Question bank — Fusion — solar fusion, tokamak (concept)

False — zyada $B/A$ ka matlab hai ki usse tod ne ke liye har nucleon ke liye zyada energy chahiye, isliye woh zyada tightly bound aur zyada stable hai.

False — sirf [[Binding Energy per Nucleon Curve|$B/A$ curve]] par iron se neeche wale light nuclei ke liye. Iron se bhaari do nuclei ko fuse karna curve ko neeche le jaata aur energy cost karta.

True in effect — products sach mein reactants se halke hote hain; woh missing rest-mass kinetic energy aur radiation ke roop mein wapas aa jaati hai. Mass aur energy ek hi quantity ke do accounts hain.

False — $\sim 1.5\times 10^7$ K Coulomb barrier ko brute force se beat karne ke liye bahut kam hai; fusion sirf usi barrier ke through quantum tunnelling se hota hai.

False — magnetic force $q\vec{v}\times \vec{B}$, $\vec{v}$ ke perpendicular hai, isliye woh zero work karta hai aur speed nahi badal sakta. Heating currents, RF waves, aur neutral beams se aati hai.

False — use zyada temperature (~$10^8$ K) chahiye, precisely kyunki hamare paas fuel ko compress aur confine karne ke liye Sun jaisi crushing gravity nahi hai.

False — plasma mein electrons strip ho jaate hain, isliye yeh free ions aur electrons ka ek soup hai; charged hone ki wajah se yeh magnetic aur electric fields par strongly react karta hai, jo ordinary gas nahi karta.

False — p–p chain mein neutrinos sirf ~26.7 MeV ka lagbhag 2% carry karte hain; baaki photons aur kinetic energy ke roop mein nikalti hai jo ultimately sunlight ban jaati hai.

True — yahi unifying idea hai. Light nuclei iron ki taraf upar fuse hote hain; heavy nuclei iron ki taraf neeche split hote hain. Iron woh valley hai jismein tum dono taraf se girte ho.

Error: fusion light nuclei ko join karta hai, kuch bhi split nahi karta. Sirf fission (heavy nuclei ka) split karta hai. Dono direction share karte hain (iron ki taraf), mechanism nahi.

Error: yeh $\Delta m{c}^2$ hai, $\Delta mc$ nahi. Light ki speed ka square conversion factor hai jo ek tiny mass ko large energy mein convert karta hai.

Error: pehla step ($p+p\to {}^2\text{D}$) extremely slow hai (weak-interaction bottleneck). Sun isliye bright hai kyunki usme protons ki astronomical number hai, na ki kisi ek reaction ke fast hone ki wajah se.

Error: Earth par hum Sun ki gravity supply nahi kar sakte, isliye hum magnetic confinement use karte hain — field lines ko torus mein bend karte hain taaki charged particles wall se takraaye bina along them spiral karte rahein.

Error: mass exactly conserved nahi hai — helium nucleus chaar protons se halka hai, aur woh mass defect ($\approx 26.7$ MeV ki value ka) energy ban jaata hai.

Error: particles field lines ke along spiral karte hain lekin ek straight field ke ends se drift kar jaate hain. Loop ko torus mein close karna padta hai, aur tab bhi vertical drifts cancel karne ke liye twisted (helical) field chahiye.

Error: yeh ek triple product $nT{\tau }_E$ hai. Density, temperature, aur confinement time teeno milke bade hone chahiye; blazingly hot hone par bhi agar bahut thin ya bahut leaky ho toh fail ho jaata hai.

Kyunki [[Binding Energy per Nucleon Curve|$B/A$ curve]] iron se neeche rise karti hai aur usse upar fall karti hai. Light nuclei ko combine karna curve par chadna hai (products zyada tightly bound → energy out); heavy nuclei ko combine karna use utarna hoga (energy in).

Kyunki [[Mass-Energy Equivalence (E=mc^2)|$E=m{c}^2$]] mass aur energy ke beech exact exchange rate hai; $c^2\approx 9\times 10^{16}{\text{m}}^2/{\text{s}}^2$ enormous hai, isliye ek fraction of a u ka mass loss millions of eV produce karta hai.

Dono nuclei positively charged hain aur Coulomb force se repel karte hain. High temperature ka matlab high kinetic energy hai, jisse nuclei itne paas aa sakein ki short-range strong force (aur tunnelling) unhe bind kar sake.

D–T ka sabse kam temperature par sabse bada reaction cross-section hai aur 17.6 MeV release karta hai, isliye yeh sabse aasaani se ignite hota hai — tritium ke radioactive hone ki takleef ke bawajood yeh worthwhile hai.

Ek charged particle ek field line ke along spiral karta hai (gyration). Ek closed loop ke koi ends nahi hote jisse leak ho sake, isliye particle endlessly circulate karta hai bajaye escape karne ke.

Kyunki $\vec{F}=q\vec{v}\times \vec{B}$ velocity ke perpendicular hai aur zero work karta hai — yeh particles ko steer karta hai lekin kabhi speed nahi badhata. Fusion ke liye energy alag heating se aani chahiye.

p–p chain ka rate-limiting pehla step slow weak interaction aur rare tunnelling par depend karta hai, isliye fuel gradually jalta hai. Uski self-gravity bhi ise runaway se stabilise karti hai.

Kuch useful nahi — iron $B/A$ curve ki peak par hai, isliye uska koi bhi fusion ya fission downhill jaata hai aur energy release karne ki jagah absorb karta hai.

Nahi — triple product $nT{\tau }_E$ phir bhi ~zero hoga kyunki $n\to 0$. Ek perfect lekin empty trap kuch bhi fuse nahi karta; teeno factors milke bade hone chahiye.

Koi $q\vec{v}\times \vec{B}$ nahi redirect karne ke liye, charged particles seedhe fly karte hain, chamber wall se takraate hain, instantly cool ho jaate hain, aur fusion conditions collapse ho jaati hain — ek disruption.

Nahi — break-even ($Q=1$) ka sirf matlab hai output equals input; useful net power (aur ignition, $Q\to \infty$, jahan reaction self-heat karta hai) ke liye isse exceed karna padta hai.

Classically effectively zero — koi kinetic energy nahi toh woh Coulomb barrier ke through approach hi nahi kar sakte; sirf finite energy plus tunnelling nonzero chance deta hai.

Nahi — neutral atoms koi Lorentz force feel nahi karte ($q=0$), isliye magnetic field unhe steer ya trap nahi kar sakti. Ionisation hi woh cheez hai jo magnetic confinement possible banati hai.

Temperature itna hona chahiye ki kaafi nuclei tunnelling threshold se upar rahein taaki reactions energy leaks se tez sustain ho sakein. Usse neeche jaane par losses jeet jaate hain, plasma cool ho jaata hai, aur fusion quench ho jaata hai — isliye D–T ka low ignition temperature itna valued hai.