Simulation of the Electrostatic field in GTO stack, mechanical modifications with the goal to reduce
E-field to the safe values, simulation of the consequences to MKD current vaweforms
Surface sparking measurement at laboratory
Example of discharge at laboratory with 2 GTO stacks in parallel; Ch1 = Cts1A, Ch2 = Cts1B, both 5mV/A;
Ch3 = Cts 2A, Ch4 = Cts 2B both 250 mV/A
Sparking between GTO deflector and surface of plexiglas isolator – plexiglas surface charging
Corresponding Cts1ab and Cts2ab, Icts1Amax = 10 A, Icts2Amax= 600 mA, U = 29 kV
GTO stacks with return bars withot plexiglas tude
Sparking between GTO deflectors and silicon surface (plexiglas tube removed) even more intense – most
likely due to lower surface and volumic resistance of silicon polymer compare to plexiglas
Corresponding Cts1ab and Cts2ab, Icts1Amax = 13 A, Icts2Amax= 900 mA, U = 29 kV
2-D simulation of an electric field between upper GTO deflector smalest radius (1.5 mm!) and return metal
rod with silicon rubber and plexiglass tube. For simplicity reson the small radius is parallel with metal rod.
Emax > 7.8 MV/m!
2-D simulation of an electric field between upper GTO deflector (not taking into account small radius on
deflector) and return metal rod with silicon rubber and plexiglass tube. Emax > 7.4 MV/m!
3-D simulation of electric field arround 2 uppermost GTO deflectors (smalest radius of 1.5 mm!) and return
metal rod with silicon rubber and plexiglass tube. Emax ~ 8.1 MV/m!
Situation after removal of plexiglass insulation of return metal rod (leaving silicon insulation) and a metal
rod with insulation; Emax > 5.5 MV/m still too high leading to air ionisation
One of possible solutions: reduction of the E-field intensity in the air by hybrid insulation: allowing
accumulation of a surface charge on insulations of both conductors. Problem: initial surface charge
deposition most likely by ionization of the air between insulators surfaces and hence resulting displacemnt
current. Necessity to ensure high volumic and surface resistance of insulators to reduce frequency and
intensity of microdischarges keeping surface charge on the limit of the air ionisation – might be not easy due
to ambient humidity (and surface afinity to it) and dust deposition
Stack return bars (original) displaced by 5 mm form actual position, insulation on the surface of the original
GTO deflectors; Emax > 2.7 MV/m; value higher than air ionisation limit
Stack return bars (original) displaced by 7.5 mm form actual position, insulation on the surface of the
original GTO deflectors; Emax > 2.3 MV/m; value close to the of the air ionisation limit
Stack return bars (original) displaced by 10 mm form actual position, insulation on the surface of the
original GTO deflectors; Emax ~ 2 MV/m; sufficiently low to avoid air ionisation
Stack return bars displaced by 5 mm form actual position, GTO deflector diameter reduced to 104 mm
(from actual 107.5 mm); Emax > 2.4 MV/m; value close to the air ionisation limit
Stack return bars displaced by 8 mm form actual position, GTO deflector diameter reduced to 104 mm
(from actual 107.5 mm); Emax ~ 2 MV/m; safe value!
Situation with original return rods and with original but screened GTO deflectors; Corner effect is clearly
removed but field is still too high Emax > 4.7 MV/m; solution is usable only with accepting the surface
charging effect.
Situation with actual return rods moved by 8 mm away from actual position and with screened original GTO
deflectors; Emax ~ 2.26 MV/m; solution is probably acceptable but increases the stack inductance.
Situation with actual return rods moved by 10 mm away from actual position and with screened original
GTO deflectors; Emax < 2 MV/m; solution is acceptable but increases the stack inductance.
Stack return bars displaced by 8 mm form actual position, GTO deflector diameter reduced to 104 mm
(from actual 107.5 mm); Emax < 2 MV/m; safe value!
6 mm metal rod + 2 mm silicon (ε=3) + 5 mm teflon tube (ε=2.1); GTO deflector outer radius reduced to
100 mm (GTO dimensions) with 2 mm araldite coaling (ε=4.8); Emax ~ 2.25 MV/m
6 mm metal rod + 2 mm silicon (ε=3) + 5 mm teflon tube (ε=2.1); GTO deflector outer radius reduced to
100 mm (GTO dimensions) with 2 mm araldite coaling (ε=4.8); Emax ~ 2.2 MV/m
Actual situation; Emax ~ 8.1 MV/m!
Solution I.: Modified metal returned rod (diam 6 mm + 2mm silicon + 5 mm teflon) in actual position,
modified GTO deflectors (100 mm instead of actual 107.5 mm); Emax ~ 2.2 MV/m
Solution II.: Original metal rods but moved 8 mm away from GTO deflectors; modified GTO deflectors
(104 mm instead of actual 107.5 mm); Emax < 2 MV/m
Solution III.: Original metal rods (with silicon and plexiglass) moved 8 mm away from GTO deflectors;
insulated original GTO deflectors; Emax ~ 2.26 MV/m
Magnetic flux density at I = 1A for actual situation; Corresponding GTO stack inductance L = 132 nH (at
100 kHz).
Magnetic flux density at I = 1A for solution I.: Metal rod diameter reduced to 6 mm, kept position relative to
GTO; Corresponding GTO stack inductance L = 152 nH.
Magnetic flux density at I = 1A for solution II.: Metal rod displaced by 8 mm further relative to GTO;
Corresponding GTO stack inductance L = 146 nH.
18.91KA
18.90KA
Diff. Tovershot1 = + 6.7 ns
Diff. Iovershot1 = - 20 A (-0.11%)
18.88KA
18.87KA
4.46us
4.48us
4.50us
4.51us
-I(L9)
Time
Influence of GTO stack inductance to MKD current waveform; Actual stack with inductance of 132 nH
(red) and modified one with inductance of 146nH (green); MKD supply voltage of 29 092 V (7 Tev);
Overshot1 value differs by - 0.11% and is delayed by + 7 ns for modified stack. Solution: increasing of the
MKD voltage by 0.1%:
18.91KA
18.90KA
Tovershot1 = 4.485 us
Iovershot1 = 18.899 kA
18.88KA
18.87KA
4.46us
-I(L9)
4.48us
4.50us
4.51us
Time
MKD supply voltage increased to 29 121 V (+0.1%); Modified GTO stack with increased inductance (146
nH) used; Iovershot1 = 18.899 kA, Tovershot1 = 4.485 us
17.93KA
17.92KA
Diff. Tref 100% = -3 ns
Diff. Iref 100% = - 18 A ( -0.1%)
17.90KA
17.89KA
9.36us
9.40us
9.45us
9.50us
-I(L9)
Time
Influence of GTO stack inductance to MKD current waveform; Actual stack with inductance of 132 nH
(red) and modified one with inductance of 146nH (green). Iref 100 % value differs by - 0.1% and is
advanced by 3ns for modified stack.
17.93KA
17.92KA
Tref 100% = 9.426 us
Iref 100% = 17.916 kA
17.90KA
17.89KA
9.36us
-I(L9)
9.40us
9.45us
9.50us
Time
MKD supply voltage increased to 29 121 V (+0.1%); Modified GTO stack with increased inductance (146
nH) used; I ref 100% = 17.916 kA, Tref 100% = 9.426 us
36.0A
Ttreshold = 1.061 us
35.9A
Diff. Ttreshold = + 44 ps
35.8A
35.7A
35.6A
1.06125us
-I(L9)
1.06140us
1.06160us
1.06175us
Time
Rise time to Itreshold (35.8 A) difference due to increased stack inductance ~ +44 ps, Actual stack with
inductance of 132 nH (red) and modified one with inductance of 146nH (green)
36.0A
Itreshold = 35.8 A
35.9A
Ttreshold = 1.061 us
35.8A
35.7A
35.6A
1.06125us
-I(L9)
1.06140us
1.06160us
1.06175us
Time
MKD supply voltage increased to 29 121 V (+0.1%); Modified GTO stack with increased inductance (146
nH) used; I treshold = 35.8 A, Ttreshold = 1.061 us
17.93KA
Diff. in Time to reach Iref 100% = + 0.6 ns
17.92KA
17.90KA
17.89KA
3.780us
3.782us
3.784us
3.786us
-I(L9)
Time
Difference in Tdelay (time to reach Iref 100% for the first time) between actual stack (red; Iref100% =
17.9155kA) and modified one (green; Iref100% = 17.898 kA) is + 0.6 ns; Trise value is hence 2.72 us and
2.73 us respectively;
17.93KA
Tdelay = 3.784 us
17.92KA
17.90KA
17.89KA
3.780us
-I(L9)
3.782us
3.784us
3.786us
Time
MKD supply voltage increased to 29 121 V (+0.1%); Modified GTO stack with increased inductance (146
nH) used; Tdelay = 3.784 us. Ttreshold = 1.061 us, Trise = 2.723 us
18.95KA
Diff. Tovershot2 = + 100 ns
18.94KA
Diff. Iovershot2 = -14A (-0.08%)
18.93KA
54.0us
-I(L9)
55.0us
56.0us
56.7us
Time
Influence of stack inductance on Overshot2 value. Actual stack with inductance of 132 nH (red) and
modified one with inductance of 146nH (green)
18.95KA
Iovershot2 = 18.947 kA
18.94KA
Tovershot2 = 55.05 us
18.93KA
54.0us
-I(L9)
55.0us
56.0us
56.7us
Time
MKD supply voltage increased to 29 121 V (+0.1%); Modified GTO stack with increased inductance (146
nH) used; Iovershot2 = 18.947 kA , Tovershot2 = 55.05 us