National Spherical Torus Experiment

NSTX NB

DESIGN SPECIFICATION
FOR
GRADIENT GRID RESISTANCE CHANGES

03/03/99

PREPARED BY:_____________________________
 (Guy Rossi)

REVIEWED BY:_____________________________
 (S. Ramakrishnan)

APPROVED BY:_____________________________
(T. Stevenson)

PRINCETON UNIVERSITY
PLASMA PHYSICS LABORATORY
(PPPL)
P.O. BOX 451
PRINCETON, N.J. 08543
609-243-2000

Gradient Grid Resistor Divider

1.0	Scope:

	1.1 Replace the existing scheme with an air cooled device.

2.0	Background:

	2.1 The existing device is a series of resistors totaling 25k ohms across the 120kv accel output 			and rated for 2.88 Megajoules (5 seconds) every 150 seconds.  These resistor are freon-			immersed in a large tank and are water cooled through a series of heat exchangers.  The 			expense and strict handling of freon has made this divider impractical to use.

3.0	Proposed Design:

	3.1 	The proposed scheme is to build a forced air cooled device, (see drawing).  The structure must fit inside the modulator/regulator, and rest on top of the filament transformer tank.  
		The structure will be octagon in shape, this will  allow a large  mounting area in a relativly small enclosure.  It will be constructed from fiberglass (G-11),  which has high heat resistance (160 degree C), and has excellent mechanical and dielectric properties.
		
		This divider must standoff 120kv, and operate at 80kv for a conditioning duty of 1 second and a repetition rate of 180 seconds.  The injection duty will be 5 second every 900 seconds. 

		The structure will hold  100 or 200 resistors in 4 banks, based on the configuration that is 			chosen.  Each bank will have 50 resistors, 25  on the outside and 25 on the inside. (see 			drawing),

		There will be ten taps available for the gradient grid voltage, tap 1 will be 90% of accel 			voltage, and tap 10 will be 80% of the accel voltage.  The taps will be in 1% increments. 		
		The resistors are manufactured by Vishay/Dale and are wirewound, but utilize Aryton-			Perry windings, (two single-layer parallel windings in opposite directions) to cancel most of 		the inductive effects. 
 
		The typical inductance for resistor values of 250 ohms to 500 ohms is .25 microhenrys.  

		Each resistor will have a tolerance of 1%, a temperature coefficient of 20PPM/degreeC.

		Each resistor is rated for 175 watts and can handle 10 times its rated power for 5 seconds, 			and can sustain temperatures up to +325 degrees C. This shall be derated to 96% at 			32.2°C (168W)

		
3.2 Configuration Options

		The following configurational options can be adopted to build a multi-resistive divider.

	Option 1-	
		100, 500 ohm resistors in series for the 50k ohm divider at 80kv.
		Each resistor will have to dissipate 1280 watts of power, (800v at 1.6 amps);
		Total average power ( for the bank of 100 reistors) will be (711 watts) for a duty cycle of 			1second every 180 second. 
		
	Option 2-	
		Use 200, 500 ohm resistors in series-parallel for the 25k ohm divider at 			80kv.
		Each resistor will have to dissipate 1280 watts of power, (800v at 1.6 amps)
		Total average power ( for the bank of 200 reistors) will be (1422 watts) for a duty cycle of 			1second every 180 second.

	Option 3
		Use 200, 1000 ohm resistors in series-parallel for the 50k ohm divider at 			80kv.
		Each resistor will have to dissipate 640 watts of power, (800v at 0.8 amps)
		Total average power ( for the bank of 200 reistors) will be (711 watts) for a duty cycle of 			1 second every 180 second. This opton enables operation up to 120 kV. It may be noted 			that the 1000 ohm resistor has appreciably low inductance as compared to the 500 ohms 			unit. See chart attached.

3.3	Prototype

		It is proposed to build a prototype of Option 1 in NB 5A, which will allow us to test a 50k 			ohm divider using the least amount of resistors, and not exceed their power ratings
		
3.4 Final Scheme
		
		After the prototype tests following schemes can be adopted.

	3.4.1	If the mod/reg tube does not turn on, Option 2 can be readily adopted by providing 				an additional bank of 500 ohm resistors in parallel to get an effective value of 25 				kohms.
		It may be noted that this divider can be operated only up to a level of 92kV dc  for 5 seconds without exceeding the power rating of the individual 	resistor.

	3.4.2	If the tube performs satisfactorily Option 3 will be adopted for the rest of 				the two units. Eventually the prototype can be reserved as a spare and 				NB5A also can be built per Option 3

3.5		Increasing the resistor bank from 25 kohms to 50 kohms will result in a very small increase 		in the change of gradient grid current with changes in the resistor values due to 				temperature and other conditions. For a 25kohm bank the % change in Igg with a 5% 			change in the resistor bank value is about  0.04% as compared to 0.08% if the 50kohm 			bank is used. See attached analysis for details.

4.0 Cooling requirements:

	4.1  Using configuration 2 as an example, the extra cooling needed to maintain a 10 				degree rise in temperature is as follows:

		Using 80kv,  the power in each resistor will be, (800v)(1.6 amps) = (1280 watts).

		The total average power will be (1280 watts)(.00555 duty cycle)(200 resistors)=
		(1421 watts) of extra heat that will need to be removed from the enclosure.
		The cooling requirements can be calculated from the equation, (Btu/hr)/(1.94)(Tc)=
		(1760)(kW)/(Tc)=(1760)(1.421)/(10)=250 cfm+25%. 
		
		The  mod/reg enclosure will require an extra 313 cfm to maintain a (10 degree 				C) rise in temperature.

		Direct resistor cooling will also be required to avoid any local hot spots. (At this time 				thermal time constant data is unknown and  not available form the manufacturer.  We 			have placed an order for sample resistors so that we may perform in-house thermal 				analysis.).  
		
		Small fans in the range 	of 250 to 500 cfm can be used for direct cooling.

5.0 	Reference Documents:

	5.1 New design mechanical drawing
	5.2 Schematic diagram
	5.3 Vishay/Dale resistor data sheet
	5.4 Dielectric sciences product document DS-105 wire data

6.0	Bill of Materials:

	6.1  200 resistors (NHL-175-07Z)
	6.2  4’x8’x3/8”  fire retardant  fiberglass sheet (G11)
	6.3  2’x2’x3/8”x 40’ long, 135 degrees fiberglass angle
	6.4   mounting hardware
	6.5  high voltage wire
	6.6  14 (7” fans)
	6.7 Labor (8 MD)

7.0 Cost:
7.1 Materials
	Construction materials          (@ $1000)
	200 resistors 1% tolerance   (@ $3000)
	14 fans (200/500 cfm)            (@ $1100)
7.2 Labor:
	Tech:    15  MDs

8.0 Schedule:

Resistor lead time, 5 to 8 weeks
Prototype fabrication & testing by June 99.
Design Review based on Prototype results - July 99
Installation of Gradient Grid Assembly -  by Jan 00

NB GRADIENT GRID DIVIDER MODIFICATION

*	EXISTING DIVIDER 25 KOHMS IMMERSED IN FREON

*	PROPOSED DIVIDER 50 KOHMS AIR COOLED

*	GUY HAS PREPARED A PACKAGE FOR THE CHANGE

*	VISHAY-DALE RESISTOR MODEL NHL-175-07Z
	100, 500 OHMS IN SERIES

		* 175 WATTS CONTINUOUS RATING
		* 1750 WATTS FOR 5 SECONDS
		* 8750 WATTSECONDS
		* INDUCTANCE < 1.2 uHENRIES

*	A FIBERGLASS STRUCTURE WILL BE MADE TO INSTALL
	THE RESISTORS.

*	PER CALCULATIONS VERY LITTLE CHANGE IN Igg WITH 	RESISTANCE VARIATION
		* 5% CHANGE IN RESISTANCE --- 0.008% CHANGE
			IN Igg WITH 50kOHMS BANK
		* 5% CHANGE IN RESISTANCE --- 0.004% CHANGE
			IN Igg WITH 25 kOHMS BANK

RECOMMEND - 50 kOHMS bank for prototype & test