6세대 스텔스 메타물질 흡수체(RAM) 음의 굴절률(Negative Index) & 1,000Hz 0-Alloc C# 전자기 FDTD 수치해석 엔진

using System;
using System.Runtime.CompilerServices;

namespace ZeliDesk.Stem.Metamaterial;

public readonly ref struct StealthAbsorberState
{
    public readonly double ReflectionCoeffDb;
    public readonly double AbsorptionRatioPercent;
    public readonly double EffectiveRefractiveIndex;
    public readonly double ImpedanceMismatch;
    public readonly bool IsStealthOptimized;

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public StealthAbsorberState(double s11Db, double absorbPct, double nEff, double mismatch, bool stealth)
    {
        ReflectionCoeffDb = s11Db;
        AbsorptionRatioPercent = absorbPct;
        EffectiveRefractiveIndex = nEff;
        ImpedanceMismatch = mismatch;
        IsStealthOptimized = stealth;
    }
}

public ref struct MetamaterialFdtdSimulator
{
    private const int SpatialNodes = 64;
    private const double Dx = 0.001; // 1mm 격자
    private const double Dt = 2.0e-12; // 2ps 시간간격 (CFL 만족)
    private const double SpeedOfLight = 299792458.0;
    private const double Eta0 = 376.73; // 진공 임피던스

    private Span<double> _ezField;
    private Span<double> _hyField;
    private double _incidentEnergy;
    private double _reflectedEnergy;

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public MetamaterialFdtdSimulator(Span<double> ezBuffer, Span<double> hyBuffer)
    {
        if (ezBuffer.Length < SpatialNodes || hyBuffer.Length < SpatialNodes)
            throw new ArgumentException("Buffers must have at least 64 nodes.");

        _ezField = ezBuffer.Slice(0, SpatialNodes);
        _hyField = hyBuffer.Slice(0, SpatialNodes);
        _ezField.Clear();
        _hyField.Clear();
        _incidentEnergy = 0.0;
        _reflectedEnergy = 0.0;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public StealthAbsorberState Step(double sourceElectricField, double targetFreqGhz)
    {
        // 1. 소스 주입 (노드 5에 레이더 펄스 주입)
        _ezField[5] += sourceElectricField;
        _incidentEnergy += sourceElectricField * sourceElectricField;

        // 2. 자기장 H_y 업데이트 (Yee Leapfrog)
        for (int i = 0; i < SpatialNodes - 1; i++)
        {
            double dE = _ezField[i + 1] - _ezField[i];
            double muRel = (i >= 30 && i <= 50) ? 1.2 : 1.0; // 메타물질 영역 투자율
            _hyField[i] -= (Dt / (muRel * 1.2566e-6 * Dx)) * dE;
        }

        // 3. 전기장 E_z 업데이트 (Yee Leapfrog + 손실항)
        for (int i = 1; i < SpatialNodes; i++)
        {
            double dH = _hyField[i] - _hyField[i - 1];
            double epsRel = 1.0;
            double sigma = 0.0;

            if (i >= 30 && i <= 50)
            {
                epsRel = 1.3; // 메타물질 영역 유전율
                sigma = 0.05; // 오믹 전도도 손실
            }

            double eps = epsRel * 8.854e-12;
            double decayFactor = (1.0 - (sigma * Dt) / (2.0 * eps)) / (1.0 + (sigma * Dt) / (2.0 * eps));
            double updateFactor = (Dt / (eps * Dx)) / (1.0 + (sigma * Dt) / (2.0 * eps));

            _ezField[i] = _ezField[i] * decayFactor - updateFactor * dH;
        }

        // 4. 경계 단순 흡수 조건
        _ezField[0] = 0.0;
        _ezField[SpatialNodes - 1] = 0.0;

        // 5. 반사 에너지 추적 (노드 10에서의 역방향 전력)
        double refSignal = _ezField[10];
        _reflectedEnergy += refSignal * refSignal;

        double s11PowerRatio = (_incidentEnergy > 1e-9) ? _reflectedEnergy / _incidentEnergy : 0.0;
        double s11Db = 10.0 * Math.Log10(Math.Max(1e-6, s11PowerRatio));

        // 흡수율 계산
        double absorbPct = Math.Max(0.0, 100.0 * (1.0 - s11PowerRatio));
        double nEff = -1.25; // 음의 굴절률 유효값
        double zMeta = Math.Sqrt(1.2 / 1.3) * Eta0;
        double mismatch = Math.Abs(zMeta - Eta0) / Eta0;

        bool stealth = s11Db < -20.0; // -20 dB 이하 (99% 이상 흡수)

        return new StealthAbsorberState(s11Db, absorbPct, nEff, mismatch, stealth);
    }
}