificantly. Different tips also differ not so much in surface losses. The nail shaped tip with wider tip--to-coupler distance is the best option among three tip geometriess if one wants to reduce surface heating while not compromising HOM damping too much.
JLAB-TN-04-027
09/11/2004
Evaluation of HOM Coupler Probe Heating ProblemTips by HFSSÔ Ssimulation . JLAB-TN-04-026027
Evaluation of HOM Coupler Probe Heating Problemtips by HFSSÔ Ssimulation
1. Introduction
The probe tip of DESY type High Order Mode (HOM) coupler is located right in the electric field minimum for the cavity fundamental mode to reduce theuce fundamental mode power transmission loss through the HOM coupler. Thise electric filed minimum is also a magnetic field maximum in this transmission line type HOM coupler. according to Maxwell’s Law for electromagnetic wave propagation.The maximum surface current on the coupler tip surface This causes the tip heating problem. To maximize the HOM damping of CEBAF Upgrade cavities both on Low Loss and High Gradient shapesof the multi-cell cavities, the HOM coupler position is was moved closer to the end cell. This would increased the magnetic field ratio between the coupler tip to the cavity equator and produced extra heat on the coupler tiptip heating. If the tip heating was is not insufficiently conducted away through the probe feedthrough, it can lower the cavity quality factor (Q) or cause the tip to become normal conductor. This note describes the simulation results on the local magnetic field around the tip calculated by HFSSÔ code.
2. Computer Model
A 3D computer model was constructed as shown in Figure 1. A The Llow Lloss shape, single cell cavity is used as the cavity resonator. A coaxial line was inserted in the left side beam pipetube to act as an input probe to transmit RF power. The output is a 50Wthe coaxial line connected to the HOM probe tip. The fillet geometriesy onof the notch rodgap and the two inductive stubs inside of the HOM coupler are eliminated to avoid dense meshes on those corners in the 3D simplify the 3D modelmeshing..
Fig. 1 3D RF model used in the HFSSÔ simulation. The HOM coupler and the right side beam pipe was cut away by 135-degree to show the inside geometry.
The model accuracy was checked through different mesh densities. Three mesh regions were chosen. Therse areare the cavity beam pipe, HOM can and the notch cap. To einsure the mesh density consistency of mesh densities, the HOM can and the notch cap regions were manually seeded as shown in Figure 2.
Fig. 2 Manual seeding of the 3D RF model. Meshes for HOM can and notch cap are shown.
When the mesh densities of the HOM can and the notch cap were increased, the S21 transmission curve remained the same for the notch frequency region as shown in Figure 3. This assures implies that thise model is accurate at least for the frequenciesy between thearound cavity frequency and the notch frequency.
Fig. 3 The transmission coefficient vs. frequency.
To calculate the local magnetic field around the tip, the notch frequency has to be tuned into the o cavity resonance. This is the most time consuming part of the simulation. Figure 4 shows the tuning process.
Fig. 4 The frequency tuning of the notch filter.
The current HOM coupler tuning procedure rule to tune the notch gap is to lset was set very close to the notch minimum very close to thefor cavity resonance. The external Q is s for different HOM probe tips were around 1012 or higher.
3. Local field
(a) (b)
Fig. 5 The electric field magnitude (a) and magnetic field vector (b) in the HOM can.
Figure 5 shows the tip locates in the electric field minimum in the HOM can and the corresponding maximum magnetic field cycled wrapped around the center conductor of the HOM coupler.
For different geometriesy, the code gives slightly different cavity stored energiesy. Typically, the cavity equator field and the tip magnetic field are identified as shown in Figure 6. Then the ratio of to the two is calculated.Typically, a cavity equator field is identified as shown in Figure 6 (a). Then the ratio of tip magnetic field to the equator magnetic field is calculated as shown in Figure 6 (b).
(a) (b)
Fig. 6 The magnetic fields at the cavity equator (a) and atas normalization factor to the tip magnetic field (b).
(a) (b) (c)
Fig 7. Three tip geometries: (a) Tip1, as standard shape, (b) Tip 2, as rod shape, (c) Tip 3, as nail shape.
Three tip geometries have been modeled as shown in Figure 7. The dimension of Tip 1 is the standard design. Tip 2 is a simple rod with a 0.059-in round fillet. Tip 3 dimensions are shown in Figure 12Appendix A. Their local magnetic field is shown in Figure 8. All the tips have a 30-mil gap between the tip and the coupler center conductor (hook). TheAll Qexts wereas tuned to around 1013.
(a)
(b)
(c)
Fig. 8 The local magnetic field around probe tip. Tip 1 (a), Tip 2(b) and Tip 3 (c)
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Evaluation of HOM Coupler Probe Heating ProblemTips by HFSSÔ Ssimulation . JLAB-TN-04-026027
Table 1. The Tip magnetic field and their associated surface heat loss. (see following notes for details)
Note: The table is an editable spreadsheet object.
Tip 1 Detuning effect lists the parameters for different notch tuning.
Tip 1 30 mil sharp is for standard shape Tip 1 with sharper tip corner, tip-coupler gap is 30 mil.
Tip 1 50 mil is standard shape tip with tip-to-coupler gap at 50 mil.
Tip 2 30 mil is rod shape tip with tip-to-coupler gap at 30 mil.
Tip 3 30 mil is nail shape tip with tip-to-coupler gap at 30 mil.
Stored energy is for Llow Lloss shape end-cell between iris to iris.
S12 is the transmission S-parameter from input coax to HOM coax output.
Tip field is the magnetic field at the middle of the tip surface facing coupler as a percentage of the equator magnetic field.
Qext_HOM, Qext_FPC is calculated through single cell stored energy and the corresponding S-parameter for the ports.
Cap, Ring/Back, Taper/Rod, coax denotes the value of for different sections of the probe as shown in the picture below.
Total loss is the total probe heat (watt) per (MV/m)2 assuming 12mW surface resistance (Rs) and 3.8mT/(MV/m) equator magnetic fieldR/Q per unit length 3.8mT/(MV/m) equator magnetic field.
(r/Q) of 288.8 W/m.
The total loss in the last column in the spreadsheet is obtained by :
Wloss = ½Rs, 12.0A/m is the equator magnetic field when is calculated.Wloss = ½Rs, here 13.4A/m0.7 and 7 are account for the 7-cell cavity. The fcav is is the cavity’s frequency in the column “Cav. Freq (Hz)”. The Ucell is the end-cell’s stored energy in the column “stored energy (J)”.equator magnetic field when is calculated.
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Evaluation of HOM Coupler Probe Heating ProblemTips by HFSSÔ Ssimulation . JLAB-TN-04-026027
Fig. 9 The transmission coefficient vs. frequency
Table 1 lists the actual tip field ratio and the corresponding Qext. The transmission S12 is shown in Figure 9. One can see that tip 2 has lower transmission coefficient, which is due to the small tip area. The 10dB loss implies the potential 10 times higher Qext for HOM damping. When the tip 1 is pulled out 20mil more, the surface magnetic field drops a little, while the transmission coefficient becomes 5dB lower. From Figure 9, the HOM’s mode 1.82GHz peak shows higher Qext.
Fig. 10 The TM010 mode HOM Qext decreasesd when notch gap increasesd for Tip 1.
Fig. 11 The HOM Qext vs. the tip magnetic field for Tip 1.
For standard tip 1, when the notch gap wasfrequency increased to detune the notch frequencywas detuned as shown in Figure 10, the tip surface magnetic field did did drop, but not low enough (Figure 11). The HOM external Q listed in Figure 10 and ,11 is for a single cell cavity.. For a 7-cell cavity, a factor of 7 should be multiplyappliedbe applied to to the TM010 modeHOM Qext.
(a)
(b)
Fig. 12 The heat loss versus temperature: Tip 1 under different HOM tuning condition (a),
Tip 1,2,3 with 30 mil probe gap and Tip 1 with 50 mil probe gap (b).
From the numbers listed in table 1, the overall heat losses on the tip for a 20 MV/m cavity field under different temperatures were plotted in Figure 12. Here the surface resistance for niobium is calculated as the function of the surface temperature by the BCS theory. It shows that the proper thermal optimization may be more efficient compared to the geometric optimization.
4. Conclusion
The simulations results show that the notch detuning has only weak effect. The TTip 2 and the large gap Tip 1 in a larger gap sacrifices the HOM damping, but the decreaseing of the surface magnetic field was not significant. The Tip 3 that maintains the same HOM damping by the same size tip head size did not show an elevated tip magnetic field. The relative small surface area on the couplerprobe tip makes it the best option, yet not with dramatic improvement. Because the tip heat loss increases exponentially when tip temperature increases, the thermal properties of the probe and feedthrough are most important.
The dimensions of Tip 3 are shown in Figure 12.
Fig. 12 2 The Tip 3 dimensions used in HFSSÔ calculation.
5. AcknowledgementAcknowledgment
The three tip geometries were proposed by C. Reece and Co. during the HOM meeting in August 12, 2004. We would like to thanks the team of G. Ciovati, E. Daly, T. Elliott, W. Funk, P. Kneisel J. Mammosse, B. Manus, J. Ozelis, L. Phillips, J. Preble and T. Rothgeb for their assistance.
6. Appendix A The dimensions of Tip 3 used in HFSSÔ computation.
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