Computer simulations of the magnetically insulated transmission lines and post-hole convolute of ZR

Abstract

An important consideration for the success of the ZR project, refurbishing the Z accelerator at Sandia National Laboratories, is limiting current loss in the vacuum section, ideally to no worse than the 5 -10% seen on Z. The primary source for this loss is electrons flowing into the post-hole convolute from the four magnetically insulated transmission lines (MITLs). The MITLs on ZR have larger gaps to reduce the electron flow to values comparable to Z when operating at ~40% higher voltage and ~30% higher current.Electron flow in the vacuum section is analyzed with electromagnetic, particle-in-cell simulations, using two complementary simulation setups. First, the exact MITL profiles are modeled with high-resolution 2-D simulations out to large radius (typically r = 60 cm), providing accurate values for the electron flow into the convolute. Second, the convolute is modeled in 3-D, but with MITLs extending out only to r ~ 30 cm. The 3-D MITL geometry is modified to provide the same electron flow into the convolute as the 2-D simulations. The 3-D simulations have detailed diagnostics for current loss and surface deposition heating in the convolute.

Computational analysis of current-loss mechanisms in a post-hole convolute driven by magnetically insulated transmission lines

“…(For examples of numerical simulations of specific Z shots, see Refs. [12,[16][17][18][19][20].) The model presented here does enable the study of the complex interaction between magnetically insulated sheath currents generated upstream of the convolute with the post-hole convolute.…”

Steady-state modeling of current loss in a post-hole convolute driven by high power magnetically insulated transmission lines

“…The vacuum sections of the Z [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] and ZR [19][20][21][22][23][24][25] accelerators have been successful, providing efficient, high-current delivery from the generator to a variety of loads. On ZR, the upstream transmission lines are 3 m in diameter, the DPHC begins at a radius of approximately 11 cm, and the final radial transmission line begins at a radius of $5 cm.…”

“…At Z and ZR peak operating levels with load currents up to 26 MA and load voltages up to 6 MV [21,27], magnetic insulation of emitted electrons results in a portion of the cathode current being carried in a sheared E Â B flow within the anode-cathode (AK) gap [28]. On ZR the AK gaps of the four radial MITLs upstream of the convolute are tapered at relatively large distances from the convolute to obtain constant impedance profiles.…”

New self-magnetically insulated connection of multilevel accelerators to a common load

“…These experiments require tens of millions of amperes to be delivered to a common load through selfmagnetically insulated transmission lines (MITL) and a self-magnetically insulated convolute to a single disk current feed [6][7][8][9]. For the highest power accelerators, the currents from multiple modules are combined in parallel, outside the vacuum insulator, on each of two or more levels.…”

“…The posthole convolute (PHC) [5][6][7][8] is the most developed convolute geometry and works well when the impedance of the load is low. However, the complex 3D distribution of the magnetic field in the PHC design is accompanied by current loss for higher impedance loads [10,11].…”