1 QXF heater proposal M. Marchevsky, H. Felice, T. Salmi, D.
Cheng, G. Sabbi, LBNL
Slide 2
2 General considerations Active quench protection: to create
the largest normal zone in the shortest possible time; distribute
stored magnet energy dissipation as uniformly as possible. Heater:
layered geometry means that heat from comes from the surface to
generate a bulk transition in the coil inherently not very
efficient method. Furthermore, the insulation barrier and the heat
capacity associated with heater material slow down the heat
transfer to the cable. Bulk heating could be a better idea (eddy
currents, etc) QXF heaters should be: -Powerful enough to meet the
quench protection challenges -Similarly designed for inner and
outer layer -Redundant -Scalable : same pattern for long and short
QXF Surface power density of > 100 W/cm 2 is desirable, based on
HQ / LQ experience
Slide 3
3 LARP magnet heater options a ) HQ-style heater a single strip
meandering along the coil inner and outer surfaces b) LQ/LHQ style
a meandering strip with varying cross-section heating station
concept c) Straight strips separately covering the high field and
low filed zones and separately powered d) A modification of c) with
sections lengths optimized according to the superconducting margin
of each section
Slide 4
4 Choosing the layout The only layout that was successfully
tested in long magnets if the LQ-style one (b). It allows extension
over large distances by spacing the heating stations further apart
Its first alternative is the pattern c that is planned to be
checked against the pattern b in the upcoming test of the LHQ. The
trace containing both patterns is being fabricated :
Slide 5
5 Optimizing period of the LHQ-style heater R1R1 R2R2 R3R3 R4R4
R3R3 R2R2 period r2r2 U1U1 U2U2 hot spot at the inner side of the
curved segment! r1r1 W1W1 L1L1 W2W2 L2L2 r1r1 r2r2 L3L3 Dimensionmm
L2L2 5 r1r1 2.5 W2W2 8.98 d0.000025 L4000 Optimize power per unit
area of the heating station for L 1, W 1 LHQ Coil 1 heater
pattern(LQ-style)
Slide 6
6 Possible design parameters (QXF) N=17 W 1 = 44 mm, L 1 = 210
mm p HS =127 W/cm 2 Assuming =5 10 -7 (SS304 at 100 K), U 0 =350 V
and L= 4 m: R heater = 4.81 For W 1 = 44 mm we can then have the
heating station coverage from the second turn from the pole to the
third turn from the outer turn same as in HQ.
Slide 7
7 Same design parameters for SQXF For the SQXF length of 1.3 m
and same heater design parameters we have a large reserve in heater
power: p HS =1500 W/cm 2 R heater = 1.42 SQXF heaters can be then
powered in series with a resistor to simulate the QXF heater
behavior.
Slide 8
8 Further steps on optimizing heater performance Reducing heat
capacity of the heater and increasing heat diffusivity of the
insulation is the most straightforward path Heater powering is done
by discharging a capacitor through it - technically simple, but not
optimal for the achieving the fastest heat transfer. Making heater
hot in a shortest possible time is needed Heater geometry should be
further adjusted based on the quench propagation velocity, as the
timescale for the active protection is the sum of time needed for
the heat to reach the cable edge plus the timescale for the quench
propagation between heating stations.
Slide 9
9 Heat transfer basics is thermal conductivity (W/(mK)) is
density (kg/m) is specific heat capacity (J/(kgK)) - thermal
diffusivity Materials with high thermal diffusivity: Pyrolytic
graphite, parallel to layers 1.22 10 -3 m 2 /s Silver, pure (99.9%)
1.65 10 -4 m 2 /s Silicon 8.8 10 5 m 2 /s Helium (300 K, 1 atm)
1.910 4 m 2 /s Can we introduce voids in the heater insulation
layer to benefit from high thermal diffusivity of the helium
gas???
Slide 10
10 Thermal diffusivity of the coil materials SS304 Kapton Cu
G10
Slide 11
11 Transverse heat diffusion H. S. Carslaw and J. C. Jaeger,
Conduction of Heat in Solids (Oxford University Press, New York,
1959), 2nd ed., p. 101. If the initial temperature distribution
within a thermally insulated solid of uniform thickness L is
T(x,0), the temperature distribution at any later time t is given
by: x L 0 surroundings heater insulation cable Can be solved
recursively for = (T(x,t)), using small time increments QQ The
amount of heat introduced in the heater zone at each step is
calculated based on the heater resistance R(T(x,t)), heat capacity
c(T(x,t)) and current I(t)
Slide 12
12 Heater simulation tool
Slide 13
13 Simulation of heater operation U 0 =100 V C=50 mF Heater
(SS304) thickness = 120 micron Heater length = 1 m Heater width =
10 mm Insulation 140 micron of Kapton 59 ms to reach 18.6 K at ~0.8
mm depth into the cable
Slide 14
14 Heater delay studies % of Iss T. Salmi, H. Felice HQ01e
Experimental verification of heater performance in and calibration
of delay versus heater power, magnet current and ambient
temperature was conducted for HQ01 and is in progress for HQ02.
These data are of great value for calibrating numerical tools and
optimizing heater geometry based on variation of the quench delay
values for different sections of the winding.
Slide 15
15 Heater temperature evolution in HQ01d 5 % Heater temperature
rises to 90 K This is still a low temperature for preserving the
integrity of heater material 5 % Heater temperature reaches a
maximum 35 ms after HFU firing. This is a long time for protection!
Resistance of the four-heater circuit after HFU firing
Slide 16
16 Conclusions Heater design work is in progress at LBL,
involving -development of the simulation tools -verification with
current and future magnet tests (HQ, LHQ) -search for the better
heater material and doing evaluative studies -new ideas about
optimizing heater powering scheme LQ/LHQ style heater pattern is
proposed as basis for the QXF heaters; its further optimization
will be done using existing and newly developing tools. It is also
pending experimental verification of performance in the upcoming
LHQ mirror test.